THE EFFECT OF LIGHT INTENSITY ON THE TRANSPIRATION OF CERTAIN PLANTS by Ben Nelson Stuckey A THESIS Submitted to the Graduate School of Michigan State College of Agriculture and Applied Science in partial fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Horticulture 1 9 /y;/fry 4 1 ProQuest Number: 10008436 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest, ProQuest 10008436 Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346 TABLE OP COl'ITEM’S I* Introduction.......... 1 II. Review of Literature.............. 2 III. Preliminary Experiments IV. Apparatus *......... ..*.*.11 .... *.......*...... 16 V. Method........................................ 27 VI. Presentation of D a t a . ............ VII. Discussion.......... VIII. Summary............ IX. Literature Cited........ X. Acknov/ledgments 29 ..63 66 ..... 134246 .......67 .............70 (1) Introduction Transpiration may be defined as the loss of water in the form of vapor from the stem and leaves of the living plant. In general, trans­ piration is affected by those external changes which affect evaporation from an open water surface, amount of water available and by internal changes in the plant itself. Light intensity is known to affect transpiration as well as other plant processes. Since the greater amount of transpirational water is lost during the day, the natural assumption is that with increased light intensity the transpirational curve would rise. When the light inten­ sity is increased, however, more infra-red rays are given off and a rise in temperature is the result. Since transpiration is known to vary directly with the temperature, this transpirational increase, caused by increasing the infra-red rays, would be only indirectly caused by a change in the light intensity. The increase in temperature, caused by higher light intensities, is thought to account for only a part of the total increase in transpira­ tion. The remaining portion is generally thought to be produced by the opening of the stomata when subjected to high light Intensities which Increases the transpiring surface of a plant. In view of the fact that the literature on the above generalities was found to be quite varied and In some cases contradictory, a plan of study was formulated in an effort to determine the effect of light intensity on transpiration while keeping all other factors constant. f2) Review of Literature Kiesselbach (9) reports the first plants was made by Woodv/ard of England transpiration experiments as early effort to measure water loss from in 1699. an 1736. 394 publications on transpiration and Stephen Hales conducted By 1909 there were at least since that date many more have been added. The effect of light on transpiration and other plant functions has been studied extensively during the last 30 years. These studies have been con­ ducted on the effect of total light intensity and certain wave lengths of light on water loss and on the opening of stomata, since stomatal action is thought to be a controlling factor in water loss. Livingston (14), using Tradescantia zebrina measured the stomatal pores and found the capacity for diffusion to range from 1.92 to 6 times the evaporating power of the air. By using diffused light he concluded that the variation in size of the stomatal pores of Tradescantia zebrina was great enough to explain that portion of the daily rise in the transpira­ tion rate which is not dependent upon the variation in the evaporating power of the air. In direct sunlight, however, he found that the variation in the stomatal opening is not large enough to account for the daily rise in trans­ piration. In further studies Livingston (15, 16) found that those en­ vironmental conditions which affect the rate of vaporization of water act directly to influence transpiration. found to differ Different species of plants were in their response to solar radiation and the same individ­ ual responds differently according to its stage of development, end to the previous treatment to which it has been subjected. He also reports that the influence of indirect solar radiation should not be neglected in discussions of the influence of sunshine on transpiration. Livingston concluded that the total amount of (5) transpirational water loss from a plant for any given period may be considered as a summation of the effects of the evaporating power of the air and of the radiant energy absorbed throughout the period, modified by certain secondary effects of these conditions and by cer­ tain responses to other conditions, Bakke and Livingston (2), using Livingston’s cobalt chloride method of measuring transpiration, studied the transpiration of Xanthium canadense and Helianthus annuus. They, as well as Blaydes (3) and Shreve (£1), found that the transpiration of each varied greatly during the day. A large increase in transpiration occurred first at sunrise, in some cases amounting to 24 per cent of the average for 24 hours. They attributed this to the opening of the stomata with increasing light and to the excess water in the leaves which had accumulated during the night. These workers also pointed out the fact that the transpiration of a leaf varies inversely with its age. Martin (17), using different thicknesses of cheese cloth to vary light intensity and, with the sun as his light source, obtained results which differed from those of Livingston. He tested Helianthus annuus for 4 to 5 hours on cloudless days by determining the loss in weight of plants growing in sealed cans. Records were kept on temperature, humidity and wind velocity for every hour and light was measured by a Smithsonian pyranometer 3 to 4 times hourly. Martin concluded that the acceleration of transpiration was due to the heating effects of the infra-red rays rather than the light intensity alone. He also states that the fraction of transpiration rate due to the direct effect of radiation for plants in full sunshine was found to vaiy 50 to 81 per cent, depending largely on (4) the evaporating power of the air. Le Clerc du Sablon (15), who tested a large number of plants by essentially the same method as Martin, reported as did Martin that the accelerating effect of radiation on transpiration is due to the heating of the leaves. He also states that this increase in leaf temperature probably causes an increase in the permeability of the protoplasm which would aid in increasing transpiration. Sir Francis Darwin (5) studied the effect of light on transpira­ tion at about the same time as did Martin and Le Clerc du Sablon. He measured transpiration by the potometer and weight methods on cut branches of Prunus laurocerasus and Hedera helix, comparing the north light of a room with total darkness. The stomata were closed with cocoa fat and slits were made through which transpiration took