/

THE EFFECT OF RADIANT ENERGY ON THE GROWTH
AND DEVELOPMENT OF RED KIDNEY BEAN
(Phaseolua vulgaris)

By
Dwight Douglas Murphy

A THESIS

Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
In partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
s

Department of Horticulture

1950

ACKNOWLEDGMENTS
The writer wishes to express his deep appreciation to Dr.
C. L. Hamner, professor of horticulture, for his help in the
direction and {guidance of this thesis, to Dr. R. E. Marshall
for his help in the administrative details of this project, and
to Professors C. E. Wildon and Paul Krone for their help re­
garding the use of florlcultural greenhouses and their equip­
ment.
The writer further wishes to extend sincere appreciation
to Professor Farrall, head of the department of agricultural
engineering, for suggestions, encouragement and cooperation,
especially in regard to the assembly and designing of all con­
trol equipment.

Other members of this department who have

helped to make this project possible also deserve mention—
particularly Mr. F. J. Hassler and Mr. Clarence Hansen.
The writer is grateful to Director V. R. Gardner

of the

agricultural experiment station, who applied for and obtained
the federal funds for this work which was listed as Agricul­
tural Engineering Experiment Station Project Number 65.
The writer wishes to thank the numerous members of the
horticultural and agricultural engineering departments and all
members of the guidance committee who have given their able
assistance.

TABLE OF CONTENTS
Page
. 1

I N T R O D U C T I O N ......................

REVIEW OF L I T E R A T U R E ................... ‘ ...............
Terminology .........................................
Temperature Measurements
Methods.

6

. .......................

......................................

7

Plant t e m p e r a t u r e s .............................10
The Influence of Light on Pl a n t s ....................15
A

Ultra-violet r a d i a t i o n ...........

28

The Use of Radiant E n e r g y .............

29

Controlled Conditions

56

. • .

EXPERIMENTAL
Selection of a Radiant Energy Source.

. . . . . .

Selection of an Adequate Reflector.

............. 46

Nutrient and ’Watering Requirements...............

43

54

Temperature Measurement and Control
Thermocouples................................... 61
P o t e n t i o m e t e r s ................................. 69
Converted Air Temperature...............
Soil temperature
Thermostats.

...........

=>77
82

........... ,.................... 90

Globe thermometers
Illumination...........

*I 92
94

GENERAL PROCEDURE.............. '........................... 98
RESULTS.................................................... 116

TABLE OF CONTENTS
(Continued)
DISCUSSION...........................
Temperature Relationships.

. .

Soil Temperature Relationships
Vapor Pressure Deficit
Plant Movements.

...

.

. ...........

S U M M A R Y ........... .. ............. ..
FUTURE SUGGESTIONS..................
LITERATURE C I T E D .................

.

LIST OF FIGURES
DIAGRAMS
1.
2.

3.

Page

Electromagnetic spectrum and the spectral
distribution
of a few radlatore.

3

Figure showing how a "heat trap" is created
hy the relative difference In transmission of
radiation and re-readlatlon through glass.

4

Graph showing the principle of the StefanBolzman law which states that the total
radiation from a black body varies as the
fourth power
of the absolute temperature.

34

Types of reflectors used In these radiation
studies and their respective distribution
patterns.

37

Diagram showing the principle of the inverse
square law of radiation distribution from a
point source of origin.

37

6.

Eppley Radiation Meter.

47

7.

Construction details of electrical radiation
unit u s e d for frost protection and later
used for radiation studies on Red Kidney
beans.

38

4.

5.

8.

Design of radiation house used for radiation
studies in the greenhouse during 1948.
Another
house of same design without reflector served
as h control house.
■'
50

9.

Method of automatic watering by the constantlevel method used for these radiation studies.
Diagram showing use in the greenhouse.
56

58.

Wiring diagram.

119

PHOTOGRAPHS
10.

11.

led Kidney beans, at the age' of 45
days,
^after growing in the radiation room, for 30
days.

20

Red Kidney beans, at the age of 45
days,
after growing in the control room for 30
days.

21

LIST OF FIGURES (Continued)
PHOTOGRAPHS (Continued)
12.

13.

14.
15.

16.

17.

18.
19.
20.

21.

Red Kidney beans, at the age of 29 days, after
growing in the radiation room (converted cold
storage) for 14 days.

22

Red Kidney beans, at the age of 29 days, after
growing in the control room (converted cold
storage room) for 14 days.

23

Radiation meter a nd thermopile used for measur­
ing radiant energy directly.

24.

25.

'48

Radiation room (converted cold storage in which
the air temperature was held at 40° F.) during
1949, showing six inch wide, parabolic reflector

51

The 12-inch wide elliptical reflector with the
cromolox heating unit mounted between two banks
of flurescent lights in the cold storage room
during 1950.

52

View of radiation room (converted cold storage)
in which the air temperature was maintained at
400 p.

53

Automatic watering system used for the radiation
house In 1948.

57

Automatic watering system used in the control
room (converted cold storage room) during 1950.

58

Example of Red 'Kidney bean seedling selected
for experimental work, photographed the same
day it was brought into the cold storage room.

60

One method of attaching thermocouple to lower
surface of geranium leaf for temperature
measurement.

64

22. * Upper surface of geranium leaf showing attach­
ment of three thermocouples by the loop-clip
method of attachment.
&
23.

Page

65

Lower surface of geranium leaf showing the same
three thermocouples Illustrated in Figure IS.

66

Method of mounting thermocouple in 1950.
Con­
trol leaf six days after moving into storage
room.

67

Method as above. ‘ Thermocouple attached to
radiated leaf.

67

LIST OF FIGURES (Continued)
PHOTOGRAPHS (Continued)
26.

Manually operated potentiometer with ten-point
selector switch for measuring temperatures from
ten different thermocouples. -

27.

Twelve-step recording potentiometer.

28.

Recording and controlling equipment in dry
greenhouse adjoining the two greenhouse rooms
used for radiation studies In 1948.

29.

Brown circular-chart potentiometer controller
shown mounted In the greenhouse 1948.

50.

Close-up of circular-chart potentiometer con­
troller showing pointer needle and recording
pen.
.

31.

.

.

.

Controlling equipment installed in horticulture
laboratory adjoining cold storage rooms used in
radiation experiment during 1949 and 1950

32.

Top view of Variac and Motortrol with rack and
pinion gear connections.

33.

Side tfiew of Variac V-20.

34.

Air pressure tank, gauges, valves, etc. used
for pneumatic operation of control equipment.

35.

Pressure reducer unit used for lowering pres­
sure to the desired 15 pounds per square Inch
necessary for pneumatic operation of poten­
tiometer and Motortrol.

36.

Lower view of model A convecter In the radia­
tion room of the greenhouse.

37.

Upper view of model A convecter mounted near
thermostat.

38.

Model B convecter shown mounted in the green­
house control room near the thermostat.

39.

Model C convecter which was used for measure­
ments of convected air temperature in the
radiation and control house.

40.

Double row spacing of pots containing Red K i d ­
ney beans used in 1949.

LIST OF FIGURES (Continued)
PHOTOGRAPHS (Continued)
41.
42.
43.
44.

Page

Radiation house used In the greenhouse during
1948.

87

Control house in early morning half hour kfter
sunrise.

95

Soil thermostat with electrical connections used
for the soil-heating cable.

89

Temperature pattern from the circular chart
potentiometer controller showing fluctuations of
temperature on the control point (lower surface
of a selected leaf in the radiation room).

91

47.

Control house mounted in control room 1948.

101

48.

Lower vieiv of parabolic reflecter mounted In
top of radiation house.'

102

49.

Lower view of top of control house showing board
placed to permit the same amount of verttilation. 102

50.

3lde view of upper half of radiation house show­
ing the wiring of the radiator.
Note also the
103
thermocouple wires coming into house.
J'

51.

Front corner of control house showing how the
. desired amount of ventilation was obtained.

104

63.

Potteft%olants used in radiation house In 1948.
Red Xldn&j^beans, geraniums, fuchsias, ireslnes,
106
coleus and"X tomato are represented.

64.

Top view of Red Kidney bean plants in control
room.

108

Complete assembly of frame, lights and tank
with Red Kidney beans as they appeared the
sixth day after setting in this control room
(converted cold storage room).

109

65.

TEMPERATURE CURVES
52.

Temperature curves In the radiation room of the
greenhouse at night when the radiator was set
for' full-capacity operation.
Test made in 1948..120

LI3T OF FIGURES (Continued)
TEMPERATURE CURVES (Continued)
53.

54.

55.

56.
57.

Page

Temperature curves showing the effect of solar
radiation on the leaf temperature in both the
radiation and control houses.

122

Normal night temperature curves in the green­
house when the controller was set for 60° F.
operation and the greenhouse heat turned off.

123

Temperature curves'in the radiation room show­
ing the changes in temperature before and after
the radiator was disconnected.

126

Temperature fluctuations in radiation room on
average day.

127

Normal temperature fluctuations In control
room (converted cold storage compartment).

128

BAR GRAPHS
59.

60.
61.
62.

Relative height of Red Kidney beans grown in
radiation and control rooms (converted cold
storage compartments) at the end of each
experiment.

112

Per cent total nitrogen of Red Kidney beans
(above hypocotyl) on dry weight basis.

113

Per cent ash of Red Kidney bean tops (all
parts above hypocotyl) on dry weight basis.

114

Analysis of Red Kidney beans (above hypocotyl)
after eight days growth in the radiation and
control rooms showing chlorophyll content on
green weight basis compared with the per cent
dry weight.

115

INTRODUCTION
The Intentional and controlled use of radiant energy has
found few applications in horticulture.

Although radiation

affects all living forms, little effort has been directed to
Intelligently apply it where it will do the most good.
Pew horticulturists realize how radiation affects the
temperature relationship of plant life.

It is true that mea­

surements of 10° C. or more above air temperature have often
been quoted in the literature, yet the idea that all physio­
logical processes are more or less dependent on plant tempera­
ture rather than on air temperature has seldom received the
attention it deserves.

This is reflected in the mass of

horticultural data in which writers use air temperature as
the criterion on which they form their conclusions.

In fact,

few of these temperatures even correspond to the true air
temperature since the mercury thermometer or thermograph was
used as the measuring or recording device In most of the
experiments.

These Instruments are sensitive to a certain

amount of radiant energy, the amount depending upon the ex­
tent to which they are protected from bodies of unlike temperature.

The relative amount of radiant energy absorbed by a

temperature-measuring device seldom or never approximates the
amount absorbed by growing plants.
Temperature can be derived in several ways.

First, the

soil may transfer a part of its temperature to the plant

directly through the root system.

Second, temperatures may

be distributed through convection currents referred to by
Foley (49).

Third, evaporation of water and other physical

changes of state either absorb or emit energy in the form of
heat.

For example, transpiration, which occurs at all times

in rapidly growing plants, lowers the temperature to some
extent in plant leaves.

Fourth, energy may be lost to colder

objects or gained from warmer objects.

Fifth, heat may accu­

mulate due to the absorption characteristics of such materials
For example,

the difference in the rate of transmission of

light of different ftave-lengths of the-electromagnetic spectrum (Figure 1) through glass causes the heating effect found
inside a glass enclosure (Figure 2).

Visible light and the

shorter infra-red waves pass without obstruction through the
glass, whl"’e the reradiated heat-waves from the plant— in the
form of the longer infra-red waves— are reflected Instead of
being transmitted through that medium.

This causes a condi­

tion Imown as a “heat trap*, an asset for greenhouse growers
in cold weather but a liability in hot3 weather.

The heat

trap makes scientific temperature control difficult.
The purpose of this experiment was to investigate how
radiant energy may be used by controlled equipment, both as a
supplementary source of heat in the greenhouse,
control rooms with no solar radiation.

and later in

Plant material was

chosen that was highly responsive to temperature changes and

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- 4 -

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uniform In its growth under controlled growing conditions.
The Red Kidney bean (Phaaeolus vulgaris) met these require­
ments.

6 REVIEW OP LITERATURE
Terminology
Radiation is the process by wh i c h radiant energy is
generated and emitted by a source and propagated through space
(Withrow, 145);

the source is referred to as the radiator.
'i
Radiation from the sun is termed solar radiation.
Although
the term radiant heat is commonly u s e d in the literature,

and

referred to in this thesis occasionally for purposes of clari­
fication, heat is not radiated but rather heat is produced in
an object when radiant energy strikes it, if that object has
the proper absorption characteristics.

The proper term for

the measurement of radiant energy is radlometry,
strument so used is a radiometer.

and the In-

A narrow band of electro­

magnetic waves which produce the sensation of light to man's
sense of sight is referred to as luminous energy or light.

In

a less technical sense, the term light Is used by many authors
to refer to those waves lying adjacent to both sides of the
visible spectrum— namely, ultra-violet and infra-red.

Luminous

energy emitted by fluorescent low-pressure mercury tubes Is
also referred to as fluorescent light.

It is measured by

means of a photometer.
Irradiation is the process of the Interception of radiant
energy by an object.

The re-radiation of some of this energy
»
at a longer wave length is termed fluorescence.

- 7 Radiant energy in the visible spectrum extends from .39
micron in length in the violet to .76 micron in the red;

the

infra-red rays extend up to the region of 100 microns, where
radio waves begin.

In this thesis the term near infra-red

refers to that band in the spectrum from visible red light to
waves 2 microns in length.

Below the blue light lies the

ultra-violet extending down to .01 micron.
rays are located,

Below this the X-

then the gamma rays, and, finally,

finitesimal cosmic rays.

the in­

Much of the energy of the sun* s rays

is absorbed by the atmosphere,

especially by water vapor.
' N 'v.

'~

Those rays penetrating in greatest abundance lie in the region
from .3 micron in the ultra-violet to 2.6 microns in the Infrared.
All electromagnetic .waves travel through space at a speed
of approximately 186,000 miles a second.

The frequency of the

wave is Inversely proportional to the length of the wave.
Barnes (12) refers to heat as the frequencies of oscillations
of atoms around their positions of equilibrium.

These fre­

quencies are of the same order of magnitude as those of infra­
red radiations.
Temperature Measurement
Methods
Radiant energy absorbed by matter is measured in terms of
s
.

the heat generated when it impinges on that matter.

Heat is

commonly measured by means of the mercury thermometer or some

- 8 instrument dependent on the expansion of gas, liquid, or
metallic plates.
Leaf temperature was first determined by pressing the
thermometer against the leaf or by wrapping the leaf about
^ the thermometer.

This method may still be used where only

approximate results are desired and where the relatively slow
response'of the ordinary thermometer is of no concern.

How­

ever, the most accurate and responsive method is to employ
the thermocouple.

This method,

invented by Seebeck (Allen

and Maxwell, 4), was first used on plants by Dutochet (Sachs,
s 116).

Blackman et al. (15) made their famous study on cherry

laurel leaves by comparing the difference between the tempera­
ture in the shade and that in the direct radiations from the
sun.
The thermocouple is usually made by twisting together the
ends of two fine wires of dissimilar metals, for example,
copper and constantan (an alloy of 60 % copper and 40^ nickel).
A difference in potential is set up between the two wires
thus brought together.

The magnitude of this electrical po­

tential is very nearly directly proportional to the temperature
of the thermocouple Junction.

A reference Junction, which is

necessary for measuring temperature differences,

is usually

made by putting a Junction into an ice-water bath (Forsythe, 50).
This is also called the cold Junction.
Instead of an ice-water bath,

Shreve (119) used the air

inside a thermos bottle and calibrated his readings for that

- 9 particular temperature.

3hreve* s Innovation merely complicated

the process because of the uncertainty of a steady air tempera­
ture inside a thermos bottle.

Wallace and Clum (136) placed

the reference Junction underneath the leaves of the plant whose
temperature they were measuring to find the magnitude of the
temperature difference between the leaf and the surrounding air.
It is debatable whether the air temperature they used was accur­
ate or whether it was typical of a true convected air tempera­
ture.

Their recording potentiometer was built so that the cold

Junction could be used in this manner.

%

Since most galvanometers and potentiometers used to measure
the electrical potential through the termocouple are read in
millivolts, a chart must be referred to in order to calculate
the temperature.

However,

some recent potentiometers are

calibrated at the factory to read directly in either the
Fahrenheit or Centigrade scale of temperatures;

others are

built to record either one or a large number of readings
simultaneously for an extended period of time.
The technique for making thermocouple Junctions varies
with the Investigator.

Miller and Saunders (90) used No. 36

copper and constantan wires braided together and Joined securely
with acid-free solder.

A pair of crucible tongs was fitted

with two small wedged-shaped pieces of cork.
these tongs the thermocouple was attached,

On one side of

and the completed

device held the leaf firmly while the temperature was read.

- 10 Curtis (40, 41) placed the Junction In direct contact
with the leaf and held It firmly by threading the lower p o r ­
tion of the thermocouple through the leaf.
Wallace and Clum (136) found It more desirable to use
several Junctions.

They assembled a thermopile composed of

five Junctions, which were mounted on a wire clip within a
* six mm. square frame.

When this clip was opened, pushed across

the leaf’, and allowed to close, the five Junctions were pressed
against the lower surface of the leaf and held In place by a
similar frame pressing down from the top.

Wallace (135) now

*

considers this thermopile obsolete since the five Junctions
Increase the probable error five times.
The placement of the thermocouple has been of considerable
concern to investigators.

Clum (35) obtained the temperature

of the interior of the leaf by inserting the Junction into the
intercellular tissue of the leaf.

This method was very tedious

and caused an inner metabolic disturbance of the leaf.

Wallace

and Clum (136)
reported that It was more satisfactory to attach
■
»
the thermocouple to the lower epidermis of the leaf.
G-ibson (55) designed some thermocouples for leaf inser­
tion by filing the thermoJunction to a point.

This device

injured the leaf, being entirely too large for the grass blades
used in his frost-prevention studies.
Plant temperature
Plant temperature is governed by a complex group of fac­
tors.

Miller and 3aunders(90)

list five that are influential

- 11 under field conditions:
moisture in the soil,
of the air,

(1) air temperature,

(3) air currents,

(5) light intensity.

(2) supply of

(4) evaporating power

They reported that the

temperature will rise more rapidly In a wilted leaf than in
■\

a turgid leaf.
Leaf temperature also fluctuates markedly and suddenly
w h e n air moves because of the difference in temperature of air
pockets.

The fact that temperatures may be higher at the tip

than at the base of the leaf blade,

they suggested, was due

to the difference in available water supply.

The lower surface

of the leaf was usually 1° C. higher than the upper surface.
Normal turgid leaves showed a leaf temperature approximately
the same as that of the air during the night but slightly lower
during the early morning and evening.
Leaf temperature (Meyer and Anderson, 89) may be regarded
as conditioned by four influences:
(2) thermal emission,

(1) thermal absorption,

(3) internal endothermic processes,

(4) Internal exothermic processes such as respiration.

and

Thermal

emission refers to the loss of heat from the leaf by conduction,
convection, and radiation (Brown and Escombe,

26).

Thermal

absorption refers to the gain of energy or heat by the leaf.
According to Maxwell (88), all objects tend to come into
thermal equilibrium with each other.

For this reason the

weather bureau uses instrument shelters to screen instruments
from objects of higher or lower temperatures (Blair, 16).

All

objects lose heat to interstellar space when the air is clear.

- 12 Tyndall (134)

states that the dryness of the air, rather than

the clearness,

causes the loss of heat.

Clear air may contain

much aqueous vapor which will intercept radiation even though
the sky be dark blue in color.

The heat lost by terrestrial

radiation is greatest when the atmosphere is dry.
Farrall et al.

According to

(48) and Hassler (67), heat lost by terrestrial

radiation is the cause of much frost injury.

Frost caused by

this means is called "radiation frost."
When a leaf is exposed to the full impact of solar radia­
tion, the temperature of the leaf normally rises markedly
above that of the surrounding air.

Clum (35) measured leaf

temperatures 5 to 10° C. higher than the surrounding air
temperatures.

Miller and Saunders (90) found little temperature

difference with the thinner-leaved plants they were using.
Blackman et al.

(15), using detached leaves,

in direct sun­

light detected a rise in temperature of 4 to 13° C. over that
of the surrounding air; in the shade the leaf temperature rose
only 1 to 1.5° C. higher than air temperature.

The maximum

temperature was recorded by Wallace (135), whose potentiometer
registered a gain of nearly 20° C . , although an earlier paper
(Wallace and Clum, 136) showed only a maximum 13° C. increase
outdoors and 16° C. increase in the greenhouse over air tem­
perature.

Moreland (92) measured the leaf of the sugar cane

and found a maximum temperature differential over air tempera­
ture of 8.5° C. on a still day and 4.5° C. increase on a windy
day.

- 13 Solid fruit can accumulate more beat than thin leaves,
as demonstrated by Brooks and, Fisher (22), who found that the
temperature of the exposed side of an apple may rise from 12
to 25° C. higher on the exposed side than on the shady side.
Anderson (4) reported that the Internal temperature of cotton
balls usually followed the general air curve but might exceed
the air temperature by 5 to 10° C. in direct solar radiation.
At night the balls were usually below air temperature.

He

remarked that respiration can produce very little heat.

Mil­

ler (91) suggested that absorption of radiant energy is in­
creased In thicker-leaved plants.

He quotes Askenasy (12),

who found that leaves of Sempervlvum attained a temperature
18 to 25° C. higher than that of Its surroundings.
Leaf temperature is also governed by air movement.

Brown

and Wilson (27) stated that air velocity Increased the emlsslvity of the leaf.

Smith (120) observed that breezes reduce

the temperature of the leaves In solar radiation from 2 to 10°
C. and that thin leaves are more responsive to temperature
fluctuations than thick ones..

Similar results were reported

by Miller and Saunders (90).
Curtis (40, 41) claimed that transpiration Is of great
importance as a cooling agent In leaves because of the change
of water from the liquid to the vapor state.

But he also made

the statement that some of the benefits of cooling by this means
were greatly exaggerated.

Curtis suggested that higher leaf

temperature relative to air temperature in humid regions,

- 14 usually ascribed to diminished transpiration,

is due in part

to the humidity of the atmosphere in preventing loss by thermal
emission.
energy,

He also stated that the angle of exposure to radiant

or the angle of incidence upon the leaf surface, greatly

affects the capacity to absorb heat.

He urged that all measure­

ments of temperature should be made with reference tb leaf
position,

or, unless otherwise mentioned,

the leaf position

should be understood to be at right angles to the Incident rays
of the radiator whether it be the sun or some artificial source.
Transpiration, according to Brown and Escombe (26), is
responsible for 80$ of the total heat given off by the leaf.
Clum (35) found little correlation between transpiration and
leaf cooling, because major temperature changes may occur
within a few seconds.
Copeland (36), working with chaparrel,

concluded that

transpiration decreased the heating of the leaves by solar
radiation, and observed that the cooling of actively transpir­
ing leaves amounted to more than 10° C.
Vatson (138) concluded that transpiration and thermal
emission from the leaf surfaces were the only significant
factors that tend to lower the temperature of the leaf.
son (139)

Wat­

later reasoned that transpiration accounted for more

than 50$ of the heat loss of Llrlodendron leaves only when both
of the following conditions obtain:
exceeds .71 gm./dm®/hr,

and,

(1) the transpiration rate

(2) the difference in temperature

between leaf and air temperature is less than 7° C.

Thus, it

- 15 Is evident that thermal emission becomes increasingly important
with an Increase In temperature differences.
\
The influence of transpiration on temperature changes in
still air is not clear.

Martin (85, 86) found that transpira­

tion increased in a linear relationship with the incident energy
absorbed by the leaf, but varied with relative humidity.
(131, 132)

Thut

found that the relationship between relative humidity

and water lose is more or less linear, but falls below the
expected value at low relative humidity.

It also varied with

wind velocity.
The Influence of Light on Plants
Although the sun is considered to be absolutely essential
for plant growth, man has been able to grow plants under arti­
ficial conditions with artificial lights only, but the most
satisfactory growth still results from solar radiation (Parker).
Artificial light for experimental plant studies may be
used for one of the following purposes:
sole source of light,

(l) to serve as the

(2) to lengthen the day,

(3) to serve as

an additional source of light in the middle of a dark period,
as used In some photo-periodic experiments,

(4) to serve as

the source of minute amounts of light to which seedlings grown
entirely in the dark are exposed, and (5) to serve as a source
of light of different wave lengths.
Light,

like all forms of radiant energy, varies in inten-

sity, quality, and duration,

and these factors have a profound

Influence on the growth and development of plants.

- 16 The intensity of solar radiation varies w i t h the altitude,
the clarity of the atmosphere, and the angle of Incidence of
the sun's rays upon the recipient of that radiation.

It may

exceed 10,000 foot-candles at times and yet be extremely weak
on the forest floor of a tropical rain forest (Oosting, 98)•
Plants vary in their ecological adaptability to different
environmental habitats, and survive where they can suitably
compete with existing weather and biological factors.

The

observed light conditions under w h i c h shade plants grow may not
indicate the most favorable growing conditions for those plants,
since many of them will grow better where they receive a higher
intensity of solar illumination.

On the other hand, many plants

of the xerophitlc type have developed a tolerance for high light
intensities which are not necessary for their optimum growth.
Holman (71) and Emerson (46) found that injury to the photo­
synthetic mechanism may result from exposure to intense radia­
tion.

They found that there existed a maximum point where the

starch ceases to be deposited and then disappears.

This

phenomenon was called Msolarlzatlon" because its reversal nature
reminded the authors of a similar response known in photography
by that name.

Solarlzation Is probably associated with the COg

concentration in the air.

Hoover et al.

(72) found that Increases

in light Intensity accelerated the photosynthetlc process u p to
the point where COg became the limiting factor.
low light intensities,
limiting factor,

At relatively

as long as the carbon dioxide is not the

the rate of photo-synthesis was approximately

- 17 proportional to tlie light intensity.

According to this study,

light was not the limiting factor under conditions of intense
solar radiation.
On the other hand, Heinicke and Childers (70)

showed that

the above statement did not apply to the normal apple tree,
probably because of the decreased light intensity in the in­
terior of the tree where the heavily shaded leaves reduced the
light intensity to one per cent of the total direct solar
radiation.
Brackett (21) found that COg became a limiting factor for
plant growth of wheat at high light intensities and that the
optimum COg concentration these plants could use varied with
the intensity of light available.
Matthaei (87) discovered that the utilization of C0£ by
plant tissue varied not only with light intensity but also with
the temperature.
Harvey (64) found that many'plants grew and set seeds in
light intensities from 400 to 10,000 foot-candles.

Shirley

(118) grew redwood seedlings under light intensities varying
from 33 to 300 foot-candles, and found that all Intensities
\
above 100 foot-candles had a similar effect on their growth,
while plants growing at lower intensities gave a less favor­
able growth.

G-eum showed a gradual reduction in growth as the

light intensities fell from 470 to 41 foot-candles.

The

author concluded from his experiments that all plants with which
he experimented,

except sunflowers,

can survive on less than

- 18 40 foot-candles of light.

At light Intensities below twenty

per cent of total solar radiation,

the Increase In dry weight

of plants so exposed was proportional to the light Intensity.
At higher light Intensities there was an Increase in dry
weight which was not proportional to light intensities.
S'

Ghithrle (63) and Shirley (118) both found that chloro­
phyll concentration increased with decreasing light intensi­
ties until the low light intensities threatened survival.
Post (107) reported that florlcultural crops,
those of bronze and pink shares,
at low light intensities.

especially

show poor color development

At high levels of light intensity,

there is doubtless a reduction in color intensity of the
petals as well as in that of the leaves of most crops.
Naylor and G-erner (95) found that 600 foot-candles of
fluorescent light for 16 hours duration a day caused excellent
growth in Red Kidney beans.
When lights were used to lengthen the day for long-day
plants, Withrow and Benedict (147) found that from .1 to .3
foot-candle illumination was sufficient to cause flowering
stimulation in the China aster.

They found that other long-

day plants require a higher initial light intensity.

This

was substantiated by Porter and Lee (103), who recommended
the use of 5 to 15 foot-candles of illumination for the
lengthening of the day period of a short-day plant.
The most economical utilization of light used to prevent
floral initiation in a short-day plant was to interrupt the

- 19 dark period by giving brief* periods of illumination about the
middle of the night.

Time intervals of only a few seconds

were effective (Parker et a l . , 99),
et al,, 101)

Later studies (Parker

showed that the total amount of illumination used

in the experiments was the same regardless of the time of
exposure or intensity of light.
Quality of light is classified as to color temperature
(79),

The numerical value indicates the predominating light

wave in the spectrum which is emitted by a black body when
heated to the Kelvin temperature of that number. *,The color
temperature is often referred to as being either warm or cool
depending on the physiological response or psychological re­
action of the observer.
Solar radiation changes its color temperature with the
angle of incidence, being warmer when the sun forms a more
acute angle with the horizon.

On the other hand, light radiated

from clouds or similar objects of white color will be cooler
to the visual sense.

Note the influence of a white background

in Figures 10, 11 as contrasted to a black and non-reflective
background in Figures 12 and 13.

However, when light is r e ­

flected by a colored object or filtered through the trees, it
does not conform to the true color temperature scale.
Plant pigments differ in the relative proportion of dif­
ferent wave lengths of light absorbed or reflected.
phyll (Miller, 91; Frank and Loomis,
tion bands.

Chloro­

51) showed five absorp­

The two which predominated were the red rays of

«

Figure 10. Rad Kidney beans, at the age
u5 days, after growing
4n the radiation room for 30 days. Photographed Marsh 16, 1950.

Figure 11. Red Kidney beans, at the age of 45 days, after growing
in the oontrol room for 30 days. Photographed March 16, 1950,

Figure 1$. Red Kidney beans, at the age of 29 days, after growing
in the radiation room (converted cold storage) for 14 days.
Photographed February 28, 1950,

Figure 13. Red Kidney beans, at the age of 29 days, after growing
in the control room (converted cold storage room) for 14 days.
Photographed February 28, 1950.

- 24 .651 micron to .680 micron and the blue, Indigo, and violet
raya of .44 micron to .510 micron.

The blue band shows a

slightly higher absorption coefficient (Meyer and Anderson, 89).
The relative rates of photosynthesis also differ with
the different regions of the spectrum.

Hoover (73) found that

the maximum photosynthesis occurred at the wave length of .655
micron in the red, and the secondary maximum at .440 micron in
the blue region of the spectrum.
In the spectral greenhouses at the Boyce Thompson Institute,
Popp (102) grew a number of different plants in light under
filters of different absorptive capacities.

His general con­

clusion was that plants growing in red light and the long end
of the spectrum were spindly, had weak stems, had good develop­
ment of chlorophyll, and had small thin leaves which tended to
roll or curl.

Plants grown in the short end of the spectrum

were sturdy, with well-formed leaves, but low stature.
Etiolation occurs when plants with sufficient storage of
food materials are grown for some time in total darkness.
Went (141) described another type of etiolation which is pro­
duced by plants grown under red light only.

This etiolabion

is typical of plants without reserve food storage, and is
entirely different anatomically from true etiolated plants.
Under conditions of true etiolation, the red rays are most
effective in inhibiting elongation.

Went prefers to call

etiolation of plants developing under red light without food
supply “excessive growth."

- 25 Went and Thlmann (144), speaking of the effect of light
on plant hormones,

state that its influence is twofold:

it is

first required to form the auxin precursors, and later it causes
destruction of the auxin produced from this precursor.

Appar­

ently the longer wave lengths favored the production of an
auxin precursor, thus enabling plants to grow taller when kept
in red light.

Yet the results seemed to indicate that those

same red and yellow rays were also the most effective in auxin
destruction.
Went (142) grew peas in darkness and then exposed them for
various time intervals to dim red light.

The longer the ex­

posure to the red light before the return to darkness,
greater the decrease in the length of the pea stems.

the
The

effect was traceable to that state of growth in which the
exposure was made; for example, if the plants were retained in
total darkness until the third Internode started to elongate,
then subjected to a small amount of red light, the third inter­
node would later show a reduction in length while the first two
Internodes would remain their original length; furthermore the
effect did not carry over to the fourth internode.

Blue light

did not produce as marked a reaction, and green light was
practically ineffective in reducing stem elongation.
Went (140) concluded that cotyledons contain hormone-like
factors which are indispensable for leaf growth and which are
'

\

rendered effective when subjected to the presence of small
amounts of light.

- 26 Priestly

(111) f o u n d that v e r y brief

exposures of light

to etiolated seedlings c a u s e d t h e m to b e c o m e less elongated
even w h e n such exposure w a s i n s u f f i c i e n t to p r o d u c e gre e n color.
H e concluded that there m u s t b e a p h o t o c a t l y t i c r e a c t i o n in
w h i c h substances of the n a t u r e of h o r m o n e s w e r e produced.
G o r t i k o v a (58), e x p o s i n g g e r m i n a t e d seeds to high i n t e n s i t y
colored l ights for t e n d a y s at 25 to 30° C. f o u n d that b o t h
vegeta t i v e a n d r e p r o d u c t i v e d e v e l o p m e n t w e r e stimulated by
r e d and o r a n g e - y e l l o w light.

B l u e light h a d the least effect.

R o h r b a u g h (115) f o u n d that d a y l i g h t f l u o r escent and the b l u e
fluorescent l i g h t gave e q u a l e l o n g a t i o n to the h y p o c o t y l a n d
first i n t e rnode of R e d K i d n e y beans.

'When a light source of

sufficient p u r i t y of c o l o r w a s used, h e f o u n d that r e d light
showed the g r e a t e s t effect in i n h i b i t i n g elongation.

This

a ccelerated g r o w t h is w h a t W e n t p r e f e r s to call H excessive
growth.H
Goodwin et al.

(57)

sh o w e d t h a t small q u a n tities of

m o n o c h r o m atic light i n h i b i t e d g r o w t h on the first internode
of A v e n a satlva seedlings g r o w i n g in darkness.
most p r o n o u n c e d in the r e d

The effect w a s

.623 m i c r o n r e g i o n a n d secondarily

in the y e l l o w light at .577 m i c r o n w a v e length.

Goodwin

(57)

fo u n d that u l t r a - v i o l e t light h a d a n inhibi t i n g effect on the
l e n g t h of A v e n a s a t l v a : i n f r a - r e d l i g h t h a d a slight effect on
length,

though its p r i m a r y effect w a s the heat gener a t e d in the

p l a n t tissue.

W a v e l e n g t h s over 1 . 6 m i c r o n s gave no me a s u r a b l e

gr o w t h difference.

- 27 «5

Lubimenko (82) found that the greening process of etiolated
seedlings takes place within definite limits of temperature,
which do not depend on length of light exposure.
The initial work on photoperiodism was announced by Garner
and Allard (54) in connection with their work on tobacco.
Porter (105) reported that the intensity of light necessary to
lengthen the day period lies in the neighborhood of ten footcandles of illumination.
Razumov (114) used different colored light to extend the
duration of several short-day plants.

He found that radiation

of red and yellow light, when used to lengthen these short
natural photoperiods, w a s equivalent to solar radiation in its
photoperiodic reaction, whereas the shorter wave lengths were
ineffective.
Kleshnin (75) found that all parts of the visible spectrum
gave photoperiodic reactions, but that the required intensities
differed from one region to another.

Differences in growth

were most noticeable when the intensity of monochromatic light
was small.
Withrow and Benedict

(147) found that orange and red light

produced earlier and more profuse blooming of pansies,
and asters.

stocks,

Although asters responded to all types of light,

stocks and pansies gave little reaction to the shorter wave
lengths.

Withrow and Biebel (148) found that the red band was

also effective in preventing flowering of short-day plants.
Withrow and Withrow (149)

studied the effect of colored light

- 28 in more detail by eliminating all extraneous light.

Their

results showed practically no effect of blue radiation and
reaffirmed the effectiveness of the r e d region.
Parker et al. (100)

studied the effectiveness of light

derived by spectrograph!c methods and reported that the most
effective region of the spectrum for preventing flowering of
cocklebur and soybeans extended from .6 micron to .68 micron
in the red band of the spectrum.

The least effective region

for preventing flowering was located at .48 micron;

effective­

ness increased again at shorter wave lengths in the violet
region.

Irradiation with infra-red light located at the .7

micron to .9 micron band proved ineffective in preventing
formation of floral primordia of soybeans.
Ultra-violet radiation
Stewart and Arthur (124) found that ultra-violet radiation
of plants grown in soil under low light intensity or during
cloudy weather caused an increased absorption <^f ash,
ing calcium and phosphorus.

includ­

But under high light intensity

there was no response to ultra-violet exposures.
tive rays lie between .9 micron and .313 micron,
with the effective antirachitic rays for animals.

The effec­
coinciding
The ability

of plants to form vitamin D seems to be associated with their
capacity to absorb and deposit calcium and phosphorus.