place. This method was used to prevent changing of the evaporating surface by the opening and closing of the stomata. From his studies he concluded as did Martin and Le Clerc du Sablon, that the increase in transpiration caused by an increase in light is due to the chloroplasts being warmed by the absorp­ tion of radiant energy or that light produces an increased permeability of the plasma membrane to water. Henderson (8), using somewhat the same technique as that of Darwin, studied the effect of light on water loss from the mesophyll. He used 100—watt gas-filled lamps suspended 3 ft. above the plants, which gave intensities sufficient for stomatal change. In using Hedera helix, Eupatorium adenophorum and Aster sp. he smeared the leaves and stems with vaseline to prevent stomatal transpiration and made slits in the leaves down to the mesophyll layer of cells. Leaf temperatures were (5) recorded “ by the use of thermocouples* Henderson found that apart from the temperature increase caused by light there was a slight increase in transpiration from increased light. This increase was found to vaiy greatly from plant to plant and even in individual leaves. Transpiration was also found to depend on the amount of water in the mesophyll cells# Henderson found that when the light intensity increased there was very little effect up to a certain point and then the transpiration rate increased rapidly until a certain light intensity was reached* At this point the curve gradually straightened out until no transpirational increase resulted from increased light. The curve was found to vary greatly with different plants* Henderson also studied the effects of intermittent light on trans­ piration. In giving plants intermittent light of 20 second intervals the transpiration was found to be about half that obtained if continuous light of the same intensity was given the plants* However, by increas­ ing the speed of a rotating disk between the light source and the plants, which gave the plants shorter periods of light and darkness, the trans­ piration rate rose directly with the speed of the disk. Thus it is seen that light of the same intensity is more efficient in Increasing trans­ piration with short rather than with long intermissions. From this Henderson concludes that conditions favorable to increased water loss induced by illumination are produced almost at once but a reversion to conditions less favorable for water loss is produced more slowly. Thomas and Hill (23) made a continuous measurement of the trans­ piration of alfalfa and wheat growing under field conditions. These plants were grown in six-foot plots covered with cellophane and transpira­ tion was calculated by measuring the amount of moisture in the incoming (6) air and the outgoing air and comparing the two. They found that trans­ piration reached a maximum at 2:00 PM., which they attributed to rising temperatures and increased stomatal aperture. At night the transpiration curve was a straight line, due to low temperature and closed stomata. It was found that the transpiration curve followed the temperature curve, whereas photosynthesis followed the light curve. Lachenmeier (12) made a study of the effect of light on transpira­ tion after Veronica So., Hiercium and Myosotis palustris were kept in the darkness for one night. To maintain constant conditions the experiments were carried out in a cellar where temperature and humidity were prac­ tically constant. For a light source a 1 J-watt Osram lamp and 1 normal opaque lamp of 40 watts were used. To vaiy intensity the distance of these bulbs from the plants was varied. water to decrease infra-red rays. The bulbs were kept in running Only 1-2° C. difference was found in the air temperature at low and high light intensities. sity was measured by an Eder-Hect-Erayredge photometer. was measured with a potometer and by weighing. The light inten­ Transpiration Lachenmeier reports that during the day the transpiration of Veronica increased slowly from 3 to 5 hours to a maximum where it remained constant for 13 hours. Arcvanthium reached a maximum in one hour and then slowly decreased with fluctations. Light Intensity seemed to have veiy little effect either way. Shull (22), using a large number of species, measured the light reflected from both the upper and lower surfaces of leaves, since the lower surface was found always to reflect more light than the upper sur­ face. He measured the light with a spectro-photometer at an angle of 90° to the leaf surface. Shull found that the amount of reflection varied with the wave length, the maximum reflection being usually at 540-560 Mu. (?) The value of reflection in this region ran from 6 to 8 per cent in the darkest green leaves and in the lighter leaves £0 to £5 per cent, but neither haimess nor smoothness of cuticle necessarily mean high reflec­ tion* Leaves of Verbascum thapsus and Abutilon theophrasti show veiy little more reflection than non-haiiy plants. The amount of reflection was found to decrease with the age of the les.f which Shull associated with the development of chlorophyll. Anthocyanin development was accompanied by a shift in the position of the maximum reflection to longer wave lengths. In Psedera the maximum reflec­ tion occurred at 640 Mu*, while the normal reflection for green leaves is 540-560 Mu. In a large number of cases there is a depression of the reflection curve at 680 Mu. which corresponds to the maximum absorption band of chlorophyll* Pokrowski (18), using practically the same technique as Shull, found that in the blue region of the spectrum there was 4 to 5 percent reflection, in the green 8 to 17 per cent, and in the red 4 to 5 per cent* These percentages are averages from the leaves of Populus tremuloides, Tilia europeae. Fraxinus excelsior. Plums pubescens. and Acer platanoides. Knight (IQ) used Helianthus tuberosus and Upatorium adenophorum in studying the interrelations of stomatal apertures, leaf water content and transpiration. He concluded that under ordinaiy conditions changes of stomatal apertures and transpiration did not run parallel. The stomata were chiefly influenced by conditions of illumination rather than by small changes in the water content. Knight found that in a bright light the stomata are wide open and therefore changes in the transpiring power are brought about chiefly by changes in the leaf water content. (8) On the other hand under low light intensity the lower transpiration rate is a result of a failure of the stomata to open widely. Gray and Peirce (7), working on certain grains, measured the effects of light intensity on stomatal openings. They measured stomata at changing light intensities and found that at equal illuminations all stomatal openings were of equal size. They concluded that there is a minimum and maximum