Later

Stewart »and Arthur (125) found that the relative absorption
of calcium and phosphorus of irradiated tomato plants depended
upon the ratio of the two elements in the nutrient solution.

- 29 ** •

Arnold (18) found that ultra-violet ray a do not apparently
kill chlorophyll on Chlorella, a green algae, but that the
injurious reaction to these cellular plants caused by these
rays may be due to the destruction of another substance in
these plants.
The Use of Radiant Energy
Prior to m a n ’s arrival at t h e - stage of intellectual
maturity though he was unaware of the fact, he nevertheless ,
was emitting radiant waves which centered in the 10 micron
area of the infra-red portion of the spectrum.

He was like­

wise receiving radiations whose spectral characteristics
depended upon the nature and temperature of the objects about
him.

Even today man is not always aware of these temperature

relationships.

'

In an early stage of history h e became aware that the sun
produced heat, and, later,

that a fire

same sensation in his body.

could also produce

the

Realizing the great benefits to

be derived from such heat-producing sources, he felt that a
great power must reside therein.

Thus history records the

religious adventures of man in w h i c h both sun and fire play an
Important part.

He has sought to deify and personify them,

e. g . , the ancient Hindu god

for fire, Agni, and the great

god of the sun in Egypt, Ra.
Although modern man does not usually deify the sun, he
recognizes its great source of power, and has at times tried to
capture some of this energy to heat water (Brooks,

23) and to

- 30 heat hie homes (Telkes,

128, 129),

Solar radiation has a

great Influence upon the terrestrial temperature of the earth,
and even local disturbances on the sun known, as sun spots may
have far-reaching effects.

Burkholder (28) stated that 60#

of the energy from the sun is in the Infra-red region of the
spectrum, and that plant growth is roughly doubled for every

10° c. rise in temperature.
Adlam (1) and Chase (32) both suggest that the earliest
man-made form of radiant energy should be attributed to the
Romans, who ingeniously designed the first form of artificial
heating,

the hypocaust system.

The Romans built masonry ducts

behind and within side walls or beneath floors through w hich
hot flue gases were made to flow.

This system heated the

adjacent walls or floor surfaces, which,
buildings the Romans wanted heated.

in turn, heated the

The remains may still be

seen today at the Roman Baths at Bath,

England,

and at the

Baths of Caracolla in Rome; these baths probalbly date back
more than 1500 years.
The modern use of radiant heating for homes began a few
miles from where the Romans built their famous baths in
England.
ing",

This system, more appropriately called "panel h e a t ­

spread from England to continental Europe (Adlam, 1;

Byers,30) and to the Un i t e d States, and is now one of the
accepted methods of heating homes.

In this design hot-water

pipes are usually imbedded in plaster,

concrete,

etc.

The

temperature of the panels is low since the temperature of the
water circulated seldom exceeds 135° P.

- 31 The advocate of panel heating stresses comfort.

He des­

cribes how man loses heat by a complex combination of respira­
tion,

evaporation, and radiation.

Radiation Is usually respon­

sible for three times as much heat loss as respiration and
evaporation combined.

The rate of radiation loss depends upon

the average temperature of the surroundings.

When the tempera­

ture of the air is reduced and the average wall temperature In­
creased, man loses less heat by radiation and proportionally
more by respiration and evaporation.

Increased heat loss due

to respiration tends to give man an invigorating feeling simi­
l ar to the sensation of walking briskly on a cool morning.
panel heating, heat, usually in the form of hot water,
ducted through one or a combination of walls,

In

is con­

celling and floor

to attain an average wall temperature about 5° F., more or less,
above the so-called comfort temperature of man, while the air
temperature is usually reduced proportionately as much below
this temperature.

A typical panel-heated room has an air temper­

ature of 65° to 68° F. (18° C.), with the relative humidity
registering 55 to 65#.
Comfort conditions during the summer months can be attained
by radiant cooling In the home.

In this system reducing the

relative humidity of the air Increases the rate of respiration
and evaporation from the skin.

A lower moisture content In the

air creates a cooling sensation even when the air temperature
is above the usual comfort level.

By this means man can move

in and out of such an environment into-hot summer weather without

I

- 32 the feeling of entering an oven or a refrigerator.

This sya

ardless of the propaganda of

t em of cooling

air conditioners who advocate increasing the water content of
%
the air by washing it.
The botanist would like to know whether
the relationship between radiation and transpiration under these
different conditions affects plant growth.
Low-efficiency radiation has long been used by gardeners,
whether they knew it or not, when they planted a vine or
espalier next to a warm building.

Hotbeds also give off some

radiation, and all greenhouses which use glass or its equiva­
lent in their construction accumulate an excess of radiant
energy (Figure 8).

Steam pipes and other radiators give off

a small amount of radiation, but since radiation travels in
straight lines,

the usual design of greenhouses (Wright, 150;

Post, 107; Lau&le and Kiplinger,

90) does not permit much of
'i
this energy to travel to the plants directly.
Some attempts

to heat greenhouses by aerial pipes can be seen in the Hill
Greenhouses at Richmond, Indiana, and the new greenhouse range
at Michigan State College.

The efficiency of heating by radia­

tion is extremely low at the temperature of these pipes because
most of the heat is transferred by means of convection.

The low

temperature of most steam or hot water pipes emits waves in the
8 to 9 microns region of the spectrum, according to calcula­
tions from Wein* s law."*"

1.

Welns law —

2884
f (Kelvin)

33 An adaptation of the usual panel-heating system through
the concrete floor is used In one Of the rooms of the~greenhouse range of Michigan State College.

In this r o o m hot water

is run through pipes imbedded in the concrete.

The efficiency

of heating by radiant means as compared to other sources of
radiation is well demonstrated by the Stefan-Bolzman law^ as
Illustrated in Figure 3.

The efficiency of this system cannot

be shown on this graph, which starts at the boiling point of
water.

It Is difficult to ascertain the true efficiency at

such low temperature levels because so many factors distort
the picture.

It is suggested that this type of heating be

known simply as floor heating or Its equivalent, a n d that the
name radiant heating,

should that name be acceptable, be re­

served for a system in which the surface temperature of the
heated object Is above 100° C.
Meteorologists,

geographers, and ecologists study the

effects of solar radiation and how It affects plant life
(Oostlng, 98).

Dinger (42), who measured the spectral absorp<
S
>

tlon pattern of near infra-red radiation,

;

concluded that plants

absorb those rays, namely 1.5 and 2.0 microns most strongly in
a pattern similar to the absorption pattern of water.
Massachusetts,

In Dover,

an attempt has been made to store solar energy

In a house which collects its heat by black-surfaced plates
and transfers this heat to cans containing sodium sulphate, a

1.

S^efan-Bolzman law = Calories per sec per cm^ = 1^36 X lO- ^

- 34 -

Cal./ cmf-sec

1.36

x

10

-12

T

A

*

Kelvin temperature- Centigrade ♦ 273

21

18

(*1 ' 1

It
i

to

773

12

rr <
•ii

«J

r

C.)

373'

300

500

1000

1000

2000

f i g u r e 3. O r a o h s h o w i n g t h e p r i n c i p l e o f t h e S t e f a n ;,o I z ~ s n l aw w h i c h s t a t e s t h a t t h e t o t a l r a d i a t i o n
f r o m a b l ^ c k b o d y v a r i e s as t h e f o u r t h p o w e r o f t h e
absolute t e m n e n t u r e .
*

- 35 substance well suited to this purpose since Its heat of fusion
Is about 88° to 90^ F.

(Telkes, 128, 129).

Perhaps the same

method could be used for storing heat for growing plants.
The earliest attempt to study the radiation of plants by
heated panels was published by Bigeault (16) and Chauard (33).
Their panels were spaced 20 inches apart and located in &
greenhouse at Saint Agnan near Oise, France.
panels were installed steam pipes for heat.

Inside these
The radiation was

slightly more efficient than the panel type of heating pre­
viously mentioned which is used in the building industry; h o w ­
ever, it lacked the efficiency that a high-temperature radiator
possesses.

While the roots were heated by the use of soil

heating units,

the leaves were partially heated by radiation

from the sides and overhanging surfaces which shaded out part
of the light from the sky.
Gray (59) built a similar panel heater using lead heating
cables for the heat source.

Two thermostats were used, one

for the two coils which operated at temperatures below 60° F.
and the other for -the two additional lead coils set at 55° F.
Temperatures of 5° to 30° F. above air temperature were recorded
by means of a thermocouple.

The test plant, a geranium, was

killed by a heavy snow at an air temperature of 8° F.

The most

noteworthy observation was the rapid drying of this plant.
Professor Farrall was instrumental in starting the first
large project on high-efficiency radiation.

His work with

frost protection commanded world-wide attention.

The first

-36
tests with a rod-shaped electrical heating unit (Figures 4, 7)
showed definite promise of protecting plants from frost on a
still night

(Farrall et a l . , 48; Gibson, 55).

Later tests

made with more practical gas-burning units offered promise to
orchardlsts,

truck gardeners,

(Farrall et a l . , 48; Hassler,

and other horticulturists
67; Hassler et al.,

68).

The

plants can be protected from radiation frosts when the tempera­
ture descends to approximately 26° F.
Other types of electrical resistors were tried by Gibson
(55).

Of particular Interest was a heating element with its

coil wound on a cylindrical refractory form which was mounted
in an aluminum reflector.

Heat lamps (1700° C.; 2000° Kelvin)

prevented frost on a dahlia plot at Lakeside Gardens, New
Baltimore, Michigan.

The visible light emitted by this type of

radiator makes it unsuitable for use on plants in the dark
period In photoperiodic studies when not suitably filtered.
Otherwise,

If breakabllity is no objection,

it is a highly

efficient source of heat producing rays.
Controlled Condit ions
In scientific research, as in the solving of an algebra
problem,

the experimenter tries to reduce the number of un­

known factors to the very minimum.

The perfect method is one

variable with perfect control of all other unknowns.

In biolog­

ical science this cjui never be completely accomplished, although
the probable error may be reduced by careful control of all

i

T?lliptical

Parabolic (behind focus)

F i g u r e 4. T y p e s o f r e f l e c t o r s u s e d i n t h e s e r a d i a t i o n
s t u d i e s an d t h e i r r e s p e c t i v e d i s t r i b u t i o n pa tt er ns .

F i g u r e 5. D i a g r a m s h o w i n g t h e p r i n c i p l e o f
law o f r a d i a t i o n d i s t r i b u t i o n from a p o i n t

the inverse seuare
source of origin.

- 39 environmental factors.

If the Investigator starts with homo­

zygous plant material of the same size and age and also can
control all exterior Influences, he Is likely to realize some
benefit from his investigation.
Crocker (38) described the effort of the Boyce-Thompson
Institute to control the living conditions of plants.

Two

constant-conditlon rooms were built, one with continuous illu­
mination and the other without light.

Photoperiodic studies

were made by shifting plants from one room to the other.

In

these rooms temperature was accurately controlled; all lights
were cooled by means of circulated water behind a plate-glass
celling.

The rooms were air-conditioned with air in which

the COg concentration could be enriched and regulated and the
relative humidity controlled.

In the two 11 x 11 foot rooms

the light intensity was held at 900 foot-candles.
Boyce-Thompson Institute also uses (1) two greenhouses
equipped with a movable gantry crane having a battery of
lights which can be moved over the greenhouse when additional
light is needed to lengthen the day,

(2) an insulated green­

house (Arthur and Porter, 9), which is used to keep out un­
wanted temperature differences, and (3) a spectral greenhouse
(Popp, 102) designed for plant study with different-colored
light s.
At the California Institute of Technology, Went (142)
described two air-conditioned greenhouses each of which was
Joined to a dark laboratory.

The temperature was accurately

- 40 -

i
controlled,

the greenhouses being heated by air circulated

through controlled baffles and being cooled by a circulatory
system of cold water which flowed over the glass on the roof of
the greenhouse.

The thermostats were actuated by a photoelec­

tric light meter at a 200 foot-candle light intensity reading
which changed the setting of the thermostats from that for the
night temperature to that desired for the day temperature.

The

air supply was also humidified to the proper degree of relative
humidity.

With these controlled conditions, Went did some

exacting experiments.
The 3mithsonian Institute built several small plant growth
chambers for studying the effect of different colored lights
on plants (Brackett and Johnson, 20).

The octagonal cylinders,

15 inches in diameter and 22 inches high, were surrounded by a
water jacket for accurate temperature control.

The same tank

supplied all the water used in this circulatory system;
same air supply was forced into all chambers.

the

The nutrient

solution was also heated and circulated throughout all chambers.
The United U t a t e s Plant, Soil and Nutritional Laboratory at
Ithaca, New York,

installed in a basement-room lOjjt- x 16% foot

eight panels of fluorescent lights,

each of which contained

twelve 30-watt 36-inch cylindrical lamps (Hamner,

65).

The

reflection from these panels was slightly increased by painting
the under surface with white paint. ... By the close arrangement of
lights the laboratory was illuminated by a light intensity of
2000 foot-candles, which grew better plants during the months

- 41 from November to April than the greenhouse did at the same
season of the year.

|Jeat from the fluorescent tubes was reduced

by means of a fan and the humidity was regulated by a humidistat.
Temperature and relative humidity were h e l d within a few degrees
of the desired setting.

A combination of white and daylight

fluorescent light gave the best results for most plants,
ing the Red Kidney bean.

includ­

It was suggested that more uniform

temperature conditions could be maintained if the lights could
be made to burn continuously rather than intermittently;

the

experimenters had found Intermittent light necessary to simulate
the duration of solar radiation or the particular duration of
light required for any particular photoperiodic crop.
In Oklahoma,

Chester (34) installed a temperature regulator

in two greenhouses.

A n elevated heater in the center of each

house supplied the heat by means of a series of steam colls
mounted in an enclosure;

a fan circulated the heated air through

anemostats or diffusers with cone-shaped baffles.

The heater

was operated b y a thermostat; when the temperature rose above
a predetermined level, the ringing of a bell called an attendant
to open the ventilators.
Hartman and McKinnon (66) described an environmental control
cabinet built at Davis,

California,

for the study of the r e l a ­

tionship between temperature and the photoperlodic response of
plants.

Four cabinets,

a refrigerator,

3 x 4 x 5

feet,

constructed adjacent to

supplied cool air for reducing temperatures

around the lights,

which were mounted in a compartment over the

- 42 glass-roofed cabinets.

The twelve 40-watt fluorescent light

tubes supplied a light intensity of 500 foot-candles to the
plants, although it was suggested that a higher intensity w ould
be more suitable for the two cabinets which were heated to a
higher air temperature.

Relative humidity was ra i s e d in the

warmer compartments by keeping water on the floor until suf­
ficient water was absorbed so that the relative humidity
equaled that in the cooler rooms.
Withrow and Withrow (149) grew plants in air-conditioned
chambers controlled within a variation of .5° to 3° C.

The

relative humidity was..maintained within a variation of 70 to
80$ saturation at the temperature studied.

In each case the

rooting medium was subirrigation gravel culture.

All lights

functioned satisfactorily, but the incandescent lamps were most
difficult to cool because of the high infra-red radiation.

Most

of the lights were mounted behind water-filled b e l l s five Inches
deep.
Parker (100) worked w i t h plants in many controlled experi­
ments and came to the conclusion that the population of plants
is limited in a control chamber, but that the results regarding
light and environmental reactions are of great value.
Naylor and G-erner (95) and latet* Naylor (94) found that
fluorescent lights gave them the best results of all lights
tried, and that 600 foot-candles gave good growth to Red Kidney
beans.

Both white and daylight fluorescent lights appeared to

be equally effective with little heat being dissipated.

Various

- 43 colored fluorescent light tubes,

they said, were also available

for special purposes.
Recently the greenhouse industry has become interested in
control equipment.

Thermostats for heat control,

and the

trombone style of heating pipes, w h i c h are more responsive to
regulation, will make greenhouse heating easier.

Automatic

ventilation will help to solve the cooling problem, and auto­
matic watering will aid the growers in maintaining an even w ater
supply at the roots of their crop.
EXPERIMENTAL
Selection of a Radiant Energy Source
As previously stated,
is the sun.

the primal source of radiant energy

Its illumination is indispensable for the process

of photosynthesis as well as the heat it produces in plants.
The energy utilized by plants directly or indirectly from the
sun enables them to carry on their normal physiological r e a c ­
tions— to grow, develop, and reproduce.
Radiant energy is being transferred from and to all places
and at all times, but the rate and wave-length may not be easily
recognized.

Wherever there Is a temperature difference— such

as between a fireplace and the objects surrounding it— there is
a transference of energy between the two objects with the major
share of energy leaving the hotter object.
radiator in relation to the plant,
tor will be as a heating agent.

The hotter the

the more effective the r a d i a ­

Panel heating should be called

- 44 a low-efficiency radiator because the temperature of the
panels or walls Is only slightly above the objects which It
surrounds.

The heating panel of Bigeault (16) and Chauard (33)

a n d those of Gray (59) were slightly more efficient than most
panels as a radiant source.
To compare the efficiency of radiators at different temper­
atures, reference to Figure 3 will show the relationship between
the energy produced at all temperatures from 372° K.
to 2000^ K.

(100° C.)

It Is evident that the efficiency Increases almost

directly with the fourth power of their Absolute (Kelvin)
temperature.
Because of the difference of temperature the most highly
efficient source of radiant energy that might be u s e d for
plant radiation studies Is the heat lamp (74).

When new,

this

lamp burns at slightly above 1700° C., and its spectral pattern
extends
well Into the visible - spectrum (Figure 1).
'v

But because

It gives off a measurable quantity of Illumination, which Is
present at all hours of the day that it Is used for heating,

It

Is not beneficial to plants which demand a certain period of
darkness;

1. e. a short-day plant,

like some varieties of soy­

beans, will not produce seed when the number of hours of the
light period exceeds an amount known as the critical period.
If these plants are exposed to light more than this number of
hours they will remain In the vegetative condition.
In this problem a radiator was needed whose radiation In­
cluded no visible light,

or one whose illumination was so

- 45 negligible that it would produce no light stimulus in the
plants studied.

It was desired to eliminate all effect of

photoperiodism, a n d at the same time to heat the plants by a
radiator of the most efficient type w h i c h still emitted no
light.
C.

This requirement indicated an emitting surface of 500°

Since b o t h the electrical and gas-burner types of radiators

used by Parral et al.

(48) for frost protection were used at

this temperature and higher temperatures,

they w e r e considered

ideal for this experiment.
A gas-burning radiator, however,
quarters w i t h plants,

since gas

is undesirable at close

injures plants.