light intensity below and above which light has no effect, however, within this range the stomatal openings va.ry directly with the light intensity. This held true for all the grains studied. They believed that intense artificial light disturbs or interferes with the natural action of the guard cells. Sayre (19), who studied the stomata of Rumex patientia. concluded in contrast to Knight (10), that the opening and closing of the stomata are the principal causes of the periodicity in leaf transpiration. When open the stomata modify the rate of water loss from the intercellular spaces of the leaf in proportion to changes in the perimeter, not to changes in area. Sayre states that sunlight is the principal environ­ mental factor concerned in the opening and closing of the stomata while the amount of water in the leaves and acidity of the guard cells are the two internal conditions directly concerned with stomatal measurement. Sayre (19) in further work on Rumex patientia studied the effects of different wave-lengths of light on the stomata. He concluded that the stomata of patience dock do not open in wave lengths longer than 640 Mu., which is the limit of the red end of the spectrum. Eltinge (6), working on the effect of ultra violet light on higher plants found that when leaves of Phaesolus. Cucumis. Lactuca. (9) Coleus were subjected to ultra-violet rays, transpiration was not effected immediately. This was followed by a period of very little or no loss in weight, which was in turn followed by a period in which transpira­ tion equaled that of leaves in total darkness* When the stomata were examined those which had been subjected to ultra violet rays were com­ pletely closed, while those in total darkness were partially opened* Arthur and Stewart (l), studying the effects of visible rays and infra-red on transpiration, worked under constant conditions and used 1000 watt Mazda bulbs as a light source. They found that when the energy was increased 2.3 times, the transpiration rate was doubled at 73-78° F. This relation appeared to be independent of a humidi-ty range of 50 to 80 per cent. At high temperatures (98-100° F.), high humidity appeared to de­ crease transpiration slightly. At a temperature of 73-78° F. plants under a total spectrum transpired 2.5 times more than those under infra-red alone. At high temperatures (98-100° F.) however, the infra-red transpira­ tion rate was only 1.3 less than that of the total spectrum. Under infra­ red the stomata were completely closed. Conclusive proof of the effect of light intensity alone on trans­ piration is apparently lacking in the above literature. The general ten­ dency of these experiments has been to vary more than one environmental factor and correlate the change in transpiration with one of these factors which makes the exact determination of the effect of each factor very difficult. It is practically impossible to separate the effect of combined factors on transpiration from the effect of single factors. The object of this stuc^r was to determine the effect of a single factor, light intensity, on the transpiration of certain plants, while keeping all other factors constant* If a reduction in the intensity of (10) light causes a reduction of transpiration, shading agents could be com­ bined with certain spray materials. These shading agents would probably act as an aid to spray materials which are mechanical barriers to the loss of water vapor. By this means a greater reduction of transpiration could be obtained by applying a spray to transplants or plants having large water requirements. (11) Preliminary Experiments Preliminary experiments to determine the influence of light inten­ sity were begun in 1959. In some cases, entire plants were shaded by glass frames which had been sprayed with 6 per cent metallic aluminum, and 6 per cent carbon suspended in water. Light intensities, tempera­ ture and relative humidity were recorded along with the rate at which these plants transpired. In other experiments these aluminum and carbon sprays were applied directly to the plants. The spray coatings, as applied above, provided shade but likewise served to reflect or absorb infra-red waves and thus increase leaf temperatures as measured by a thermometer. Leaves which were sprayed with the carbon suspension were from 6-10° F. warmer than those sprayed with aluminum. This spray method, therefore, was discarded as temperature is one of the controlling factors in transpiration. A second preliminary method which followed closely the technique of Martin (17) was tried. In this method plants were shaded with differ­ ent thicknesses of cheese cloth to vary the light. Light intensity reduction was determined by use of a photo-electric cell and transpira­ tion measurements were recorded daily. Leaf temperatures were obtained by use of a thermocouple and air temperatures were recorded hourly. The data from an experiment using this method are presented in Table III. As may be seen from Tables I and II the leaf temperatures of the shaded plants were consistently lower than those of the unshaded plants. Table III shows that when the light intensity was lowered by shading with cheese cloth a reduction of transpiration took place. However, since the (12) leaf temperatures of the shaded plants were consistently lower than those of the unshaded plants this reduction of transpiration may have been due to a reduction of the temperature. Another error found in this method was that on bright days the shading effect of the cheese cloth was much greater than on cloudy days. The values given on the reduction of light intensity, Table III, are only averages of sunny and cloudy weather. Owing to these errors this method was also discarded as a means of meas­ uring the effect of light intensity on transpiration. (15) 8 pH O Fh CD .a p cd O U•) U• 5•O i 0i0U5u:ii)O WOW^WOWU5O • • • • • • • • • * • • • • • (MHWtOtDCOtOtOiOlStOWWWWWWH^ rH CD O d Q> Fh CD C O U ? r|U ? lO O C O lD sjtW U )U 5W H 'lt 'liN H O O W H i— I i— I i— I H iH i— I H i—1 si p o 1—( o to Fh CD £? rH CC2 1 • (—1 O d • -P •H ed O Cv2 rH to Cm P CD d cd CD CD rH Fh CD Pm bO *H CD Ph TS -P 'tS TS CD 3 rQ § p o rH O fH a> T3 0 Fh d •H 1 CD u 0 p cd d CM CD Hfc: Pm E P CD cd P rH Cm Fh •H O • o to CD CD Fh t(D CD Td OlOO HHO O )O O DCV!C005CO O O U5CV!0)!M • • • • • • • • • • * * * • ■ • • • • r f O t O l O O H C O N O l O i W rHt^-CD^^^CTSCO w w w w t o t o w w w w w w w w w w w w w 5 rH Hte P cd i—1 Cm CD Fh 0 P rH ed H t: Fh CD P CM cd E rH CD Cm P Mm Fh O •H <£j • o CQ (D CD Fm bD CD ts C•v i• O•H t OtOOHffl^O^^OH^OTO'ODO • • • • • • • • « « • • • • • • W W ' « t (tDO)OtDISO)0>C£>tOO)t~U5U5(CO)a) C\iCNiC'2Cv2C\2tOC\2C'aC\2C\2Cv2C\2C\2C\2Cv2C\2C\2C\202 d •h cd 6t * Er m S & jS SB S S S S3 > c? p p X 0 0 Xt a X CO X p o p o 0 CO 0 0 X o a 0 X 03 1=1 a p Ph S-t **H I—t (DO P * P O £ Q) cS 0 O C D bO Q )T OrH 1 0 -P t t if c n( 1 ) <1) CD O Tj CD PI 03 a Ph P a p a o i a 1 C\i p a p Ph §g§g§^B§ ra c oc o c o r o o r o c o 0 P 0 1PPOOOVOOPO 6 P a WCRWWHO^CD OiCV2C\ZCViC\iCViC\iCSi 0 «aj Ph *=tj «lj «=d <3h P*i Ph Ph CO CD B Cn M a •H 5 E t O O O O O O O O OO WtOWWWO «• «• ++ •« •• »• »* »• C D P C O C T S O i — I i— ( C O , S3 «h O O *H P p two q 53 Cd ft s o a xs 3 0) In t>> si pci p to CD 05 LO CM CO♦ 05 to jSJR p Wj si CD O P ft S3 C Table III. 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I) divided the box into two A 6-inch space was left at the top and bottom of this partition end covered with cheese cloth to permit the passage of air, allowing a minimum of light to pass from one side to the other. The top of the chamber was made of two glass-bottom waterbaths (F in Figure I) which were constructed so as to keep one inch of water in them at all times. At one end of this chamber a hole of 9 inches dia­ meter was made in which was inserted a metal tube. coils (C in Figure I) were placed in this tube. Two 600—wr&tt heater Behind these coils in the pipe was placed an 8-inch fan (B in Figure I) which circulated air over the coils into the chamber. This current of air hit a light-tight metal flange which caused the air to circulate evenly over the chamber at a velocity of 6 miles per hour without a direct current of air on the plants in the box. The 9-inch metal tube containing heater coils and fan was attached to a humidifier (A in Figure I)* This humidifier consisted of a square metal box 12" x 14” x 4" with a one-half inch drain pipe in the bottom. A hole 9-inches in diameter was cut in each end of the box and covered by a 1/4 inch mesh wire screen. The box was then filled with (17) excelsior and covered with a top containing 20- 1/4 inch holes. Water poured on this top, filtered evenly through the excelsior, and passed out the bottom drain. Air, sucked in by the fan through the excelsior, absorb­ ing moisture in proportion to the amount of water running through the ex­ celsior, was blown over the heating coils and circulated in the chamber passing out a tube in the opposite end of the chamber (K in Figure I). This'- air at the exit tube had a velocity of 6 miles per hour. The heater coils were attached to a relay (I in Figure i) which was attached to a thermostat in the chamber (H in Figure I) having an accuracy of —2° C. Eight 300-watt Mazda lamps were used for a light source. These bulbs were put in aluminum photoflood reflectors, as this type was found to give about 50 per cent more reflection than any other kind tried. These re­ flectors (A in Figure II) were arranged on movable stands (C in Figure H ) , four to a stand. All light which entered the chamber passed through the water baths which removed a large portion of the infra-red rays. These lights were cooled by three 10-inch electric fans (B in Figure II) which also aided in cooling the water baths. To vaiy the light intensity the stands holding the reflectors were moved either up or down into positions designated 1, 2, 3, 4, 5 or 6 of Table IV, thus varying the distance of the lights from the plants. All light measurements were made with a G.M. photo-electric cell and measured in foot candles. Each figure in Table IV is an average of five measurements taken at five different points on a plane at the stated distance from the light source. These intensities were checked at two week intervals, as the Mazda lamps were found to deteriorate after a short time. To obtain low light intensities a ground glass plate was put between the light source and the (18) CQ o ■ i Vi w t 1 of Apparatus Excepting Light Arrangement 1 1 Diagram < CD 2. d 0 •H Vi •H "O »H e d m w l 1 1 I 0 rH V O -P CD V -P -P rH rH o o -p td>JO 0 e X P SC o d -p -P -p E n op •h d -p d cd — CD d T3 O d -p a Cd 1 —1-P o o JO rH Oh rH ra •H 0 rH •H •H ,0 u o -P H cd X -p CQ 0 rH X e 0 tlfl 0 •H -P CQ -p cd *- v p 0 CO 2? -P 0 ab ,Q H ' D d f-t rH £ 0 02 .d aJ d cv •H 0 ■=H P h Eh Eh s O 5£ I ■ | I i 1 1 V i i 1 1 1 —1 tsd P 3 tc 1 Oh O E Fig. E O CD a. W U 0 d -P 0 X id 0 C*H 0 CO X E CQ -P o 02 o • d CQ to HH -P d -p £ M 0 X o 0 0 O -P O •_ i — to cd o w <4- o r-P ♦— 1 c+ <4- r-P v~> O o to Q. Od <4«a a H* O O cr £3 O c+ I— ' t» o 4-,. *"i* b-t q? H* C+. « ir> I— '■ <4* H- © as © c+CD q »4 t4 CLad &> h» SO a is M3 > t-3 s i i ■ ,1 T C-. txt 5 «* to 3 QJ <4cr uil c+ 03 o a (4 © a. Hi ,i C < N ffl CO H* O o © a. a ist 3 <-H <4- u- * M S» cu a © <+ SO o <4 —» 4-r £X o — • CQ a H”, m 3 w CL, ca 4 a X v—, © CO X © •4 <4- eo cr © «* <4- © a <4 ft? Q ta to ta 0 i 1 1 1 C 1 X a o * m3 « 4 © q © © © m3 •4 © CE O Hr* ■ Hi* h P (4 Hr* a* <4- Cr C5 H» O © a O 1— i c Q? P*3 ee 1 ) | C3 CQ O CQ PT^P£ V^»&g2»6jj£ O fH O DjsgLSiij 01 v&bsrx.s£rra m3 VO Of} Hr m3 h»3 EE O (19) water bath. This reduced the light intensity but had very little effect on the quality of light falling on the plants. With the above lighting arrangement light intensity could be varied from 350 F.C. to 1700 F.C. The method of measuring transpiration used in this chamber was actually a measure of absorption. Kramer (11) found that the rate of intake of water was determined by the amount of transpiration. Changes in trans­ piration, however, usually preceded changes in absorption by from 2 to 4 hours• Two steel flats (G in Figure I), 2 1 x 1*6” x 6”, were placed in the chamber. These were later replaced by four flats having the same total area and constructed similarly. The bottom of these flats had a 1" slope to the middle at which point was fastened a copper drain tube. The inside of the flats was coated with neutral asphalt to prevent action of the nutrients on the metal and filled with-washed gravel, 1/4” - 1/8" dia­ meter. Three-gallon bottles containing nutrient solution were connected to the drain tubes of these flats. The bottles were connected in such a manner that compressed air could be forced in all bottles from one source. Potted plants were washed free from soil and planted in gravel. This method was later changed as the transplants were veiy slow in starting growth. The later method was to wrap the plant roots and soil in burlap and plant the entire mass. Nutrient solution was then forced into the gravel flats by compressed air twice daily and allowed to drain back. That amount of solution which was absorbed by the plants from the gravel obviously failed to drain back and was a measure of transpiration. Glass wool was used to cover the top of the gravel to prevent excessive evapora­ tion. The glass wool was found to reduce evaporation from the exposed (2 0 ) E O m -P o M tC CQ CQ to cd i-t O •H X! -P s § o O -P cd CQ V CD -P cd {£: V» xt: xo *H XI Vi O Vo o tO * Ph -P S CO -P 3 O o § m -p • CU E o I O a' ft Pi 0 m P3 a (3i s •4 m S- i". o © g. c* H* m 