Until an

efficient gas-burner is designed without the danger of escap­
ing gas, it will be of questionable value for the heating of
-plants.
4

The electrical design was therefore the most practical of
these two heaters;
controlled.

the heat output could also be more readily

The design used by

Farrall (48) was

based on an

electrical cromolox heating rod 4 feet long mounted in an
x
»elliptical reflector.
Its heating capacity could be regulated
by a change in wattage in the electrical circuit.
it was decided that, by using this heater,

Therefore

the emitting surface

could be regulated by the suitable manipulation of a Variac.
At an amperage of 12.5 coulombs per second and an average vol­
tage of 120 volts the wattage was 1500,

which, when divided

by the conversion factor 4.18^ gives 358 calories per second.

1.

There are approximately 4.18 watt-seconds per calorie

- 46 Selection of an Adequate Reflector
The electrical cromolox heating element which was mounted
In an elliptical reflector used for frost-prevention studies
(Figures 4 and 7 ) was the logical choice for the study of
radiant energy on the growth and development of R e d Kidney
beans.

,The distribution pattern of this reflector, with its

polished aluminum undersurface, was studied by means of an
Eppley thermopile (130) and a radiation meter (Figures 61 and
14).

Although the distribution of radiant energy was satis­

factory for all purposes, uses in this problem, the 12-lnch
wide design shaded the plants considerably.

Therefore a 3-

inch elliptical reflector was made, modeled upon the 12-inch
one.

This miniature #odel was not so satisfactory as the

larger one but permitted more of the sun* s rays to fall on the
plants when it was used.

However this elliptical reflector

had one undesirable feature; i. e . , the radiation distribution
pattern covered too wide an area.

A narrower and more con­

centrated pattern was needed so that one could place the re­
flector at a greater height above the plants and thereby de­
crease the difference in temperature between the upper and lower
parts of the plants.
This need led to the design of the near-focus parabolic
reflector (81; Figure 4), with the help of F. J. Hassler.

1.

Used by permission of Mr. F. J.

Hassler

lade

o

©

o

o

O

D

POTENTIOMETER
NO. 328
THERMOPILE
NO. 1369 *

FIG. 6

E P P LE Y

RADIATION

METER

- 48 -

1 v '; Ik

Figure 14*
Radiation meter and thermopile used for measur­
ing radiant energy directly.

- 49 from double-weight sheet aluminum with an anodized center,
on the under side, this reflector was six inches wide, and
the radiation house (Figure 8) used in the greenhouse was
designed with the correct width at the top to h o l d this
particular reflector.
during 1948,

Besides being used in the greenhouse

it was mounted on a frame in the cold storage room

during 1949 (Figure 15),

Several irregularities developed in

its distribution pattern known as "hot spots", while other
areas had a lower-than-desired exposure to radiant energy.
The main reasons for this irregular distribution were:

(1)

the faulty design due to the difficulty of designing a re­
flector which would give a good distribution pattern;

(2)

the failure of the metal to hold its original shape because of
its lightness.
While the elliptical reflector was being used,

it was

necessary to move the potted plants frequently to avoid local
overheating of some plants and underheating of others.

Pots

were also moved to prevent the roots from growing into the
sand beneath the pots.
The principal reason for returning to the parabolic re­
flector in 1950 (Figure 16, 17) was to use its more even rate
of energy distribution.

The main disadvantage found was the

necessity of lowering the reflector to within fourteen to
sixteen inches of the tops of the plants.

- 50 -

F i g u r e P. D e s i g n o f r s c i a t i o n h o u s e u s e e f o r r a d i a t i o n
s t u d i e s i n t h e g r e e n h o u s e c u r i n g 1G4E . A n o t h e r h o u s e
of sorne d e s i g n v/ithout r e f l e c t o r s e r v e d a s o c o n t r o l
house.

Figure 15.
Radiation room (converted cold storage in
which the air temperature was held at 40° F.) during
1949, showing six inch wide, parabolic reflector.
(Note framework supporting assembly of lights, re­
flector and tank.
Two rows of Red Kidney beans were
used due to the narrow width of radiant energy dis­
tribution. )

- 52

Figure 16.
The 12-inch wide elliptical reflector with the
cromolox heating unit mounted between two banks of flurescent lights in the cold storage room during 1950.

i

- 53 -

a # * ''

&

Figure 17.
View of radiation room (converted cold
storage) in which the air temperature was maintained
at 40® F.
The twelve-inch elliptical reflector here
shown was used during 1950.
Red Kidney beans on
sixth day under radiation treatment.
Taken Feb. 20, 1950.

- 54 Nutrient and Wate r i n g Requirements
The quantity of wa t e r lost through transpiration of the
aerial parts of the plant and by evaporation f r o m the soil is
extremely h i g h w h e n the pl a n t is continually exposed to r a d i a n t
energy.

This h i g h water loss was m e n t i o n e d by Gray

(59) a n d

w a s verified in the present experiments on b e g o n i a a n d g e r a n i u m
p o t t e d plants during the early testing period In w h i c h r a d i a n t
energy w as used.

R a d i a t e d plants u s e d from two to six times

as much water as control plants
in all of these experiments.
■»
When infra-red radiation was used,

to prevent wilting,

it

b e c a m e imperative to adjust the w a t e r supply at the roots of
t h e radiated plants to approximate the supply to the control
plants.

It w o u l d be difficult to hand-w a t e r pl a n t s f r e q u e n t l y

enough by casual observation to insure an adequate a n d u n i f o r m
supply.
It was necessary then to l o o k for an artificial means of
supplying a continuous and uniform supply of water.

Two

systems of supplying water were m e n t i o n e d by P o s t and Seeley
(109) and Post
Post and Seeley
for pot plants.

(106):

Injection a n d automatic watering.

Later

(110) reported on an automatic w a t e r i n g system
Seeley (117)

of pot plants by this system.

d i s c u s s e d in detail the w a t e r i n g
The Post and Seeley met h o d

seemed acceptable for the conditions of the present experiment.
Therefore it w a s u s e d throughout all of these studies.
This m e t h o d depends upon the princ i p l e of capillarity of
water.

In the case of pot plants where the pot Is not over

- 55 four Inches In diameter,

a water level an inch of two below

the bottom of the pot was reported satisfactory.

This distance

between the pot and the water level will vary, however, with
the size of pot,

the size of plant,

the type of soil in the

pot, the type of medium in which the water table is Immersed^
and the compactness of the soil.

(In short it will depend upon

the capillarity of the soil used.)
In the experiments reported in this thesis,

it was neces­

sary to raise the water level slightly higher in the radiation
room because of the more rapid loss of water.

In the method

followed, recommended by Post and Seeley, a layer of gravel
t

was placed in the bottom of the bench,
cold storage experiments,
galvanized tin tank.

or, in the case of the

in the bottom of an asphalted, painted,

In the first group gravel was used alto-

gether with the pots immersed in this medium (Figures 13, 14, 15,
46).

Since a few pots dried out, the gravel was later covered

with sand,
deep.

in which the pots were pressed about a half Inch

Water was applied at one end of the tank automatically

by the constant level system (Figures 9, 18, 19).
To Insure better contact with the water and to maintain
capillarity,

all pots were filled with soil without the cus­

tomary drainage material in the bottom as suggested by Cornell
research workers

(37).

A guard row of pots was added to each

of the cold storage experiments to replace any that failed to
establish good capillarity in the tank.
Figure 17).

(Observe first row in

At the time the pots were installed at the be­

ginning of each experiment,

they were watered from overhead

figure 18.
Automatic watering system used for the
radiation house in 1948*
The Red Kidney beans outside
the house were used as checks.

Figure 19.
Automatic watering system used in the con­
trol room (converted cold storage room) during 1950.
Some of the fluorescent tubes were removed to balance
the light intensity in the radiation room.

- 59 until they were completely saturated.

To Initiate capillarity

during the first two weeks after seeding, the pots of bean
seedlingB which were started in the greenhouse were watered by
hand with a sprinkling can.

This was iirue both for the four-

inch pots planted in 1948 and the three-inch pots in 1949 and
1950.

The root system developing under hand watering conditions

was slightly more branched and had more root hairs than the
root system of those plants developing under automatic watering
conditions.

Both root systems are equally beneficial to the

plants growing under those conditions.

The fleshy condition of

the roots Is especially noticeable when plants are growing under
*■

water culture or “hydroponics" (Eastwood, 44).
ments reported herein,

In the experi­

the seedlings were moved at the age when

the new set of leaves were Just assuming their full size,
about two days after they first unfurled

(Figure 20).

They

were selected at this age because they could be chosen for
uniformity of age and development and also because the imma­
ture root system could easily produce the more fleshy root
system which is more suitable for constant watering conditions.
The nutrient level of the soil in all pots was approxi­
mately equal in each experiment,

since the soil was always

mixed in one large pile before each seeding.

A good grade of

greenhouse loam was mixed with a small proportion of sand and
peat (approximately 15$ each), with superphosphate added.

No

further nutrients were added after the bean seeds were sown.

- 60 -

Figure 20.
Example of Red Kidney bean seedling selected
for experimental work, photographed the same day it was
brought into the cold storage room.
All seedlings were
chosen at this age, .lust as the first leaves were
approaching full size*

- 61 Temperature Measurement and Control
0

Thermocouples
The unreliability of the mercury thermometer and the other
methods of measuring temperature which depend on a change in
volume or pressure makes necessary the thermocouple when one
wishes to learn the precise temperature.

The thermocouple can

measure the thermal conditions in a minute spot within a few
seconds.

Its use Is particularly helpful when it is necessary

to record the rapid fluctuations existing in the leaf under
intense radiation.
The sensitiveness of a thermocouple depends on its size;,
the higher numbers correspond to the smaller diameter.

Finer

wire than number 40 is not required for measuring leaf tempera­
tures.

In fact, In this experiment all sizes from number 24 to

32 were used.

Numbers 28 and 32 seemed to be the best sizes

both for accuracy and for a sufficiently firm attachment to the
lower surface of the leaf.
The earliest measurements in this study were made with a
minute thermocouple inserted within the leaf.

This thermo­

couple had to be made very small so that it could be inserted
within a leaf blade from .015 to .018 inch in thickness.

To

do this the smallest wire available at the time had to be
rubbed down with crocos cloth and, after being soldered with
non-acid solder, again smoothed down until a diameter o f *010
inch was produced.

If the wire were smaller it was likely to

- 62
break.

Particular care had to be exercised to push the ther­

mocouple all the way into the leaf until the first contact of
the copper and constantan wire was located where a temperature
reading was desired.

Throughout this work particular attention

was always given to this first meeting of these two wires, for
the difference in electrical potential is measured on the
potentiometer at the place where the two wires first make con­
tact.

All work had to be carefully executed with the aid of a

magnifying glass.
It is questionable whether the internal leaf temperature
derived in this manner represents the true plant temperature
as accurately as the temperature taken on the lower epidermis of
the leaf blade.

The injury to the leaf increased with time so

that the contact could be used only for a few hours.

It posi­

tively could not be used where a continuous reading was needed.
Since the point of insertion was not sealed from air movement,
there might have been a lowering of the true temperature be­
cause of the evaporation of cell sap on the thermal Junction.
After the preliminary testing period with the injection
type of thermocouple,

it was decided that these experiments

should be based on temperatures taken from the lower surface of
the leaf.

As mentioned in the literature,

the contact on the

lower surface of the leaf seemed to be the most feasible.

This

syetem of recording temperature causes no injury to the plant
and can be used for long periods of time.

Regarding the

temperature relationship of different parts of the leaf, Miller

- 63 and Saunder (90) indicated that the lower surface was 1° C.
higher than the upper surface.
Thermocouples for use on the outside surface of the leaf
were made by twisting the copper and constantan wires as in the
insertion type of thermocouples; larger-sized wire was used.

A

few were made with the contact point slightly flattened to
Insure better contact against the leaf surface.

To attach these

thermocouples to the leaf or leaves, the wires were bent around
both sides of the leaf (Figures 21, 22, 23, 24" and 25) to insure
the best contact with the Junction.
Representative leaves--those that represented the average
heat absorption for the group of plants— were chosen for all
temperature measurements.

Other leaves were chosen for the

study of temperature variations within a given group of plants
to determine the length and breadth for each experimental' plot,
and to discover any variations in temperature produced at
different heights and leaf positions.

Most readings were made

with the leaf in the horizontal position— that is, with the
leaf at right angles to the direction of radiation and at a
height representative of the group of plants.

These readings

were made in approximately the center portion of the leaf.

A

location on one of the leaves was selected to serve for tempera­
ture control after careful comparisons of numerous temperature
readings.

This was necessary to insure the best possible

temperature regulation of the radiator in the radiation room,
so that the radiated plants would be heated as nearly the same
as the control plants in the control room.

- 64 -

Figure 21.
One method of attaching thermocouple to lower
surface of geranium leaf for temperature measurement.
A
spring-like tension was created by bending the wire into a
clip-like attachment.
1946.

i

- 65 -

Figure 2 2 . Upper surface of geranium leaf showing attachment of three thermocouples by the loop-cllp method of
attachment.

1

Figure 23.
Lower surface of geranium leaf showing the same
three thermocouples illustrated in Figure 19.

- 67 -

Figure 24.
Method of mounting thermocouple in 1950.
Control leaf six days after moving into storage room.

Figure 25.
Method as above.
to radiated leaf.

Thermocouple a t t a c h e d

i

- 68 When the parabolic reflector (Figure 15) was used, only
two rows of plants were inserted in the tank, for the distri­
bution from this reflector,

as measured with the radiation

meter and thermopile (Figure 11),
fell off rapidly on the sides.

showed that the distribution

With the elliptical reflector

(Figures 16 and 1?) it was possible to use four rows of H e d
Kidney beans (Figure 17).
After the twisted type of thermocouple was used for two
years,

Clarence Hansen suggested the use of a new type of

thermocouple made by silver soldering two wires end to end.
After this thermocouple was tested and found not only to com­
pare favorably but to have some advantages over the twisted
design of thermocouple,

it was decided to use this new Junction

during the 1950 experimental studies.
The technique for making these Junctions was difficult;, ^
but the following procedure was found to be the most satis­
factory.

With the use of a magnifying glass, the two ends of

the thermocouples were filed or sanded at right angles to the
length of the wire.
silver wire.

Silver filings were made by filing some

The two wires were Joined by melting a small

quantity of flux on the end of each wire,

then melting a small

amount of silver filings on top of the flux.

The two wires

were then mounted in a suitable clamp which could bring the
wires together.
or two,

The wires were held in the flame for a second

sufficient time to Join the wires together with the

silver solder.

When properly made,

the thermocouple was as

- 69 strong as a single piece of wire.

The Junction could only be

identified by the difference in the color of the copper and
the white constantan wire.
The ten thermocouple Junctions made with the silver solder
proved satisfactory for all the 1950 experiments,
age occurred.

and no b r e a k ­

They were easy to attach to the leaves at the

desired location.
All thermocouples were calibrated while they were immersed
In a 32° F. ice-water bath in which the mixture was stirred to
produce an even temperature.

The proper correction, which was

less than .5° F. in all

cases, was made to all readings of

thermocouples which did

not check with the expected reading.

No variations were noted in the silver-soldered thermocouples.
Variations in readings are usually due to the length and size
of wire used, although it may be caused by the way the Junction
is assembled.
Potentiometers
Several devices for measuring potential difference in the
thermocouples were used.

In the early tests a manually operated

potentiometer was connected to the two open ends of the copper
and constantan wires which were scraped
ing material.

Readings

to remove all insulat­

were calibrated in millivolts;

thus

it

was necessary to refer to the proper tables to find the correct
temperature.

An ice-water bath in a thermos bottle was used for

a reference Junction with this potentiometer.
and 1950 season,

During

the 1949

another manually controlled potentiometer for

- 70 making temperature measurements was loaned by the vegetable
crop section of the horticulture department.

This instrument,

with its ten point selector switch (Figure 26), was calibrated
at the factory for copper-constantan thermocouples to read
directly in Fahrenheit degrees without the need of an external
reference Junction.

With this apparatus it was possible to

estimate the temperature to within .1° F. on the temperatureindicating dial.

Thermocouple wires were attached to each of

the ten wiring posts of the selector switch.
Several recording potentiometers, based on the potential
difference between copper and constantan in a thermocouple,
were also used.

The twelve-step Brown Electronic recording

' V-V'-v-

potentiometer #153Z65 (Brown 25; Figure 27, 28), which could
record simultaneously twelve different temperature readings
from as many thermocouples,
agricultural engineering.

was loaned by the department of
A Brown circular-chart air-operated

air-o-line Electronic potentiometer controller was acquired
for use in this experiment (Brown 24; Figure 28, 29, 30, 31).
It was adjusted to operate within two or more degrees variation
in temperature (Figure 58).

It could be set to any desired

temperature by means of a pointer h a n d

(Figure 30) and the

temperature recorded by a pen carrying enough inC to last for

Jj

more than a month.
Helpful suggestions for the use of the aforementioned
controller,
radiator,

to determine the amount of energy used by the

were made by Wallace of the University of Connecticut,

F i g u r e 2c*
M a n u a l l y oper-ated p o t e n t i o m e t e r w i t h t e n - p o i n t
- e l e c t o r s w i t c h f o r m e a s u r i n g t e m p e r a t u r e s f r o m t en d i f ­
ferent thermocouples*

Twelve-step recording potentiometer.

«■

rm *T

(:£ <=*

V

F i g u r e 28.
R e c o r d i n g a n d c o n t r o l l i n g e q u i p m e n t In d r y
g r e e n h o u s e a d j o i n i n g the two g r e e n h o u s e rooms u s e d for
r a d i a t i o n s t u d i e s In 1940.

f

F i g u r e 29,
Brown circular-chart potentiometer
s h o w n m o u n t e d in the g r e e n h o u s e 1948.

controller

F i g u r e 30.
C l o s e - u p of c i r cu la r <=c h a r t p o t e n t i o m e t e r
t r o l l e r s h o w i n g p o i n t e r n e e d l e a n d r e c o r d i n g pen.

con­

Figure 31*
Controlling equipment installed in horticulture
laboratory adjoining cold storage rooms used in radiation
experiment during 1949 and 1950,
Photographed in 1949.

- 7? (137) Gaddis of the Minneapolis Honeywell Company,
members of the agricultural engineering department.

(53) and by
Since the

only controlling potentiometer that could be acquired was airoperated, it was suggested that the Motortrol Grad-u-motor
#M0900C (11) (Figure 38) could be actuated by the potentiometer
controller because it was also'air-operated.