1 <& *~3 «+. a ** EO £3 3 O ■©*+ m *4 © c+ &J a C+. a (D 3 © 0!EJ o rt *1 I > « H* 3 rjSpps CD & © CtfJ a H© t—i O £& iOAspje 2-fsiJg O o I O hf ht C+ a 3 CO Qj3 a © (t* » >4 o CD l-i M> CD PQ tr o o (•—» m 09 • CU3 H* ndj (-4 o O CD jjtj 1~* a H* a H* O 05 iBfc « I I CQ «E» CQ (2 1 ) Fig. 3* Constant Conditions Apparatus used to test the effect of light on transpiration (22) Table IV* Light Intensities as Measured in Foot Candles at Varying Distances from Light Source Plot 1 Position of Reflectors_________ 1_______2________3_______ 4_______ 5________6_______ Distance in inches from light source Intensity in foot candles as measured by photo-electric cell 30 24 20 16 12 8 4 850 * 990 1130 1160 1355 1595 1720 650* 855 910 990 1125 1330 1590 Plot 30 24 20 16 12 8 4 * 900 1085 1155 1160 1315 1640 1700 625 810 860 970 1085 1240 1495 495 * 660 750 860 1005 1215 1500 515 * 585 660 750 885 1075 1290 460 * 565 650 710 860 970 1190 282 509 334 377 425 595 680 545 630 695 775 895 1025 1265 480 565 660 720 810 950 1165 272 300 334 375 415 545 630 2 575 715 800 890 1005 1155 1385 Each of these values is an average of five measurements made at different points in the chamber. (25) gravel surface from 75 to 80 per cent* With the apparatus described above light could be closely controlled and fairly constant temperature and humidity could be maintained, as may be seen from Figure 4. To determine if increased light intensity caused increased leaf temperatures, measurements were made with thermocouples. One point of the couple was attached to a leaf under high intensity and the other point to a leaf under low intensity which gave only the difference in tem­ perature rather than the a.ctual temperatures of each leaf. These points were fastened to the epidermis of the leaves by "Scotch tape." This caused no injury to the leaves as does the older method of sticking the points under the epidermis. The deflections of the galvanometer were recorded and converted to degrees C difference. Table V shows that, regardless of light intensity, no consistent difference in temperature was recorded* In those cases where stomatal measurements were made, a Leitz UltraPak was used. The stomata were measured directly on the living leaves. This method was preferable to the alcohol strip method as the stomatal aperture may change when fixed in Chrom-Acetic or similar type of killing solution. The apparatus which has been described (pp. 15-25) was found to be very useful in studying plant reactions under constant conditions. From Tables IV and V and Figure 4 it may be seen that light intensity and temperature were controlled within very narrow limits. The relative humidity was controlled except during extreme weather changes. During long periods of rainy weather the humidity would rise as much as 20 per (2U) Table V, Leaf Temperature Differences as Determined by a Thermocouple Plants - Peaches Flat I Time 6:50 10:50 8:30 9:00 10:00 10:30 10:45 PM PM AM AM AM AM AM - 1600 F. C. Gonstant Conditions Chamber Flat II - 1000 F.C. Air Temp• I in degrees c. Air Temp. II in degrees c. Leaf temp. difference in degrees c. 30.5 30.5 31.0 31.0 31.4 31.4 31.4 30.7 30.6 51.2 31.2 51.5 31.5 31.5 0 0 #1 - .25° warmer 0 #2 — .50° warmer Plants - Catalpa Flat I 6:30 PM 7:30 PM 8:30 PM 9:30 PM 10:30 PM 11:30 PM 8:00 AM 9:00 AM 10:00 M 10:30 AM 11:00 AM - 31.0 31.0 30.0 30.0 51.0 31.0 30.5 30.7 30.7 31.0 31.0 - No lights Lights on ti ft If I! 0 No lights 0 - 1600 F.C. 31.6 31.5 30.2 30.2 30.8 30.8 30.7 30.6 30.6 31.2 31.5 Remarks Constant Conditions Chamber Flat II 0 0 .25° n #i — .25° #2 — .50° 0 0 — .25° ** .25° 0 0 - 350 F.C. warmer warmer warmer warmer warmer No lights it Lights on » ti ii it it n if No lights (25) Temperature 7 Humidity Days 4 5 1 70 80 90 Degrees Fahrenheit Figure 4. 50 60 70 80 90 Per cent Belative Humidity Chart taken from Hygrothermograph showing temperature and humidity in chamber for seven days. 3 & 5 S I Ofc 08 C? 03 03 00 08 QY evidisIsH &ttso TlsO d’isrfe'rrf.B^ 399*1390 baa 9*iirds'i9qni9d- gxiiworfs riq5*13omiericf0*1 3 mo'i‘1 naiad- d'iBrfO • s x fc b n 9 V 9 e *1 0 1 la d n r s f f o n i T c t if o if s u it .£ (26) cent. This amount, however, was not enough to disrupt transpiration studies. The light intensity attained on each side of the chamber was practically equal when the distance from the light source was equal. no case was the difference in Intensity over 100 foot candles. In Differences which were this great were at high intensities where the accuracy of the photo-electric instrument used in this study becomes a factor to be con­ sidered. At low intensities the difference in light between the two sides was negligible* No chlorosis appeared even when plants were grown under the Mazda lamps for 8 weeks. Spindly growth resulted, however, since this light source was very strong in the red end of the spectrum and weak in blue and violet. This weakness could probably be overcome if mercury vapor lamps, complete spectrum carbon arc lamps, or some other type having a more complete spectrum than the ordinary Mazda lamp had been used. The method of measuring transpiration used in this apparatus was found to be rapid and accurate when large numbers of plants were used. Better results were obtained when plants whose transpiration was over 300 c.c. per flat were tested. For those plants which lose very little water, such as conifers, a large number of plants per flat should be used. (27) Method The general method of procedure used to determine the effect of light on transpiration was as follows: The plants to be tested were planted in the metal flats and allowed to grow in the greenhouse until they had overcome the setback from transplanting. The metal flats were then moved into the constant conditions chamber and the leaves were counted. The leaf count was used in determining the total area of transpiring surface. Transpiration measurements were then made with the light Intensity on each side of the chamber equal. These measurements were continued for four or five days until the ratio between the transpira­ tion of the two sides was constant. Then the light intensity on one side of the chamber was changed and the transpiration measurements were made for three days. The light intensities were then changed eveiy three days and records were kept of the transpiration of each flat. Three control periods were made in each experiment by having the light in­ tensity equal throughout of the flats. the chamber and comparing the transpiration These periods were spaced so as to have one at the be­ ginning, one in the middle and one at the end of each experiment. One flat could not be kept under a constant light intensity during the entirety of the experiment and used as a check flat because large differ­ ences in the growth occurred unless the total light given each flat was approximately equal. In all cases humidity and temperature were kept constant during each experiment and the plants were given 15 hours of light and 9 hours of darkness. Leaf counts were made at three to four day intervals and at the end of the experiment leaf areas were obtained from these. In the case of the California Privet and Maiden Hair fern the areas (as) of 500 representative leaves were measured and the total area obtained by the leaf counts. Vitae. No effort was made to obtain leaf area in the Arbor In all other cases the total area was measured after the experi­ ment was finished and the areas during the experiment were calculated from the leaf counts. Stomatal measurements were made only on those plants having large and unobstructed stomata. Measurement of very small stomata or of those having obstructed perimeters was not 8-ttempted, as the accuracy of the Leitz Ultra—Pak method is questionable under these conditions and the alcohol strip method was not feasible in this case. The stomata- were measured by putting an entire leaf under the Ultra-Pak and measuring the stomatal aperture on both the upper and lower leaf surfaces. five measurements were made in each case. Twenty- At no time was more than ten minutes allowed to elapse between the time the leaves were taken from the chamber and the stomatal measurements were made. This was done to pre­ vent a change in the stomatal opening after removing the plant from the chamber. The chief trouble found in using this method for measuring stomatal apertures Is that the perimeter of the stomata is difficult to distinguish due to the structure of the guard cells and other surrounding tissue which obstruct the view. (29) Presentation of Data In the following tables, wherever feasible, transpiration measure­ ments are given in cubic centimeters of water lost per square inch of leaf surface. The water loss of the petioles and stems was considered too small to change the transpiration trends as indica/ted by the leaf blade areas. All light measurements are given in foot candles and calculated as percentage reduction of the lower light intensity from the higher light intensity. Transpiration reduction was calculated as percent, by compar­ ing the ratios of the transpiration of each flat when under different light intensities to that when they were at the same intensity* The average of these ratios was obtained by use of the geometric mean. Experiment I Forty Michigan State Forcing tomato (Lvcopersicon esculentum) plants about six inches high were planted in two metal flats after the soil had been washed from their roots. Daily transpiration records were kept for 12 days, by which time the leaf area had approximately doubled. A temperature of 80° F. and a relative humidity of 65 per cent was main­ tained throughout the experiment and the light intensities tested were 725, 1060 and 1085 foot candles. Transpiration data are presented in Table VI and shown graphically in Figure 5. The transpiration of tomato plants, data of Table VI, was found to decrease with a reduction of light. The wide variance in the transpiration reduction is probably due to the short length of this test and to differ­ ences in growth of the two lots. Experiment II In this test 50 bean (Phaseolus vulgaris) plants of the Fuby Dwarf Horticultural variety were employed. They were 12 days old and (30 ) ti TO O 0 d p p 0 TO p a cd d d d 0 0 d P •H P p C l, P P TO O O p d d d P cd nd np bl d TO TO P e-> d d rH d ♦ft d O P *P P O P d bD t 3 P TO P d p d TO 0 d Influence of Light Intensity Rate in the Tomato on Transpiration TO >» c? w P £ 1 P TO TO top ra !> d •• -p o P) •• • p TO Ph e TO E-) P d TO a P d TO d TO P. d 02 O to to to to p to cn a> c00 ccc 00 to CO 00 02 CO to 02 C- 00 p to to 00 to Cxi tr­ io to 02 C- co M o S SJ 0 P P cd d a TO d d p cd d 0 Cd -P CD T3 Cd d PH O O 00 P d TO P *HO*fH TrJi K O CD C2 (h bG Ph P TO J> •H P cd P TO « ud O M M P TO •H gj M Table VI* • CO PU CO CO t>* **-< d P cd -P o & § i a> Pt o •P cd f n CD *H cd b O Q, p Pd 03 as P P a l -P d a CD 3 > d PO o P P P cd TO d TO TO § TO P d cd P 0-4 o dI TO P-. 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As may be seen in Table VII the data of this investigation showed that transpiration of the bean plant varied directly with the light intensity striking the plant. As in Experiment I, the gradual rise of the transpira­ tion corresponded to the increase in leaf area due to the growth of the plants. Experiment III In Experiment III 50 may were tested. They were about two apple (Podopbyllus peltatum)plants weeks old, contained one andtwo leaves and had an average height of 14 inches. They were tested at a temperature of 70° F. and a relative humidity of 70 per cent, given light ranging from 354 foot candles to 1155 foot candles in intensity and the experiment lasted for 21 days. Stomatal measurements under different light Intensities were taken end the results are shown in Table XVI. At the end of the experiment, leaf areas were measured and the transpiration per square inch was obtained for the entire trial as the leaves of the may apple had grown very little during the test. The transpiration measurements are recorded in Table VIII and are shown graphically in Figure 7. Table VII and Figure 7 show that with may apple, just as with tomato and bean, transpiration when plotted against light intensity would give a straight line curve. the light intensity. The stomatal aperture as well as transpiration paralleled (33) p pj CD o Pi £3 . s * -p «J PI a Pi o o CD Pi 00 ♦H P -.P P O P Cvi to t- CO C\> c\> E Q O Table VII. Influence of Light Intensity on Transpiration in the Horticultural Bean Rate PA 9 € td bd U CD CD *H e-t p (h rH p O Pi CD Pi IP CO § • H p P o rbPjQT£3 •H CD P Pi CO & rcJ 00 rH •H Td •H Si CD i> -P cd rH CD p 05 CD P O CD o Pi 3 p cd Pi CD P. 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O o o *£» o Cl* Q> ■* HI ft o jEj^rf ■' O f-f o * hJ cu OJ Hi H .* O O * * CD *Q m CD h3 s 03 CM H» o h» UG3 O H* "►* 03 63 £3 CD 2 ■ —•3 £3 CD O % HI w t-4 cw £U h* S» Oi o ■ j3 a? H 09 Hr CM o o m rj£| IO *fcr CVS CM 8 Q> & S3 HI o cu Q> OJ CD O £ 3 CD o h3 o 09 Q> ,*a3 ac5 H* O O cw iO ii.SFuebji.fi.p^oii |>©x, aditsj.