In order to make

this instrument regulate the amount of electricity passing to
the radiator, the Varlac was recommended as being best adapted
for the job because it could vary the amount of voltage passing
through it.

The instrument, Variac V-80, which was acquired

for this purpose, was ingeniously connected to the Motortrol
by means of a rake and pinon gear (Figure 32, 33).^

Since no

air line of sufficient pressure was available in the green­
house or horticultural building to operate these instruments,
a tank and compressor we^e installed together with the valves,
pressure reducer,

etc. (Figures 34 and 35) necessary to operate

the clrcular-chart potentiometer and the Motortrol.
Convected air temperature
To determine the actual temperature effect of radiation,
it was necessary to know precisely the true temperature of the
convected air.

Consequently, It was necessary to protect the

thermocouple from the direct rays of a radiator and, at the

1.

Appreciation Is extended to Professor A. W. Farrall, Mr.
Clarence Hansen, and Mr. F. J. Hassler, who, in conjunction
with other members of the agricultural engineering department
of Michigan State College, helped to assemble this radiation
control apparatus.

4

- 76 -

Figure 32.
Top view of Varlac and Motortrol with rack and
pinion gear connections.

i

- 79

Figure

33

Side v i e w of V a r l a c

V-20

Figure 34.
Air pressure tank, gauges, valves, etc.
used for pneumatic operation of control equipment.

- 81 -

Figure 35.
Pressure reducer unit used for lowering pressure
to the desired 15 pounds per square inch necessary for
pneumatic operation of potentiometer and Motortrol.

<

- 82 same time, to allow a free passage of air over the thermo­
couple.

Several metallic contrivances were built out of sheet

aluminum with an anodized surface to achieve this purpose.
For the needs of this paper they shall be referred to as
•’convector" or " c o n v e c t e r s . S i n c e

three designs were tried

they shall be designated models "4", "B", and "C" (Figures 36,
3?, 38 and 39).
Soil temperature
The mercury thermometer was used in a few cases to check
the temperature of the soil (Figure 40) and air.

It was far

less responsive to temperature changes and was used mostly for
comparison.

The advantage of this measuring device was the

ease by which readings could be frequently checked.
Soil temperatures were usually measured in the center of
one of the pots used in the experiments.

In some oases, as

during the 1949 experiments, an extra pot in which the original
plant had been cut off was used for this purpose.

This method

eliminated the danger of injuring the roots of the growing
plants, especially when the mercury thermometer was used.
In the greenhouse experiments, which were carried out in
the hand-made radiation and control houses (Figures 8, 41, 44)
no soil heat was applied.

1.

In this case the central position

The word"convecter" is used in this,thesis merely for the
purpose of convenience, indicating ishe three designs used
for measuring convected air temperatures that were used in
these studies.
No effort is made to coin a new word.
The
word convector, which Is listed by Webster, means an agency
of convection and has a "different meaning.

- 83 -

F i g u r e 36.
L o w e r v i e w of m o d e l A c o n v e c t e r
r a d i a t i o n r o o m of the g r e e n h o u s e .

F i g u r e e?.
Upper
n e a r th er mo st a t.

view

of mo d e l

A

in th e

c o n v e c t e r mourtt ed

Y

F i g u r e 38.
greenhouse

M o d e l B c o n v e e t e r s h o w n m o u n t e d In
c o n t r o l r o o m n e a r the t h e r m o s t a t .

the

Figure 39. Model C conveeter which was used for measurements
of convected air temperature in the radiation and control
house.
(It was also used above the greenhouse for determinin
the outdoor temperature.)

Figure 40*

D o u b l e row spacing o f p o t e c o n t a i n i n g R e d
K i d n e y b e a n s u s e d in 194S.
(M ot e h o r i z o n t a l p l a c e m e n t
of l e a v e s on s i x t h d a y a f t e r they w e r e p l a c e d in r a d i a ­
tion room.
T h e t h e r m o m e t e r in t h e e x t r a p o t w a s u s e d
f o r m e a s u r e m e n t of aoll t e m p e r a t u r e . )

- 8? -

F i g u r e 41.
Radiation
l u r i n g 1948.

house

used

in

the g r e e n h o u s e

- 88 of the four-inch pot— used at that time for the bean plants—
%

was selected as the position for measuring the soil temperature.
This position was chosen because of the temperature pattern,
which was progressively colder toward the bottom of the pot
and warmer toward the top where the soil surface absorbed some
of the radiant energy from the radiator.
As will be mentioned subsequently, the original plan of
the experiments was changed so that the soil was heated by a
soil heating cable 60 feet long.

In this case the temperature

pattern was the reverse of the former pattern in the radiation
room, with a higher soil temperature toward the bottom of the
pot than at the top.

This was due to the effect of the heating

coil which was placed near the bottom of the tank and to the
cooling effect of the 40° F. air of the room.

The heating of

the soil by the radiator on the soil surface was small.

As

in the other temperature pattern, the center of the pot was
considered the most logical position to represent the average
soil conditions existing around the root system.

Most of these

readings were made with a thermocouple.
The soil thermostat (Figure 43) was regulated by a heat
sensitive bulb connected to 18 Inches of flexible tubing, which
was inserted in the sand and the gravel, where the greatest
temperature fluctuations occurred.

The setting on the soil

thermostat was regulated by checking it with a 60° F. reading
on the thermocouple placed in the center of one of the pots
used in the experiments.

The above technique was necessary in

- 89 -

f i g u r e 43.
t'icnc u s e d

Soil t h e r m o 8 t a t 'w i t h e l e c t r i c a l
f o r the s o i l - h e a t i n g ca ble.

eonnec-

i

»

- 90 order to make the soil temperature the same in the radiation
room as it was in the control room.
Thermostats
Thermostats were used throughout these experiments to
control the air temperature of the rooms in which the experi­
ments were conducted.

In the greenhouse a new type of green­

house-designed thermostat (G-addis, 53) was installed in each
of the two rooms used (Figures 37 and 38).

They were manually

aet for a 46° F. night temperature in the radiation room and
60° F. temperature in the control room.
setting was raised 8° higher.

During the day the

The variation in air temperature,

as recorded by the sensitive response of the thermocouple sus­
pended in the center of the model B conveeter (Figure 38) when
attached to the twelve-step recording potentiometer, was 2° F.
(f’igure 44).

Temperature variation of less than one degree

has been claimed for this thermostat when Installed with suit-^
able heating equipment and according to factory instructions.
The thermostats used in the cold storage rooms, combined
with the cooling system installed, nade close temperature control
almost impossible.

Each time the cooling system was set into

operation by the thermostat,
from five to ten degrees.

it brought the temperature down

These temperature dips (Figures 55,

56 and 57) were observed in both rooms although they occurred
more frequently in the cold radiation room (40° F.)
control room (60° F.).

than in the

In order to ascertain the average

temperatures of these rooms it was necessary to record and

F i g u r e 44.
T e m p e r a t u r e p a t t e r n f r o m th e c i r c u l a r
chart potentiometer controller s h o w i n g fluctuations
of t e m p e r a t u r e on the c o n t r o l p o i n t ( l o w e r s u r f a c e of
a s e l e c t e d l e a f In the r a d i a t i o n r o o m ) .

- 92 average numerous readings on the manually operated potentiometer
before the mean temperature could be found.

In the control

room the temperatures were nearly constant except for the cool­
ing periods which occurred about every hour.
made with the aid of thermocouples,

These readings,

showed a much greater varia­

tion in temperature than temperatures recorded by a thermograph
placed in these same rooms; for the metallic plate on the ther­
mograph was slow to respond to any temperature impulse.
Globe thermometers
Attempts have been made to measure the convected air
temperature combined with the heating effect of radiation.
An Instrument supposed to make this measurement is called the
globe thermometer.^

Several adaptations of the principle upon

which the globe thermometer is based have been used for making
thermostats sensitive to the combined influence of these two
modes of heat transfer (Adlam, 1).
The writer attempted to make a globe thermometer from a
globe purchased in a plumbing shop and a mercury thermometer
inserted into the center position through a cork collar.
a thermocouple was u s ed Instead bfc a-^hermometer.

Later

Results by

both measuring devices showed that the globe thermometer ab-

1.

The globe thermometer was invented by Mr. H. M. Vernon,
is six to eight inches in diameter, and is made of copper
with a mercury thermometer inserted into it through a
collar*
It is painted black to simulate a perfect black
body (3).

- 03 sorbed a tremendous amount of heat when subjected to radiation.
When measured for several consecutive days, the twelve-step
recorder indicated that the globe thermometer was a very effi­
cient absorber of radiation.

The absorption was so great that

there was a definite temperature increase soon after dawn.
This suggested the idea of mounting a ’thermostat instead of a
thermocouple in the center of a globe.

By this means it was

hoped that the thermostat would show a nearly equal rl3e in
temperature, one which would be great enough to be used to
actuate the change-over in another thermostat from a night to
a day temperature control.

The usual automatic mechanisms

used for actuating this change-over are a time clock or a
photronic cell.
To test this theory, a thermostat was privately purchased
and mounted inside a globe similar to the one previously used.
Unfortunately the wrong designed thermostat was obtained which
failed to make a two-way contact.

The project was abandoned

because it did not relate directly to this experiment.
How-*
i
ever, the idea should be fully tested to evaluate its true worth.
This theory is based on the following observations.

The

thermocouple inside the globe thermometer showed a quick
response— a rapid rise in temperature— soon after the sun rose.
The difference exceeded the average air temperature variation
found under thermostatic temperature control.

a

If the thermostat

inside the globe should be set from three or four degrees above
the night temperature during the night,

it would remain in the

- 94 off position.

As soon as the dawn arrived and the sky begah to

lighten, the temperature would aBcend to a point beyond the
setting of this globe thermostat.

When this happened, the

Impulse from this thermostat could actuate a relay, which,

in

turn, would close a switch operating a thermostat with a daytemperature control*

If the idea is practical,

it would give

greenhouse growers an automatic temperature regulator cheaper
than a time clock, one that would not have to be reset frequent­
ly, and one that might be almost as successful as the more
costly photronlc cells.
Illumination
Radiation studies were made in two different greenhouse
rooms during 1947 and 1948 in which all of the Ituainous energy
came from the sun.

In the latter part of this greenhouse study,

plants were grown Inside of hand-made radiation and control
houses (Figures 8, 41, 42)

in which the light from the sun was

slightly less than in the open room itself.

During the next

two years, 1949 and 1950, all light was provided by daylight
fluorescent light tubes.
The advantages of the sun as a source of light are its
higher intensity of radiation and its economy.

The advantage

of the artificial light source is the better control of the
quantity of light (intensity) and of the duration of light
(the length of time the light is on and off as regulated by a
time clock).

Since daylight fluorescent llght*was reported to

- 95 -

frt m *

*

F i g u r e 42.
Control h o u s a in early m o r n i n g h a l f h o u r a f t e r
aunriae.
(Note f o g g i n g of g l a a s w h i c h r e d u c e d the light
Intensity.,
N o f o g g i n g o c c u r r e d In t h e r a d i a t i o n h o u s e . )

i

- 96 be as good as any other artificial source of'“illumination for
plant growth (Naylor and Grerner, 94; Withrow and Withrow, 149;
Hamner,

65),

it was used throughout the experiments where an

artificial source of light was needed.

It emits very little

infra-red light and is therefore cooler than most other arti­
ficial light sources at a given light intensity.

The f l u o r e s - ^

cent light is considered more desirable for radiation studies
since it emits less heat-producing rays Itself.

Because of

this it is possible to control the amount of radiation on the
growing plants more accurately.
One of the disadvantages of solar radiation is the excess
heating of plants,

especially when it Interrupts the effects

of a radiation study.

Sunlight is also unpredictable in nature,

especially w h e n an experiment calls for an equal supply of
radiation for two different houses placed in two different
greenhouses.

Figure 56 shows the recorded temperature curve

of two leaves on a day that was partly cloudy and partly sunny;
the leaves were shaded by some greenhouse obstructions at
different times of the day.

This graph indicates some of the

difficulties encountered when one wishes to standardize the
amount of sun light.

Another disadvantage associated with

sunlight and the wide variation of temperatures encountered in
the greenhouse under conditions of h i g h humidity is the fogging
of the glass, which reduces the light still more.

(Figure 42).

The disadvantages of the fluorescent light are the low
light intensity,

the higher cost of operation,

the breakablllty

- 97 of the light bulbs, and the dependence upon electrical current.
The plants which were used in the experiments conducted
in the converted cold storage rooms were illuminated with d a y ­
light fluorescent light tubes (Figures 15, 17, 19, 45, 46).
Although most references Indicate that 600 foot-candles or more
light should be used for Red Kidney beans,

the decision to use

only 300 foot-candles was based on the following statements.
First, a steady temperature of 60° F. was decided to be a
satisfactory temperature range for this experiment,

since that

was the ordinarily accepted night temperature for R e d Kidney
beans and since it was impossible to have two different day and
night temperature ranges in these cold storage rooms.
at 60° F. continuously,

Growing

the plants would not require so much

light aa if they were growing at a higher temperature.

Second,

the length of day was set for eighteen hours instead of for
sixteen hours as has been sometimes used for this type of study.
The Red Kidney bean is a day-neutral plant and will grow satis­
factorily on a longer duration than used here.

A most impor­

tant consideration was that all plots of beans were to receive
identical hours and intensities of light.

Third,

this experiment were not to be grown to maturity.

the plants in
Fourth,

by

keeping the light intensity low, it was possible to eliminate
the probability of COg concentration as a limiting factor for
plant growth.

It was possible that there may have been a

difference in COg concentration in the two cold storage rooms
*

- 98 used In these studies due to the poor circulation of air and
the smallness of the room.

Fifth, the cost of .lamps and the

difficulty of designing equipment for this purpose did not
warrant any greater expenditure of money.
To increase the luminous energy of each individual fluores­
cent flight tube, Clarence Hansen suggested that each be mounted
in an individual aluminum reflector.

Since a light intensity

of 300 or more f oot-candles was desired,

a plan was used to

distribute the light of the eight fluorescent tubes where they
could illuminate the bean plants evenly and be most easily
handled.

Since the radiator had to fit above the tank contain­

ing the experimental plants, only the space on the sides was
left for the placement of lights.

Therefore half of the tubes

were placed on each side, four four-foot X3-uore8cent light
tubes being mounted on each panel along with their ballasts and
starters (Figures 15, 16, 17, 19, 45, 46).

They were swung

from a framework and could be adjusted for height and angle.
GENERAL PROCEDURE
The following items are the most significant steps taken
in this problem:
1.

Testing the characteristics of the rod-shaped cromolox

heating element— its distribution of energy and effect at dif­
ferent distances from an object to be heated (I.e. the operation
of the inverse square law).

(Figures 4, 5).

- 99 2.

Acquaintance with the radiation meter and the thermo­

pile as a radiometer for measuring the radiant energy intensi­
ties (Figure 14').
3.

Experiments with the injection type of thermocouple.

4.

Collaboration with Hassler and Glbeon regarding methods

of temperature measurement and operation of equipment.
5.
Company

Consultation with Gaddis of Mlnneapolie-Honeywell
regarding the possibility of using its equipment for

the control of radiant energy from a thermocouple mounted on a
leaf (Gaddis, 53).
6.

Experiment with various types of thermocouple attach­

ments to the outside of a plant leaf.
7.
house.

(Figures 21, 22, 23).

Installation and testing of cromolox unit in the green­
This unit was mounted over a

7 x 11 foot tiled bed in a

17 x 21 foot greenhouse.
8.

Purchase of one hundred Creole lily bulbs.

They were

planted and stored in a cool cellar until time of sprouting.
They were then divided into two equal sections— fifty to a
group— half to be used as control and half to be used for
radiation studies.
9.

Measuring the height of the growing point of twelve

plants from each group of lilies at weekly Intervals and count­
ing the number of quarts of water used by each lot of plants.
The absorption of water showed a difference of two to one with
the greater water loss under the radiation unit.

- 100 10.

Making the experimental design of a three-inch ellip­

tical reflector and measuring its distribution pattern (Figure 4).
11.

Testing methods of measuring convected air tempera­

tures, which resulted in the designing of three experimental
temperature measuring units, Models A, B and C, called "convecters" in this thesis.
12.

Testing of globe thermometer and some of its adapta­

tions.
13.

Planning for and purchasing all control equipment

u s e d throughout the experiment.
14.

Installation a n d testing of control equipment which

w a s placed in an adjoining dry room (Figures 27, 30, 32, 33,

34,

35).
15.

Accumulation of useful data by means of the twelve

step recorder (Figure 27).
16.

Designing parabolic reflector with the help of Hassler.

This design was found to be suitable for not more than two rows
of plants.
17.

Construction of two houses for radiation study,

one

of which was to be used as a radiation house and the other as
a control house (Figures 8, 41, 42, 47, 48, 49, 50, 51).

They

were designed for equal ventilation at their tops (Figures 48
and 49), reduced ventilation at their bases (Figure 51), and
were mounted to face the same direction in the two adjoining
greenhouse rooms.
18.

Installation of automatic watering In both houses (Flg.J

silii

Figure 47.

Control house

mounted

In

control

room

194 8.

Figure 48.
In to p

of

L o w e r v i e w of p a r a b o l i c
r a d i a t i o n house.

F i g u r e 43.
Lower view
b o a r d p l a c e d to p e r m i t

reflecter

mounted

of top o f - c o n t r o l h o u s e ale o w i n g
t h e same a m o u n t of v e n t i l a t i o n .

- 103 -

F i g u r e 50*
S i d e v i e w o f u p p e r h a l f of r a d i a t i o n h o u s e s h o w
I n g the w i r i n g of the r a d i a t o r .
R o t e also the t h e r m o c o u p l e
wir e s coming Into house.

- 104

F i g u r e 51.
F r o n t c o r n e r of c o n t r o l h o u s e s h o w i n g
d e s i r e d a m o u n t of v e n t i l a t i o n w a s o b t a i n e d .

how

the

i

- 105 19.

Installing and adjusting thermostats in two green­

house rooms to operate at 60° F. (control room) and 46° F.
(radiation room) night temperatures with a rise of 8° F. for
the day temperatures (Figures 37, 38).
20.

Planting of certified seeds of Red Kidney beans

(Phaseolus vulgaris) . four seeds to each four-inch pot.
These pots were placed in the main greenhouse and watered
from a sprinkling can.
21.

Leaving the two most uniform plants in each pot

when the seedlings arrived at the proper age for selection.
Ten of these pots were placed in a single row in the radia­
tion house and a like number in the control house.
22.