© jircp o CXI CM (37) Experiment IV In this test 50 Elberta peach (Prunus Persica) seedlings four months old, 16 inches high and with an average leaf area of about 1200 square inches per lot were used. They were tested at a temperature of 80° F., a relative humidity of 70 per cent and at ^.ight intensities ranging from 545 to 1720 foot candles. taken daily for 27 days. Transpiration measurements were Leaf* area measurements were taken at the end of this test and the area averaged about 2200 square inches per lot. Transpiration data are shown in Table IX and graphically in Figure 8 . Stomatal measurements were made under different light intensities and the data are shown in Table XVI. This test showed that with peaches, as with the other plants tested, transpiration reduction was directly proportional to the light reduction. Tables IX and XVI show that light intensity, transpiration, and stomatal aperture run parallel with each other. Experiment V. Twenty-four catalpa (Catalpa speciosa) seedlings two years old, 16 inches high and averaging 2100 square inches leaf area per lot were tested. A temperature of 80° F. and a relative humidity of 85 per cent were maintained and the light intensity was varied from 545 to 1700 foot candles. This experiment lasted for 15 days by which time the leaves had increased in area to an average of 2500 square inches per lot* Table X and Figure 9 show the transpiration data obtained during this study. Transpiration in catalpa as in other plants tested, was directly proportional to the light intensity. 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IT CO 0 0 c+ a i— t o u> oo s co Hi CO GO It H so § 03 CO SO Hi o Hi O S 0 H a w 0 a H to tA 0 kQ a 0) t—3 hj-3 H H* a o It H to 0 H» £> m a to ►4 J-3 o 03 O ♦ CJ O a? cc o O Hi • a H O Q) Hi o * *?• a M • to t-1 o h3 o CM SO HI O Q> a? 03 It HI O CU O o o OJ SO j,i.cijabji.cj?Toir bei. sd/isxe j»cpr (53) shown on Table XVI. This test shows that with Arbor Vitae transpiration varies directly with the light intensity over a range of 415 to 1555 foot candles. As may be seen from Tables XIV and XVI, in Arbor Vitae, transpiration, light intensity and stomatal opening were directly proportional to each other. Experiment X In this test, as in Experiment VI, 48 Maiden Hair fern (Adiantum Cauillus-Veneris) plants, averaging 8 inches in height and with an average leaf area of 250 square inches per lot, were used. They were tested at a temperature of 90° F. and a relative humidity of 85 per cent and the light intensity ranged from 500 to 1150 foot candles. This experiment lasted for 27 days by which time the plants had attained an average leaf area of 550 square inches per lot. The transpiration data of this study are shown in Table XV and graphically in Figure 14. Sto- matal apertures were measured under different light intensities and areshown on Table XVI. The data in Table XV correspond to those in Table XI in that they show that transpiration does not run parallel to the light intensity within the range of 500 to 1130 foot candles. As in Experiment VI the maximum transpiration was reached between 600 and 800 foot candles at which point the curve tended to become level. The stomatal openings were also at their maximum at 554 foot candles, becoming smaller at higher in­ tensities. This test shows, as does Experiment VI, that the maximum trans­ piration for Maiden Hair fern took place between a range of 400 and 800 foot candles light intensity. (5*0 PJ I I O TO d O •H d O £ c^, •hd d P *H <0 O 0 O H P P 0 P c P J P «i P P rC Td . 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Stomatal Measurements Under Different Light Intensities Light intensity in foot candles Width of stomatal aper­ ture in microns Light intensity in foot candles Width of stomatal aper­ ture in microns Arbor Vitae May Apple 1155 15.6 1555 12.7 650 12*8 1125 10.7 554 7*9 1005 6.7 860 5.5 810 5.8 415 5.6 Peaches 1720 10.5 1700 11.5 1595 9.8 1500 7.2 1085 7.2 1585 7.9 715 6.0 1160 8.6 565 8.9 1155 6.5 405 7.9 710 6.0 500 9.6 595 4.1 0 1.2 545 4.8 Maiden Hair Fern (6o) Fig. 15. May Apple in Chamber Leaves of left side wilted due to excessive transpira­ tion of .306 cubic centimeters per square inch under light intensity of 1155 foot candles for 15 hours. Right side transpired .144 cubic centimeters under 334 foot candles of light. ( 61) Pig* 16. LIicroph.otogra.ph of Peach Stomata showing open stomata 011 left under 1 7 0 0 foot candles while closed stomata on right are under ^>bO foot candles* (62) p i c-, ]_y. LIicroph.otogra.pli of Mr id an Hair fern. Stomata at left under 1C 85 foot candles. Stomata at right •under J>00 foot candles. Very little difference in anerture• (63) Discussion Some investigators ( 5,13,16) have concluded that the increase in transpiration caused by light is due to the infra-red rays which increase the temperature of the leaves. Others (8) have found that light, apart from raising the temperature of the leaves, causes a higher rate of trans­ piration. Some (12) have found that light intensity had no effect on transpiration. The writer found, however, that when the infra-red rays were eliminated from the spectrum by use of water screens or filters, as was done in the apparatus previously described, increased light inten­ sity still caused a transpirational increase. The effectiveness of this bath may be seen in Tables I, II and V which show, that, regardless of light intensities, the leaf temperatures never varied more than 1° F. When the plants were subjected to high intensities the increased trans­ piration may have been a factor in keeping the leaf temperatures constant. However, in no place in the literature was there found a case where trans­ piration cooled leaves more than 5° F. When no water screens were used and infra-red rays were allowed to strike the leaves, as was done in certain tests the data for which are given in Table I and II, the tempera­ ture difference was as much as 8° F. So with use of water baths the leaves should have been at least 3° F. higher* This, however, did not occur in any test in which leaf temperatures were measured while the water screens were used. From Tables VI - XV it may be seen that a reduction of transpiration in the tomato, bean, peach, catalpa, may apple, privet and Arbor Vitae was caused by decreasing the light intensity. entire light range of 300 to 1700 foot candles. This held true over the Tfrhen the Maiden Hair fern was tested, however, the highest transpiration was found at an intensity (6h) of from 500 to 800 foot candles. Tables X and XV show that the results were very irregular, possibly due to the inaccuracy of measuring the leaf area of small fern leaves. From 800 foot candles to 1200 foot can­ dles, the transpiration curve tended to decrease. Since all plants have an optimum range of light for growth and other functions, these data suggest that the optimum range for transpiration in the Maiden Hair fern was from 500 to 800 foot candles. It is probable that if the light -rrA intensity were increased to 5000 foot candles or even more (which is the Intensity of the sunlight in Michigan in July) the transpiration curve of the other plants tested would tend to decrease or straighten out at the higher intensities as did the Maiden Hair fern above 900 foot candles. The reason for higher transpiration with increased light intensity was found to be due to or at