Using miscellaneous greenhouse potted plants on

each side of the bean plants as a filler to observe how they
would respond to different temperature influences (Figure 63).
23.

Setting the pointer needle on controller for opera­

tion at 60° F. plant temperature (Figures 29, 30).
24.

Plotting of temperature curves on graph paper from

data accumulated on the twelve-step recording potentiometer.
Examples are shown in thesis (Figures 52,- 57).
25.

Analysis of all individual bean plants: measuring

height of plant; weighing individual green leaves and stems
and calculating the ratio of both weights to each other;
estimating the leaf areas in square inches with the use of
graph paper; analyzing for ascorbic acid in the horticulture
laboratory (Lucus, 83).

- 106 -

F i g u r e 63.
P o t t e d p l a n t s u s e d In r a d i a t i o n h o u s e In 1 9 4 8 .
R e d K i d n e y b e a n s , geraniurns, f u c h s i a s , l r e s l n e s , c o l e u s a n d
a tomato are represented.

i

- 107 S'

26.

Moving of equipment to cold storage rooms In the

horticultural building to better control all environmental
factors, with controlling equipment mounted in dry laboratory
room adjoining storage rooms (Figure 31).
27.

Purchase of electrical lead-cheating cable to con­

trol soil temperature.
28.

Designing and use of fluorescent lamps for illumina

tion of experimental plants.
29.

Construction of framework to hold the radiator and

panels of fluorescent lamps (Figures 15, 17, 45).
30.

Construction of a galvanized tin tank four and one

half feet long,

eighteen inches wide, and six inches deep,

later coated with asphalt Inside to prevent any zinc toxicity
to the plants (Figures 15, 17, 45, 64, 65).
31.

Installation and setting of time clock to regulate

the illumination for an eighteen-hour day.
32.

Planting of certified seeds of Red Kidney beans,

one seed to each three-inch pot.

This was the system used

for all subsequent plantings.
33.

Filling the tanks with a one-inch layer of gravel,

laying lead-heating cables above this layer and filling the
tank to within one inch of the top with sand.
34.

Installation of automatic watering in both units

(Figures 15, 19, 45).
35.

Wiring of electrical cromolox heating unit in the

elliptical reflector to the automatic controller set at 60° F

108

F i g u r e 64.
T o p v i e w o f R e d K i d n e y bean p l a n t s In .control
ro o m .
(Note u se of t h e r m o m e t e r in an extra p o t to m e a s u r e
s o i l t e m p e r a t u r e a n d a n g l e o f t h e p a n e l s of l i g h t to g i v e
an even d i s t r i b u t i o n of light).

i

- 109 -

F i g u r e 65.
C o m p l e t e a s s e m b l y of f r a m e > l i g h t s a n d
t a n k w i t h R e d K i d n e y b e a n s as t h e y a p p e a r e d t h e s i x t h
d a y a f t e r s e t t i n g in t h i s c o n t r o l r o o m ( c o n v e r t e d c o l d
storage room).
T h i s r o o m w a s set f o r o p e r a t i o n a t
50° F. r o o m t e m p e r a t u r e .
1949.

- 110 This controller moved a variac which determined the voltage
going to the radiator.
36.

Adjusting room temperature in radiation room to

40° F. and in control room to 60° F.
37.

Adjusting soil thermostat to lead heating cable

to operate at 60° F. average soil temperature (Figure 43).
38.
growth.

Selecting bean seedlings at the right stage of
From this selected group twenty pots were chosen

at random for each of the two experimental rooms.

These

were brought into the converted cold storage rooms and ar­
ranged in two rows of ten pots each.
the sand, watered,
leaf

They were pressed into

and a thermocouple attached to a control

(See Figures 64 and 65 for arrangement in^control r o o m ) .

An extra pot was used in some cases for soil temperature
measurement.
39.

Wiring ten thermocouples to selector switch and

extending them into the radiation a n d control rooms for
temperature measurements (Figure 26).
40.

Changing of charts daily on the controlling

potentiometer.
41.

G-rowing three different groups of Red Kidney beans

for approximately four weeks each in the cold storage room
during 1949.

The second of these groups was divided into

two sections.
42.

Measuring individual plants for height in centi­

meters at each internode.

The average of these measurements

- Ill w a s used for comparison

(Figure 59).

43.

W e i g h i n g plants when fresh and later when dried.

44.

A n a l y s i s for percent of ash content In each group

b a s e d on dry weight
45.

Anal y s i s for percent of total nitrogen in each group

b ased on dry weight
46.
forty pots
47.

(Figure 61).

(Figure 60).

Growing another group in 1950, using four rows with
(Figure 17).

"

Control of temperature in the radiation room during

the 1950 experiments by the raising and lettering of the para­
b o l ic-shaped reflector.
48.

Analysis for chlorophyll made on a green-welght

basis from plants of each group.
was compared on a dry-weight basis
49.

Analysis of b o t h groups
(Figure 62).

Anal y s i s for vitamin C in the horticultural

research laboratory.

- 112 Radiated plants
Control plants

□

15-

X

X

\
to
a> x-c'

N
X

a>
^-i
4-}
c:
<u
a
c:;

Second
node

Nl
O -

•f-i

6

Third
node

IX
\
\

"

X
X

First
node

O"
X

Grout) I

Croup II

Group III

Croup IV

Figure 59. Relative height of Red Kidney beans
grown in radiation and control rooms (converted
cold storage c o m p a rtments) at the end of each
experiment.

113 Radiated

plants

Control plants

|

|

Croup

ITT.

Croup

IV

"' k’Jre c0 . Per cent tota 1 n 'tropen cf Red ricriey
•:-e*»ns (" :,ove hypocotyl) on dry weight br.vie.

- 114 -

Radiated plant
Control plants

IP

1C

Oronn III
Figure 61. Fer cent ash of Red Kidney bean tope
(all parts above hypocotyl) on dry weight basis.

- 115 -

Per cent dry wei £ht
Chlorophyll content

|

|
-14

16 ..

■

-1

Pndif-ted ol.ant
1 2..

-

10

-

8

Control plants

1C

o-6

figure 62. Analysis of Pied Kidney beans (above hypocoV'l)
after eight days growth in the radiation and control
r o o m s ■showing' chlorophyll content on green weight b-sis
compared with the per cent dry weight.

- 116 RESULTS
A.

Test on lilies
Weekly measurements in Inches of terminal growth to

measure the effect of radiant energy on lilies in the green­
house (1947).

Linear measurements of Creole lilies In

inches.
TABLE I
M ay 14 - 19

May 14 - 27 'I

May 14 - June 2

Radiated

2.14“

6.91"

8.95"

Control

2.35“

7.01"

9.63"

B.

Analysis of individual bean plants grown in radiation and

control house in the greenhouse (1948).

(See TABLE II on

next page).
C.

Measurement of temperature fluctuations in the green

house (Figure 58)

indicates the wiring diagram used in these

equipment controlling the leaf temperature.
(1)

Testing maximum heating capacity of the radiator in

the parabolic reflector by setting the controller for full
capacity operation (Figure 52).

Note the 88° F. leaf temper­

ature while the air temperature in radiatioh house was 72° F.a difference of more than 16° F . — and the air temperature
in the unheated greenhouse room,

40° F . , in w h i c h the radia­

tion house was placed— a difference in temperature of 32° F.
from the air in the radiation room.

TABLE II
Pot Number
2
3

1

4

Average

Radiant Treated Exterior
Leaf Weight (Gms.)

8.0

8.0

9.0

10.0

8.8

Stem Weight (Gms.)

4.0

4.5

4.5

5.5

4.6

Ratio Leaf/Stem Wt.
2
Leaf Area - Inch

2.0

1.8

2.0

1.8

1.9

Height of Two Plants (Inches)

80

87
9.0
8.5

Ascorbic Acid Mg./l00 Gr.

8.2
8.5

y
li

84

75

80

8.5
9.0

9.8
9.0

8.7

88.2

89.2

88.7

Pot Number
13

14

Average

Check Plants Ext er ior
Leaf Weight (Gms.)
\
Stem Weight (G-ms.)
Ratio Leaf/Stem Wt.
Leaf Area - Inch**

11.0

7.5

12.0

11.0

10.4

6.0

4.5

6.0

5.5

5.5

1.9

1.7

2.0

2.0

1.9

73

Height of Two Plants (Inches) 10.0
10.1

65
7.5
7.7

Ascorbic Acid Mg./lOOG-r.

(Table Continued on Next Page)

83

71

73

11.5
11.0

10.5
9.5

9.7

98.6

79.3

89.2

- 118 TABLE II (Continued)
Pot Number

Z

I

£

JUO

Average

Radiant Treated Interior of Glass House
Leaf Weight (Gms.)

12.5

10.0

8.0

13.5

11.0

12.0

11.1

Stem Weight (Gms.)

5.5

6.5

4.5

7.5

5.0

7.0

6.0-v

Ratio Leaf/Stem Wt.
2
Leaf Area - Inch

2.0

2.3

1.8

1.8

2.2

1.7

2.0

105

95

83

120

100

94

100

12.0
10.5

12.5
12.0

9.5
9.8

13.0
13.0

11.5
11.0

12.9/
12.0 11.5

92.4

84.7

88.5

20

Average

Height of Two Plants
(Inches)
Ascorbic Acid Mg./lOO Gr.

15

16

Pot Number
17
15“
19

■*

Check Planta Interior of Glass House
Leaf Weight

(Gms.)

8.0

12.5

8.5

17.5 12.0

Stem Weight (Gms.)

5.0

8.0

5.5

10.0

Ratio Leaf/Stem Wt.

1.6

1.6 . 1.55

Leaf Area - Inch2
Height of Two Plants
(Inches)
Ascorbic Acid Mg./lOO Gr.

75

105

81

11.2
12.0

11.5
14.5

10.1
12.5

12.0

11.7

9.0

7.0

7.4

1.75

1.3

1.7

1.6

130

90

115

99

15.3 11.5 12.0
15.0 13.0 12.5

12.6

78.3

70.2

74.2

■V"u
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- 121 This shows that reradiation from plants can heat" the air
within a small enclosure and retain most of this heat due to
the "heat trap" effect illustrated in Figure 2.
(2)

Testing normal operation of radiation unit at night

with the

controller hand set at 60° F.

(Figure 54).

In this

case the

only leaf whose temperature was recorded on the

twelve-step recording potentiometer registered a temperature
slightly higher than that of the control plant.

Note also

that there was some accumulation of heat within the radiation
house (Figure 8, 41) shown graphically in Figure 53.
The soil temperature consistently measured lower than the
leaf temperature but was higher than the air temperature in
the same house for the following reasons:

(a) the soil

temperature tends to become warmer in the daytime due to the
higher air temperature and the radiation effects from the
sun, part of which it retains throughout the night due to
the capacity of soil water to h o l d heat;
some heat from the radiator,
degree of direct exposure

(b) the soil absorbs

the amount depending upon the

(no obstructions by leaves,

etc.),

the distance from the radiator, and the absorption factor
(varying with the color, nature and texture of the soil);
(c) the soil tends to be cooler because of the colder water,
not exposed to artificial radiation brought in by the auto­
matic water system (Figure 18), a n d because of the loss of
heat by conduction and convection and by the evaporation of
water.

- 122

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- 124 (3)
the day

Testing the normal operation of the r a d i a t o r d u r i n g
(Figure 53).

As mentioned previously,

great f l u c t u a ­

tions may occur In leaf temperature w h e n the leaf is a l t e r ­
nately exposed to and shaded from the sun.

A tremendous In-

-i

crease of temperature was often observed under Intense solar
Illumination.

Note the smoother curve for b o t h soil tempera­

tures.
(4)

T e s t i n g convecters.

Temperature r e a d i n g s from

thermocouples pl a c e d in the central p o s i t i o n of all three
m o dels of convecters che c k e d fairly close to air tempera t u r e s
at night and in heavily shaded spots during the day but
v a r i e d a little under radiant conditions.

It is sometimes

b e l i e v e d that thermocouples give the true c o n v e c t e d air
temperature.

This is not always true,

especially w h e n

r a d i ation is exposed directly upon the thermal Junction,
since a small amount of heat is a b s o r b e d by radiation.

When

the therm o couple is p r o t e c t e d from the direct r a y s of the sun
or an artificial radiator,

the temperature it r e c o r d s ap p r o a c h e s

very closely the exact air temperature.
p r o t e c t i n g device for a thermocouple,

However,

care must be

in ma k i n g a
exercised

to m a i n t a i n free circulation of air.
In d e velo p i n g these convecters,

these two factors w e r e

c o n s i d e r e d — p r o t e c t i o n f r o m all objects of unlike tempera­
tures and free circulation of the air.

- 125 The model B convecter

(Figure 38)

seemed to meet these

conditions most precisely because it was larger and most
protected from radiation.
the above only

Model A p r o t e c t e d radiation from

(Figures 36,. 37).

The model C convecter

(Figures 36, 45) was the simplest to make a n d to use.

One

of the model C convecters w h i c h w a s mounted above the green­
house, p r otected the thermocouple adequately enough to make
reliable temper&ture readings of the outside air possible.
Care was taken to Insure that no light would fall on the
thermocouple f r o m the open sides of this design.
A l t h ough all convecters were m a d e from highly polished
surfaced aluminum, which r e f l ected about 95 percent of all
radiation falling on it, the less as it became weathered,

the

convecters d i d absorb a small amount of radiation when sub­
jected to long periods of intense radiation.

This condition

w a s minimized by b u i l d i n g a larger convecter an& by prot e c t i n g
the thermocouple w i t h more layers of polished metal.

Those

were two rea s o n s why the model B convecter p r o v e d the most
satisfactory.
D.

Measurement of temperature f l u c t uations in the converted

c o l d storage rooms (1949-1950).
(1)
F igure 57,

Normal operation in the control room.

As noted in

leaves in a similar posi t i o n often registered

slightly different temperatures.

Some of the factors af fect­

ing this variation were angle of Incident exposure, height,

- 126 -

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D u r a t i on of f i n e vd.en
Le -f terser:'1tune

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Tine of day
Figure 57. If

:
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storare con-:-.

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11:00

11: ° . n

r eo. 18, 1950
t l c n s in

ontrol roo::;(converted cold

- 129 and absorptive properties.

This curve graphically illus­

trates h o w leaf temperatures were affected by the on a n d off
action of the refrigerating unit.

N o t e particularly the

constant temperature of the soil.

Not shown here,

however,

1° F. drop in temperature w h e n the fluorescent lights were
turned off by the time clock.
(£)

Normal operation in radiation room.

on the curves,
(Figure 56),

As indicated

at different locations in the radiation room

the air temperature fluctuated greatly due to

the irregular cooling ac t i o n of the refrigerator,
stat,

and the fan.

the thermo­

The differences in absorptive properties

of leaves in a similar position are again portr a y e d in these
curves.
the leaf,

The air beneath the leaf was partially h e a t e d by
as shown here "and in numerous other readings which

are not included in this thesis.

W h e n & thermocouple was

located b etween 1/2 to 1 inch b e l o w the leaf,

it registered a

temperature about mid-way between that of the leaf temperature
a n d that of the air temperature.
E.

Results of the 1949 a n d 1950 seasons.

During the 1949

season a series of mechanical disturbances varied the results
to

some extent.

For example,

the h e a t i n g load of electricity-

1600 or more watts when operating at optimum capacity— over­
loaded the circuit and bu r n e d out the fuse on a few occasions.
Little damage was done to Group I , but the experiment was
terminated a few days early when some of the equipment n eeded
to be altered.

- 130 With Group II, the variae bu r n e d out on the second day
of the experiment.

All of the p l a n t s In the radiation r o o m

were, as a consequence,

throxm out.

r e p a i r e d the following day,

When the equipment was

It was considered desirable to

f o l l o w the p l a n described b e l o w w i t h the remaining good
plants.

T h e control plants in Group II were divided into two

sub-groups,

one-half b e i n g used in the radiation r o o m and the

other half left in the control room.

Then

equal number of

b e a n plants w e r e brought in from the greenhouse from the same
original lot as the rest of the plants.
larger,

These w e r e slightly

a n d again w e r e selected for uniformity a n d divided
r

by r a n d o m c h o i c e for the two divisions of the experiment.
They w e r e d e s i g n a t e d as Group A (divided plants f r o m the con­
trol room) a n d Group B (plants brought in later f r o m the
greenhouse).

The plants from the greenhouse never adjusted

themselves so satisfactorily to the change as did those w h i c h
started in the control room.

Evidently plants can adjust

themselves b e t t e r to their environment w h e n they are younger
in age,

a n d have a less elaborate root

system.

Gr o u p III w a s also exposed to one short p e r i o d of Intense
heat w h e n the variac b u r n e d out,

and to a pe r i o d of cold

temperature when the v&riac was disconnected.

These two

exposures did not last more than ten hours and the results
f r o m the plants of this group should not be given too much
consideration.
record.

They are included h e r e for completion of the

- 131 Because of the unsatisfactory
d u r i n g 1949,
1950.

operation of the equipment

it was necessary to duplicate ano t h e r run in

This time the experiment w a s

conducted w i t h o u t mishap,

and the r e s u l t s may here be considered the most

significant

.of all groups.
The m a n n e r of h e a t i n g was changed in 1950,
stated,

as previo u s l y

by u s i n g the elliptical refle c t o r ins t e a d of „the

parabolic, w h i c h made it possible to double the number of
p l a n t s u s e d (Figure 41).

To test the temperature pattern of

a leaf u n der radiant energy a n d later after it w a s disconnected,
a test was run after the Gr o u p I was

completed in 1949.
This
»
figure shows how the temperature m i g h t have f l u c t u a t e d on
occasions w h e n the electrical current was disconnected.
All plants were individually m e a s u r e d for height at each
internode

(Figure 59).

This d e m o n s t r a t e d that the average

h e ight of the control plants was,

in each case,

significantly

higher.
Each group was analyzed for total nitrogen.

The amount

of nitrogen originally u s e d in each group varied.
cant feature here

(Figure 60)

The s i gnifi­

is the higher percentage of total

nitrogen found in all of the r a d i a t e d plants on a dry-weight
basis w i thin a given group.

These increases,

associated w i t h a reduced growth in each case.

however,

were

The m a l a d ­

justed plants brought in later from the greenhouse (group 2-B)
showed a higher total nitrogen percentage even though the p l a n t s
d id not appear

so healthy as in group 2-A.