least associated with increased stomatal aperture. The stomatal aperture, as measured by the Leitz-Ultra-Pak, Table XVI, was found to vaiy directly with the light intensity in all plants tested except the Maiden Hair fern. ported by Gray and Peirce (7). This follows the results re­ Sayre (19,20) found that the periodicity of transpiration was caused by the opening and closing of the stomata with light and darkness respectively. This work substantiates the results obtained by Sayre, although Knight (10) concluded that stomatal openings and transpiration do not run parallel. The stomata under the light intensities used in this test were completely open only at the high­ est intensities. Since Brown and Escombe (4) proved that diffusion thru stomata is proportional to the linear dimension of the aperture and not the area, these data imply that transpiration is something more than a physical process such as evaporation or diffusion. In the case of (65) Maiden Hair fern, the stomatal opening did not run parallel to trans­ piration as did the other plants tested. The largest stomatal apertures in the Maiden Hair fern, however, were found at 400 foot candles inten­ sity while the highest transpiration occurred at between 600 and 800. These differences are possibly due either to inaccuracy in stomatal measurements or to inaccuracy in obtaining leaf areas. From the data here reported, transpiration is reduced by a reduc­ tion of the light intensity. In any general effort to reduce the trans­ piration of plants under cultivation, due consideration should be given to the possibilities that lie in shading them. Light reduction would be especially beneficial to transplants, before the roots have become established, and during dry seasons to those plants which have large water requirements. (6 6 ) Summary I. A plan of study was formulated in an effort to determine the effect of a single factor, light intensity on the transpiration of certain plants. II. Preliminary transpiration experiments using carbon and aluminum sprays and cheese cloth shading to vary the light intensity are described. Ill* An apparatus to measure transpiration under controlled humidity, temperature and light intensity is described with the aid of two diagrams. IV. Tables are given to show the accuracy of this apparatus. Experiments using tomatoes, beans, May apple, peaches, catalpa, California privet, Arbor Vitae and Maiden Hair fern are described, with tables and graphs to show the effect of differences in in­ tensity of light on transpiration. V. Stomatal measurements on leaves under different light intensities were made. The transpiration in all plants studied was found to vary directly with the stomatal aperture. In all cases except the Maiden Hair fern the stomatal aperture was found to vaiy directly with the light intensity. VI. The transpiration of all the plants studied except Maiden Hair fern varied directly with the light intensity of Mazda lamps over the range of 500 to 1700 foot candles. The Maiden Hair fern showed maximum transpiration from 600 to 800 foot candles and reduced transpiration occurred at higher light intensities. (67) Literature Cited 1* Arthur, John M. and Stewart, W. D. Transpiration of Tobacco Plants in Relation to the Radiant Energy in the Visible and Infra-Red* Contr. Boyce Thomp. Inst. 2. 5:483-501* Bakke, A. L* and Livingston, B. E. Transpiring Power in Plants* 3. Blaydes, Glenn W. Further Studies on Foliar Physiol. Research. 2:51-71. 1916. A Survey of Rates of Water Loss from Leaves. Jour, of Sci. 4. 1933. 28:99-118. Brown, H. T. and Escombe, F. Ohio 1928. Static Diffusion of Gases and Liquids in Relation to the Assimilation of Carbon and Translocation in Plants. 5. Phil. Trans. Roy. Soc., B, Darwin, Francis. Eltinge, Ethel Taber. Higher Plants. 7. 1900. The Effect of Light on the Transpiration of Leaves. Ray. Soc. Lond. Proc., Series B. 6. 193:223-291. 87:281-299. 1914. The Effect of Ultra-Violet Radiation upon Ann. Mo. Bot. Gar. Gray, John and Peirce, George J* 15:169-240. 1928. The Influence of Light Upon the Action of Stomata and Its Relation to the Transpiration of Certain Grains. Amer. Jour. Bot. 6:131-156. 8. Henderson, F. Y. 1919. On the Effect of Light and Other Conditions Upon Rate of Water-Loss from the Mesophyll. Ann. Bot. 40:507-535. 1926. 9. Kiesselbach, T. A. Transpiration as a Factor in Crop Production. Univ. Heb. Research Bui. #6. 10. Knight, R. C. 1915. The Interrelations of Stomatal Aperture, Leaf Water Content, and Transpiration Rate. Ann. Bot. 31:221-240. 1917. (6S) . 11 Kramer, Paul J. Jour. Bot. 12 . Transpiration and Absorption of Water. 24:10-15. Lachenmeier, J. Amer. 1957. Transpiration und Wasser-absorption intakter pflaniff zen nach vora.usgegangener Verdunkelung bei Konstanz der Lichtintensitat und der Ubringen Aussenfaktoren. 765-827. 15. 1932. LeGlerc du Sablon, M. Sur les Causes du Degagement et de la Reten­ tion de Vapeur D feau par les Plants. 83, 104-122. 14. 15. 16. _________________ 1909. Light Intensity and Transpiration. ________________ Studies Upon the Influence of Solar Radiation Inst. Wash. Year book. Martin, Emmet V., Pokrowski, G. I. Baume. 19. Sayre, Jasper D. Forrest. 10:541-354. 1935. 165:420-426. 1925. Physiology of the Stomata of Rumex patientia.. 26:255-267. 1926. Opening of Stomata in Different Ranges of Wave Lengths of Light, . Shreve, Plant Pbys. Biochem. Zeitschr. 20. ________________ 1923. Uber die Li chtab sorption von Blaltem einiger Ohio Jour. Sci. 21 22:288-289. Carnegie Effect of Solar Radiation on Transpiration of Helianthus Annuus. 18. Bot. Gaz. 1911. on the Rate of Transpirational Water-loss in Plants. 17. 25:49- Stomata and Transpiration in Tradescantia zebrina. 29:269-270. 52: 417-438. Rev. Gen. de Bot. 1913. Livingston, B. E. Science. Jahrb. wiss Bot. 76 Plant Pbys* 4:323-328. 1929. The Transpiring Power of Plants as Influenced by Differences of Altitude and Habitat. Science N. S. 45:363. 1916. (6g) 22. Shull, Charles A. A Spectrophotometric Study of Reflection of Light from Leaf Surfaces. 23. Thomas, M. D. and Hill, G. R. Bot. Gaz. 87:583-607. 1929. The Continuous Measurement of Photo­ synthesis, Respiration and Transpiration of Alfalfa and Wheat Growing under Field Conditions* 24. Wagner, Arnold. Plant Phys. 12*285-307. Gravel and Cinder Culture for Greenhouse and Flowering Crops. correspondence)• Ohio State University. 1939. (personal 1937. (70) Acknowledgments This research was financed by funds from the Horace H*. Rackham Research Endowment of the Michigan State College of Agriculture and Applied Science. The writer wishes to express his appreciation to Director V. R. Gardner for his helpful advice and guidance throughout the course of this research* The writer is also indebted to Dr. H. G. Petering for assistance in the designing of the equipment used*