- 132 A h i g h e r ash content was also shown for all radiate d
plants w i t h a maximum difference shown In 1950 (Figure 61).
Throughout these experiments the deeper green color of
the r a d i a ted plants seemed to Indicate that they contained
more chlorophyll.

To test this,

an analysis for chlorophyll

was made on f i v e plants from each group fourteen days after
the plants w e r e set Into the control and r a d i a t i o n rooms.
The results are plotted on a bar graph against the percentage
of dry w e ight/green weight
Increase in chlorophyll

(Figure 62).

content,

Besides

showing the

the b a r graph also shows

that radiated plants h a d a higher p e r c e n t a g e of dry weight.
When compared together,

the increase in chlorophyll is not

considered significant.
Some of the plants were analyzed for ascorbic acid con­
tent a few days before the end of the experiment.

No signifi­

cant difference was detected on the basis, of dry weight.
The method followed (Lucus, 83)

consisted of macerating the

stem and leaf tissue for three minutes in a W a r i n g blendfer w i t h
a two percent

solution of metaphosphorlc acid.

The mace rated

material w a s filt e r e d through a flu t e d filter paper.
the clear filtrate a two ml.

From

aliquot was transferred to a

porcelain dish, where it was titrated against a .02 percent
solution of sodium 2, 6 - d i chlorobenzenonelnd ophenol to a faint
p i n k and point.

.

\
- 133 F. . Plant growth behavior.

Although all plants were similar

after moving (Figure 15; radiation room) and (Figures 40,
45, 46;

control room),

teristics.
observed.

they soon developed individual charac­

The following differences in leaf shape were
Those in the control room curved inwardly and b e ­

came bumpy in appearance

(Figures 13,

24; 1950 plante).

Leaves that developed later showed less of this tendency;
however,

they still faced toward the sides,

of the fluorescent lights.

in the direction

The lower leaves died and the

older remaining ones dropped (Figure 11).

The higher humidify

in the control room kept the dead leaves from drying up as
fast as they would have done in the greenhouse.
Those plants in the radiation room were smooth and dark
green in appearance (Figures 12, 17,
their growth.

25)

in the early stage of

At the same time the growing point of the

plant was yellowish and chlorotic in appearance.

The terminal

growth was much slower than was that of the control plants at
the same time.

The first pair of leaves grew in a more or

less horizontal position;

this appearance was more pronounced

at times other than those when the photographs were taken.
Later leaves that developed showed a similar but less pro­
nounced horizontal placement, while the older leaves at that
time tended to bend downward and to look more as those of the
control leaves had previously looked.

(Figure 10).

Mildew infected the second group of plants in 1949,

probably while they were

still growing in the greenhouse

before they were transferred to the converted cold storage
rooms.

The Infected plants became noticeable when they were

in the storage room about two weeks.
fastest on the radiated plants.

The Infection grew

They were sprayed and no

further Infection appeared.
DISCUSSION
The results obtained in this thesis require an explana­
tion of the operation of some of the principles,

underlying

the experiments.
Temperature Relationships
All discussions of radiant energy tend to center around
the topic of temperatures and their relationships to other
environmental factors.

First, let u s consider the temperature

of the radiator or source of radiant energy,
temperature of the air,

and finally,

second,

the

the temperature of the

plant itself.
As previously stated,

the efficiency of the radiator In­

creases w ith the temperature in a nearly direct proportion to
the fourth power of the Absolute temperature (Stefan-Bolzman
law; Figure 3; Koller, 76).
temperature of a radiator,
heated.

Furthermore the higher the
the less the intervening air is

A radiator at a high temperature— especially above

200° C.— is more effective In heating a distant object by

- 135 r a d i a t i o n than It Is at a lower temperature.

F o r example,

the electrical heating unit used by Farrall et al.
h e a t e d the air only

(48)

.5° F . , while at the same time it h eated

the g r ound f r o m 6 to 8° F.
p r esent experiments,

above the surrounding air.

In the

as substantiated by numerous temperature

readings m a d e w i t h several thermocouples,
h e a t e d only to a negligible extent.

the air was usually

A r e a s near the leaves

wh e r e the air w a s wa r m e d more than elsewhere,

w e r e heated

largely thr o u g h the effects of conduction and,

to some extent,

convection.
When a leaf is h e a t e d to a temperature above the surround­
i n g air temperajBare,

it returns a portion of this energy to

its immediate vicinity in several ways.

First, heat is lost

by r a d i a t i o n — the p h e n omenon by w h i c h heat is emitted from
the leaf,

in the infra-red portion of the spectrum,

to distant

objects.

Second, heat is lost from the leaf by conduction.

T h e f o l l owing illustration will serve to show h o w heat is
lost by conduction in still air.

A i r molecules

surrounding a

h e a t e d leaf receive a part of the kinetic energy from that
leaf and,

in turn,

release a part of their absor b e d energy

to the r o w of molecules lying adjacent

to them.

Carleton (31)

s t a t e d that heat can best be defined as the total kinetic
energy p o s s e s s e d by the molecules of a substance.

Thus it

be c o m e s apparent that this kinetic energy and heat can be con­
sidered identical as far as these discussions are

concerned.

- 136 The net result of the transferring of energy from one layer
of molecules to the next,

and so on ad Infinitum, is to give

a pattern with concentric layers of molecules whose energy
decreases as the distance increases from the leaf blade.

This

pattern seldom remains Intact without being disrupted by
other Influences.

Third, heat is lost by convection.

air is heated and gains more kinetic energy,
lighter or less dense than cooler air.

When

it becomes

Wherever air thus

becomes heated it will tend to migrate to areas of less
density and thereby create a condition known as convection
currents.
When the air is in motion— as in breezes or wind,

and

through the action of a fan— both conduction and convection
patterns become modified or partly disintegrated.

In other

words, air movement increases the relative amount of heat
lost by conduction and convection by increasing the tempera­
ture differential between the layers of molecules lying close
to the leaf.

At the same time, however,

the air movement

will decrease the temperature differential between the leaf
and the average air temperature.

Radiation loss from a leaf

depends more upon the lower temperature of distant non-adjacent
surroundings than upon the temperature of the air which
Immediately envelops it.

The heat absorption of the atmos­

phere that is absorbed by the atmosphere is largely due to
water vapor and,
and oxygen,

to a lesser extent,

themselves,

carbon dioxide.

-

Nitrogen

absorb an infinitesimal amount of energy.

- 137 Radiation studies on growing plants are altered by these
temperature patterns.

Air movement is not wanted because it

disrupts the conduction-temperature pattern.

Additional

calories of radiant energy are required when breezes remove
these heated molecules lying adjacent to the leaf;
when these particles remain undisturbed,

therefore,

it is possible to

heat the plants by radiant means using less energy, for wher­
ever the air-heated-by-conduction molecules can be held in­
tact, the heat lost by conduction and convection is greatly
reduced.

Even when all air movements are eliminated,

that were possible,
duction pattern,

if

convection currents will modify this con­

the extent of the modification varying with

the temperature differential which the experimenter maintains
between the leaf and the average air temperature.
Although it is next to impossible to eliminate the heat
lost by convection, it is possible to prevent unnecessary air
movements.

These controlled conditions are, however, not

found in many plant-growing chambers because the need is sel­
dom specified to the designers who draw up the plans for the
construction for such rooms.

Fpw experimenters have ever

asked for still air because still air does not give them the
even distribution of temperature which they require in most
of their experimental work.
In the radiant heat experiments carried on in the cold
storage rooms, a differential of 20° F. was maintained between
the leaf and the average air temperature at a distance from

- 138 the plant.
rooms,

In spite of the unwanted air circulation in these

there was a slight temperature pattern extending from

each of these plants into the adjacent air.

A difference of

10° F. was found between one-half and one inch from the lower
and protected surface of the leaf.
of 20° F.
tion.

A temperature differential

is seldom encountered in nature under solar radia­

Perhaps some of the alpine plants on the southern

exposures may be heated that much or even more, but such
examples are infrequent.

A temperature differential of 10° F.

is often found, and a 5° F. increase is commonly observed
where plants are exposed to solar radiation.
application of radiant heating,

In any practical

it Is much better to think in

terms of a small differential than in terms of a large differ­
ential,

such as was attempted in this experiment.

However,

it was important to test the effects of radiation under as
severe a test as possible in order to learn its potentialities.
The present type of radiant heating is impractical, not only
because of the high temperature differential maintained, but
because of the use of electricity, which will never be prac­
tical unless It becomes much cheaper than it is at present.
The cold storage rooms which were used in these studies
were not adequately adapted to radiation studies because of
the irregular air circulation caused by the fans in the room.
The fans were necessary in order to circulate the cooled air
from the refrigerating units.

- 139 Many attempts were made to minimize the irregular flow
of air.

During 1949 the entire framework with all the equip-

ment was moved from one part of the room to the other.
endwise position,

An

as viewed from the door, at the farthest

end of the room from the door proved the most successful of
all positions tried.

During 1950, wrapping paper was draped

around the framexirork in various ways,

and eventually a system

was provided which proved partially successful.
In designing a room for the purposes of radiation studies,
one of several different ideas might prove practical.

First,

air could be conditioned to the proper temperature before it
is Introduced into the room.

Ducts used for this purpose

could be so located and baffled that little draft would reach
the plants.

Second, a system of radiant cooling could be

used whereby pipes of a refrigerant could be imbedded behind
some highly emissive material in the wall and ceiling.

The

celling should prove most practical because the warmer air
moves to the top of a room.

The cost of construction would

be high because of the need for extra thick insulating material
behind such a panel,

if an efficient cooling system is wanted.

If these systems are Impractical the present rooms could
be converted into more desirable cold rooms by Increasing the
cooling area,

using a lower temperature refrigerant and ad­

justing the cooling system for continual operation.
The Inverse square law of radiation distribution, which
applies to perfect black bodies (Figure 5) does not apply

- 140 absolutely to plant leaves which cannot absorb 100 percent
of the radiation as does the theoretical black body.

Leaves

permit a part of the waves which fall upon them to pass through.
The transmission is small,

but it allows the shaded leaves

or the shaded portions of leaves to receive some heat.
waves of light are reflected.

Other

The amount of radiation

reflected depends upon the angle of incidence and the surface
characteristics of the leaf.

In radiation experiments the

experimenter wants to minimize the amount of reflection and
increase the degree of absorption.
when the leaf is horizontal,

i.e.,

This is best accomplished
at right angles to the

direction of radiation.
The principle of the Inverse square law answered the
question why the leaves closest to the radiator were the h o t ­
test.

The difference is theoretically quadrupled for every

doubling of the distance between the radiator and the plant.
This Indicated that the lower leaves of the radiated plants
did not receive as much energy as those higher up.

It also

meant that the stems did not receive very much direct radia­
tion because of their position.

The relatively small exposed

area per volume of both the petioles and the stem showed that
these areas were locally cooler than the leaves, which had a
relatively larger exposed surface for their volume of tissue.
Borthwick (17)

showed that localized cooling of plant tissue

can and does greatly slow up the metabolic rate of growth.
Although the growing point is closer to the radiator than other

- 141 parts of the stem and some leaves of the plant,
is not heated as much as the leaves are.

It probably

This lower tempera­

ture may help to explain the chlorotic condition seen at the
growing point of the radiated seedlings.
(89)

Meyer and Anderson

say that the dividing cells in the top of the growing

point is a region of intensive respiratory and asslmllatory
activity.

Thus any decrease in temperature at this region may

well account for any tardiness in the rate of growth.
When it was necessary to operate at such close distances
as was necessary in 1950,

14 to 16 inches from the radiator,

it was difficult to estimate the average amount of radiant
energy received per plant.

If any error was made,

it was on

the side of underheating due to underestimating the volume of
colder tissue in the stem and petioles.

Air currents which

tended to cool some plants more than others and the uneven­
ness of radiation distribution produced errors w hic h were
partially counteracted by moving the plants from time to time.
The narrow distribution of radiant energy for the two differ­
ent reflectors meant using two rows of plants with the para­
bolic reflector and four rows with the elliptical reflector.
A space had to be vacated at both ends and at the sides because
of the insufficient amount of radiant energy.

The position

of the leaves brought in an uncertain variable, but fortunately
the strong tendency for the leaves of the radiated plants to
face in one direction simplified calculations.

- 142 All light Intensities were balanced with a Weston light
meter (photometer), but it appeared that there may have been
a variation in the amount of luminous energy received by the
different plants because of their placement and the curling
of some of the leaves.

Since there was no simple way to

ascertain this difference,

it was assumed that all variations

tended to cancel each other and that whatever variation
occurred was not significant.
The leaf position constantly altered due to nutation.
This alteration could have been photographed with a timelapse camera.

The thermocouple connection with the leaf was

sufficiently strong in almost all cases to counteract any
tendency of plant movements to loosen these Junctions.

When

it was necessary to use thermocouples for thermostatic control
of radiant energy,

extreme care was taken in attaching them;

none of these attachments loosened.
Soil Temperature Relationships
Soil temperature greatly affects the absorption of water
by growing plants in general and more so by certain high
temperature crops.

Arndt

(7) found that cotton wilts when the

temperature of the soil falls to a region between 17 and 20° C . ,
while Doring (43) found that, when several plants were trans­
ferred from a soil temperature of 20° C. to one of 0° C.,
the absorption of some plants was greatly retarded and others
were scarsely affected.

Kramer (77) also found species dif­

ferences with respect to water absorption at low soil tempera-

- 143 tures.

Meyer and Anderson (89) state that the principal

factors affecting the rate of absorption by growing plants
are:

(1) available soil water,

(2) soil temperature,

(3)

aeration of the soil, and (4) concentration of solutes in the
soil solution.

All these factors likewise influence the

rate of transpiration.
In the experiments conducted in the cold storage room,
these above-mentioned factors were controlled In the follow­
ing ways.

The necessary water was made available by the use

of automatic watering;

the soil temperature was kept constant

by the soil-heating cable regulated by the soil thermostat;
the soil aeration was adequate under automatic watering con­
ditions, as stated by Post (106) under this subject; and the
soil solutes were probably nearly equal because each group
of experimental plants was potted from an evenly mixed pile
of soil, and because the seedlings were nexrer fertilized from
that date until the end of the experiment.

Quick tests by

the Spurway system of soil analysis checked perfectly each
time an analysis was made.

From the foregoing considerations

it is seen that the variability of the plants in the experi­
ments was little affected by soil conditions.
Since an increase in total nitrogen and ash was found in
the radiated plants,

the following references regarding the

Intake of nutrients at different soil temperatures are included.
Kramer (77)
ture levels.

showed that salt intake is reduced at low-temperaAllen (3) described how calendulas and snapdragons

- 144 showed, marked symptoms of nitrogen deficiency at the slightly
low soil temperature of 52° P.
Regarding the relationship between light intensity and
soil temperature Steinbauer

(123)

showed that the greatest

response to an increase in concentration of the nutrient
solutions occurred at the higher light intensities,

indicating

that a greater need for nutrients occurred under these conditions
If differences in soil temperature caused any of the
differences noted in the radiated plants,

they were probably

due to the variation in the different parts of the pots which
were warmer

at the bottom

and colder at the top while they

were in the

40° F. room.It is not known what effect

localized heating may have had on the soil roots.

this
The roots

and root hairs appeared normal and equally distributed with
the largest quantity toward the bottom.

Nodule development

appeared equally abundant in both group of plants.
Stoutemeyer and Close (127)

and Stout et al.

(126) mention

that the localized heating of a hes.ting cable laid.on the
surface of the soil gave the most growth to plants growing
nearest the

heating coil.

When the heating coil was placed

beneath the

rows of gladioli, Emswoller and Barthwlck (47)

found that these plants were forced greatly ahead of unheated
corms.

Zerfoss and Strand (152) and Porter and Dltchman (105)

mention that heating the plants with electric bulbs in the
hotbeds gave a good growth even when the soil was not heated.

- 145 Hayward and Spur (69) relate the Intake of Ions to the
osmotic pressure differences existing between the soil solu­
tion and cell sap and to the metabolic status of the roots
in respect to the carbohydrate reserves.

3ubstrates of high

osmatic concentration tend to inhibit the merlstematic activity
and elongation of the root.
Vapor Pressure Deficit
The relative dryness of the air determines,

in part,

the amount of water that is lost through the plant by either
cutlcular or stomatal transpiration.

It also determines the

amount of water lost from the soil by evaporation.

At a

given temperature of the air the relative humidity can be
used for comparing differences in the huaidlty, but it fails
to indicate the drying power of the atmosphere whe n the
temperature of the air varies.

In discussing the transpira­

tion of plants under different temperature conditions,

a more

reliable guide is the use of the vapor-pressure deficit.
This deficit is an index of the relative water-absorbing
capacity of the atmosphere— the true criterion which deter­
mines thenapldlty by which this air changes liquid water into
water vapor.

The higher the vapor pressure deficit,

the

greater the capacity the air will have to evaporate water
from a leaf or atmometer into vapor.

According to Anderson

(4) the vapor-pressure deficit is the difference between the
actual vapor pressure of the atmosphere and the vapor pressure

- 146 /

of a saturated atmosphere at the same temperature.

The vapor

pressure can he calculated from tables when the relative
humidity and temperature are known (Oosting,

98).

The vapor

pressure deficit increases with the rise in temperature and
will vary from place to place in a more or le ss direct rela­
tionship to fluctuations in temperature when transpiration is
not considered.

Air molecules adjacent to a heated leaf,

therefore, have a greater energy factor which aids the trans­
formation of liquid water into vapor.

This.energy tends to

hasten the transplratory rate of the leaves under radiation
conditions, and the vapor thus formed,

in turn, lowers the

vapor-pressure deficit in that vicinity.
Localized areas of varying degrees of moisture,

especially

around the leaf and the stomatal or transpiring areas, are
known as microclimates

(Ramsey et a l . , 113).

characterized by a humidity gradient,

These are

the vapor pressure

deficit at the evaporating area of the inter-stoma.tal spongy
cells in the leaf differing from that of the dryer air at a
greater distance from the leaf.
Meyer and Anderson (89) stated that most transpiration
occurs through the stomatas but that in some cases some water
is lost through the cuticle;

the amount lost by both methods

depends upon leaf characteristics.

Miller (91)

stated that

the amount of transpiration depends upon the intercellular
spaces, which may form between 3 to 70 percent of the total
volume of the leaf.

- 147 The leaves on plants growing under intense radiation
probably have fewer intercellular spaces because of the
difference in texture that is seen when intensely radiated
leaves are compared to leaves growing under less intense
radiation.

Oosting (98)

stated that plants growing under

Intense solar radiation have a thicker palisade layer and
less sponge tissue, while those growing under conditions of
less radiation have fewer palisade cells and more sponge
tissue.

These statements indicate that radiated leaves have

less evaporating area within their leaves, and since water is
lost in such large amounts under radiant conditions,

the

water-loss per Intercellular transpiring area must be rela­
tively large.

The probable reduced evaporating area and the

high water loss would Indicate, besides,

that the inter-

stomatal cavity is a region of higher humidity than might
otherwise be anticipated.
The experience with mildew infection in the second group
of plants during the 1949 season,

in which the mildew flour­

ished more in the radiated plants than in the control plants,
seemed to verify the idea of a higher moisture condition in
the intercellular cavity.

Mildew usually develops on the

-

surface of the leaf, and casual observation has often given the
impression that it could not grow if all water were kept off
the surface.

Under radiant energy exposure leaves are usually

quite dry on the surface due to the Increase vapor pressure
deficit resulting from a higher leaf temperature.

Actually,

- 148 however, the mildew grew a great deal.

It becomes apparent

that the spores must have germinated within the Intercellular
areas of the leaf, and that sufficient water must have been
present to make germination possible.

To enable the mycelium

to furnish enough water must have been available to maintain
a lush growth.

Further research with mildew under radiation

conditions should be made to find out the true relationships
existing under such growing conditions.
The use of radiant energy for the prevention of purely
surface-borne organisms,
supposed to be,

such as damping-off organisms are

should prove a fruitful field for future

research since it offers one of the most practical.,applications
for the future use of radiant energy.
The higher vapor-pressure deficit on radiated plants
caused considerably more water to transpire from the leaves.
This increased loss of water is frequently observed by horti­
culturists who note that plants require more water when the
solar radiation is most intense.

Under the combined influence

of both sunshine and artificial radiation,

the lily plants

grown in these experiments showed a water consumption twice
as high as those same plants growing under solar radiation
only.

Red Kidney beans growing under artificial radiation

showed only a 4:1- difference at the start and a 6:1 differ­
ence toward the end of experiment.

This difference was

partly due to the higher humidity w h i c h developed in the
control room,

since no effort was made to control the moisture

- 149 content of the air.

No effort was made to control the

humidity because of the difficulty of maintaining temperature
control and because of the poor adaptation of the available
humifiers to the purpose.
All beans growing under control conditions showed a
higher water content in their leaves as revealed in the
difference between t^ie green and dry weight of the plant
tissue.

This is graphically shown in Figure 65 in relation

to the chlorophyll analysis.

The differences, although not

shown in this thesis, were not as high when the comparisons
were made after the plaits were four weeks in the radiation
and control room. 4 The radiated plants were usually dried to
about 11 percent dry weight, while the control plants weighed
to about 10 percent dry weight.

It appears that the most

succulent pls^nts are those growing under the control condi­
tions.

Could this be due to the influence that radiation has

in influencing the intercellular structure of the leaves?
Future investigation using cross-sections of the leaves would
indicate the differences in cellular structure.
Vapor-pressure variation affects the radiation loss from
the soil (Puckett, 112), and the thermal conductivity is higher
on wet soils than it is on dry soils (Nicholas,

97).

Most references in the literature regarding the relation­
ship of Intake of sal'ts vs. water intake indicate that no
apparent relationship exists between these two phenomena;
however,

since more ash and total nitrogen were measured in

\,

- 150 the radiated plants over the control plants in these experi­
ments,

two references In partial support of the theory that

the higher water Intake Influences the Increased Intake of
solutes are offered.

Sorauer (121) found that the pea plants

growing In a dry chamber had a slightly greater dry weight
and ash content than similar plants growing In humid chambers.
Muenscher (93) obtained 5 percent less ash In plants growing
in rooms where the transpiration was reduced one-half.
he remarks,

still is not significant.

However,

This,

It seems,

there might be some slight effect on the ion intake from an
increased absorption of water which is not normally present
under the usual type of experimental designs.
Plant Movements
The Hed Kidney bean showed

some diaphototropism;

i.e.,

the plants bent their leaves in the direction of light.

This

was evident in the early hours of the day when the leaves
were facing the eastern sky.

The effect was most pronounced

when the light was in the most unilateral direction— i.e.,
when the light on one side was more intense than in any other
direction.
The experimental bean plants used in the cold storage
rooms also responded in a different way.

The lights used,

500 candle-power in the 1949 season and 240 candle-power during
1950,

seemed to cause the plants in the control room to face

toward the two sides on which these lights were placed.

In

- 151 the radiation room the leaves were more or less at right
angles to the direction of the radiation source.

It appeared

f

that the radiant heat sourc4 might have had a tropic stimulus
on these leaves in much the same way that the lights have in
phototropism.
Boysen-Jensen (18)

showed that phototropism was the

result of the unequal stimulus of auxins,

or plant hormones,

which tend to migrate to the shady side of the stem.

These

hormones cause a more rapid growth on the side away from the
light and thus cause the stem to bend in the direction of the
light.

Auxins in plant tissues can be inactivated by light

of high intensity (Burkholder and Johnson,

29).

The shorter

waves are the most efficient in causing this inactivation.
The horizontal placement of the Red Kidney bean leaves
in the radiation room probably indicates that the radiation
from the heater causes some destruction of these hormones.
It appears that there might be a two-fold reaction to
radiant energy.

First,

it seems to stimulate, the leaves

already formed and give them a very healthy and vigorous
green appearance;

and,

second,

it inhibits the terminal grow­

ing point which at certain times,

especially when the plants

are young, become chlorotic in appearance (Figure 50).

This

may indicate the reaction of two different types of growth
substances.

This inhibitory type of growth cannot be asso­

ciated with any similar response typical of that produced by
various colored lights.

This growth looked more like the

- 152 reaction to blue light, however,
were normal in length.

the Internodes that developed

The main conclusion that can be

arrived at 13 that the plants Just did not grow as fast, and
that the slow growth may have been due to such reactions as
\

the probable lower temperature of the stem, lnjurj' to the
growing point,

the higher osmatic pressure (which undoubtedly

existed at this point) which can inhibit merlstematlc growth
(Hayworth and Spurr,

69), or the lower carbon dioxide content,

which could have existed.

If any injurious gases were present

there was no way of detecting their presence.
on the radiation plants were thicker,

The leaves

and the stimulus of the

radiant heat probably diverted part of the nutrient of the
plant to form added palisdde layers rather than for use in the
merlstematlc region of the growing point.

This thicker

palisade layer is described by Turrell (133), who describes
how plants developed a significant degree of x^romorphy under
high light Intensities.
Goodwin (56) found that light inhibits the length of
growth of the first internode of Avena sativa.

However he

found no inhibition in the infra-red wave band longer than
1.6 microns at the intensities used.

Later the same investi­

gator found that the greatest inhibition was found in the
visible yellow and red light (Goodwin,

57).

The wave band

used in these experiments centered around the four micron wave
length with a broad spread extending from at least 1 to 10 or
20 microns.

153 -

/
SUMMARY
1.

Radiant energy was used from a croxnolox heating

element to heat plants above the surrounding air temperature.
A plant whose temperature requirement

such as the

Red Kidney bean, which requires a 60° F. night temperature
for optimum growth, was found suitable for most of the
radiation studies.
2.

Radiant energy can be used to heat the leaves of

R e d Kidney beans to a temperature of 20° F. above the average
room temperature in which they are growing when 60° F. is
used as the standard leaf temperature at a distance of 14
inches with the elliptical designed reflector and 24 inches
with the parabolic reflector.
3.

When the analysis was made of the plants growing under

radiant conditions and those used as control plants, an increase
of ash and total nitrogen was indicated in each case; this
increase may have been due to the greater intake of water
and to t h e ‘fact that some of extra nutrients in the soil may
have entered the plant with the extra water absorbed.
4.

No significant difference in vitamin C or chlorophyll

content was found when the analyses of the radiated and con­
trol plants were compared on a dry weight basis.
5.

Red Kidney beans grown under radiant energy displayed

a dark green, flat,

smooth leaf in a more or leas horizontal

position, while the control plants at the same age of growth

- 154 were curled, bumpy and tended to face sidewise in the direc­
tion of the two panels of light.

Terminal growth of the

plants in the radiated group was partially chlorotlc and
stunted in appearance and the growing point of this group
did not develop as fast as those in the control room.

This

difference in growth may have been due to a difference in
temperature existing at those points and the uninterrupted
presence of radiant energy.
6.

Both the 300 and 240 foot-candles of light intensity

proved adequate for the needs of plant growth In these
experiments,
better.

although more illumination might have proved

The daylight fluorescent tubes gave off a negligible

amount of heat— warming the plants approximately 1° F.
7.

Automatic equipment for the heating of Red Kidney

beans proved successful.

When the circular-chart pneumatlc-

potentiometer controller was activated by a stimulus from a
thermocouple located on the under surface of a selected leaf,
it controlled the variac through a motortrol giving the re­
quired amount of voltage to the electrical heating rod.

The

temperature variation was about 2° F.
8.

The elliptical reflector gave the best distribution

of energy but could not be used at sufficient distances to
eliminate the deleterious effects associated with the disproportlonal degree of heating occurlng at close distances.
9.

The total height of radiated plants was less in all

lots, although thp length of each internode approached a

- - 155 condition approximately normal.

The radiated plants developed

fewer internodes than the control plants.
10.

The thermocouples made from soldering the copper

and constantan wires with silver end to end proved very suc­
cessful,
11.

easy to use and accurate.
Red Kidney beans proved to be a particularly valuable

plant for such radiation studies because of its sensitive*
ness to temperature changes and the .ease of growing seedlings
that are suitable for selection purposes.
FUTURE SUGGESTIONS
The use of radiant energy to heat plants to a temperature
of 20° C. above air temperature does not look promising when
viewed from the results of this thesis.
however,

It was Justified,

in this case, to determine w h a t effects such a tem­

perature differential would have on plant material.
It is suggested that a temperature differential of 10° F . ,
such as is commonly observed in the greenhouse on most plant
material, be used in future research which may have practical
applications.

If the plants which demand a 60° F. night

temperature could be grown in a greenhouse heated to 50° F.
by usual means, xvith an extra 10° F.

supplied by radiant

means, and if this job could be done more economically than
heating the same greenhousS to 60° F. air temperature,

it

would be considered practical means of heating providing the
cost of installation would not be prohibitive.

The fuel bill

- 156 of heating greenhouses In Michigan to 60° F. Is approximately
twice the cost that of heating the same greenhouse to 50° F»
Heating plants at close distances,
the present experiments,

as was necessary in

created several undesirable features

In which the temperatures varied greatly from one part of the
plant to the other.

Future heating from radiant sources should

make use of greater distances, and ultimately the farthest
distance possible In a greenhouse— under the ridge of the
greenhouse.

The distance

the sun, 93 million miles,

ideal because it heats all parts of the plants,
exposed,

Is

equally

identically.

Electricity, as a source for radiation,

is still too

expensive to compete on an equal basis with coal or oil.
the present

studies It was the only available means of con­

trolling radiation for experimental purposes.

However,

future control may be obtained by other methods.
promising,

In

s t m

The most

In the theoretical and untried stage,

is

radiation from the flow of a liquid^ heated to a temperature
of 500-600° C., circulated by means of a pump, and heated
from a central location by whatever fuel is most economical.

1.

A ”liquld-metal coolent" Is referred to, as well as a
sodium-potasslum alloy used at Oak Ridge National Labora­
tory wh ich melts at 59° F.
Gallium melts at 86 F. and
bolls at 3600° F.
If a metal is needed to resist high
temperatures, molybdenum, with a melting point of 4750°
F. or an alloy made from it could be used.

- 157 The conduit for such a flow would have to extend from one
end of the greenhouse to the other;

It would have to be heavily

Insulated on the top to prevent heat loss and scientifically
shaped on the bottom with the proper convex curvature,

to

insure equal distribution of radiant energy to all parts of
the greenhouse.

Commercial preparations on the market— like

gallium or some alloys— may be suitable to find application
in such a system.
Air circulation should be studied to insure the efficiency
of radiant heating.

An ingenious method was used by Gray

(60, 62) to detect air movement in the greenhouse.

By this

means it might be possible to minimize unwanted air circula­
tion.

Freeland (52)

suggests one me thod of controlling cir­

culation which might be modified for these experiments.
Radiant energy experiments carried on without
radiation should be supplied with ample.light.

solar

Illumination

of 600-1000 foot-candles of daylight fluorescent or white
fluorescent light is suggested,
are to be grown to maturity.

especially if the plants used

It is desirable to measure and

control the COg concentration in order to remove it as a con­
trolling factor in plant studies under higher intensities of
illumination.
Refrigeration should be constant and controllable.
Besides modification of the present system,

two other methods

might be used which would be suitable for radiation studies.

- 158 First, air that Is conditioned to the proper temperature,
humidity,

and,

if need be,

COg concentration could be intro­

duced by means of ducts with suitably baffled outlets.
Second, refrigeration could be obtained by means of a radiant
cooling system mounted behind a highly emissive ceiling or
0

wall and with ample insulation to the outside air.

This

system should give a theoretically even distribution of cool­
ing,

especially if placed in the ceiling,

with little air

movement other than the normal convection currents.
The vapor-pressure deficit should be controlled.
tlo6)

Wallace

suggested a simple and effective humidity control which

consisted of a radlo-grld actuated by a hu mid ist at.

The most

suitable humidistat available is probably the electric hy ­
grometer.

Its accuracy is much greater than that of the

design based on the expansion of the human hair.
If a more highly efficient source of radiant energy is
desired other than the type emitted by a heated metal (black
body type),

there is the possibility that future developments

of suitable filtering devices may provide a higher concentra­
tion of energy to plants.

With their use a higher temperature

emitter could be used similar to zirconium— one of the most
concentrated forms of radiant energy that is used in industry.
Becker and Fan (15) have tested the optical properties of
different metals and report that both germanium and especially
silicon transmit a large quantity of infra-red rays in the
near infra-red region of the spectrum.

A filter made of

158 First, air that Is conditioned to the proper temperature,
humidity,

and,

if need be,

C O

q

concentration could be intro­

duced by means of ducts with suitably baffled outlets/
Second, refrigeration could be obtained by means of a radiant
cooling system mounted behind a highly emissive ceiling or
wall and with ample insulation to the outside air.

This

system should give a theoretically even distribution of cool­
ing,

especially if placed in the ceiling, with little air

movement other than the, normal convectiop currents.
The vapor-pressure deficit, should be controlled.
tlo6)

Wallace

suggested a simple and effective humidity control which

consisted of a radio-grid actuated by a humidistat.

The most

suitable humidistat available is probably the electric hy­
grometer.

Its accuracy is much greater than that of the

design based on the expansion of the human hair.
If a more highly efficient source of radiant energy is
desired other than the type emitted by a heated metal (black
body type), there is the possibility that future developments
of suitable filtering devices may provide a higher concentra­
tion of energy to plants.

With their use a higher temperature

emitter could be used similar to zirconium— one of the most
concentrated forms of radiant energy that is used in industry.
Becker and Fan (3.5) have tested the optical properties of
different metals and report that both germanium and especially
silicon transmit a large quantity of infra-red rays in the
near infra-red region of the spectrum.

A filter made of

- 159 silicon should be more practical than ordinary Infra-red glass
filters or the asphalt type filter used by Withrow (14-6).

No

visible light should pass through such a filter as its
absorption does not extend into the visible part of the
spectrum but starts at about one micron wave length.

Light

through this filter will fog special infra-red photographic
plates which are sensitive to 1.2 microns of waves.

If a

glass type of filter is needed the new glass discovered by
Dr. Frerichs (96) has the ability to transmit more infra-red
radiations than any other molded glass.

It is reddish in

color and is made with arsenic and sulphur.

It is possible

to construct the glass so that no visible light can be
transmitted.
If it is desirable to study the effects of a narrow band
of any particular radiation,

it now seems probable that the

newly described interference filter will be suitable to studies
of the effect of visible, light on plants and probably could be
likewise adapted to studies of the effect of infra-red radia­
tion.

Because of the narrow band of emission it would not be

considered an efficient type but rather one that could be used
to tag,

so to speak, the effects of each distinct band in the

spectrum.
If the effects of radiation are to be studied in the
greenhouse,

it is neewssary to determine the amount of solar

illumination falling on the plants being studied.
tain the amount of solar illumination at Ithaca,

To ascer­
New York,

I

- 160 Post and Nixon (106) built a graphic light meter which
recorded light from

\

to 10,000 foot-candles continuously

throughout the day and year.

Another method of determining

the accumulative effect of light over the entire day is based
on the light chemical reaction of oxalic acid-uranyl sulphate
by ultra-violet light (6).

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___________ ..
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-

169

-

113.

Ramsay, J . A . , et al.
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114.

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120 .

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123.

Steinbauer, G.P.
Groxvth of tree seedlings In relation
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Plant Physiol. 7; 742-744.
1932.

124.

Stewart, W . D . , and Arthur, J.M.
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125.

_____________, and ______________ . Change In mineral com­
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Ibid. 9:
105-120.
1937.

-

1950

-

126.

Stout, G.J. et al.
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127.

Stoutemeyer, V.T., and Close, A.W.
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128.

Telkes, M.
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1949.

129.

___________ , and Raymond, E.
Storing soler heat In chemi­
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Heating and Ventilating 46: 80-86.
1949.

130.

The Eppley pyrheliometer Bull. 2, and Eppley Thermopiles
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The Eppley Laboratory, Inc.
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Received In 1948.

131.

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132.

Penn.

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133.

Turrell, F.M.
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134.

Tyndall, J.
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135.

Wallace, R.H., Private conversation and collected mimeo­
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136.

______________ , and Clum, H.H.
Leaf temperatures.
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1938.

137.

______________ , and Bushnell, R.J.
A simple and effective
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Tent,
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__ _

Amer.

Specific factors
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1941.

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growth

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I."'"" t h e ’ a i r - c o n d i t i o n e d
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________________, P r i v a t e

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The M a c ­

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1947.

______ ________ , a n d B e n e d i c t , K . M .
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t i o n a p ' l i e d a s a s u p p l e m e n t to d a y l i g h t .
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.936.
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growth with
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and

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Y a r w o o d , C.E., a n d H a z e n , W.E.. T h e r e l a t i v e h u m i d i t y
leaf surfaces.
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at

Z e r f o s s , G-.E., a n d S t r a n d , A . B .
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190: 1 9 4 4 .

as

Electric light bulbs
U n i . of T e n n . B u l l .