W H W l V H W H EFFECT 6F HUMEDEW Q‘N @wwn-z cu“: SELECTEQ QRNAMENTAL gmms “west: {0? the Degree of M. S. MECHEGAN STATE UNIVERSETY H Arthur Whang 1957 ”new! LIBRARY Michigan State University EFFEC OF HUMIDITY 0N GRUfl‘H OF SELECTED ORNAMENTAL PIANTS 3y H . Arthur Nhang A THE IS Submitted to the College of Agriculture of Michigan State University of Agriculture and Applied Sciences in partial fulfillment of the requirements for the degree of 25115 TIER OF S CIENCE Department of Horticulture 1957 / , 7’52?“ _/ eagfi DEDICATION To nw parents ACKI‘EOE‘ILEDGI‘ESN T8 The author wishes to express his gratitude and appreciation for the guidance and understanding of Dr. D. P. Watson; to Dr. F. B. Nidmoyer for help in the anatomical descriptions; and to James R. Lonsway for assistance in preparation of copy. TABLE OF INTRODUCTION . . . . . . . . . . . SUMMARY OF PERTINENT LITiRaTth Macroscopic Effects . . . . . Lnatomical Effects . . . . . . Physiological Effects . . . . Nutritionrl Effects . . . . . Other Effects . . . . . . . . PROCEDURE . . .7. . . . . . . . . DIb‘CUb‘LJILN CF ltb'UL‘l‘is . . . . . . . Humidity . . . . . . . . . . . Watering . . . . . . . . . . . Growth . . . . . . . . . . . Anatomy . . . . . . . . . . . Bil’r‘LICI-JI‘ION CF RubULI‘S . . . . . . Anatomical Lgreement . . . . . Anatomical Difference . . . . Transpiraticn . . . . . . . . D UPMLIiY O O C O O O 0 O O O O O 0 LITERATURE CITED . . . . . . . . . CONT ‘r—JNTS PAGE [‘0 \JJ 0‘- 3' 10 12 15 15 15 19 31 ho 1:2 TABLE II III LIST OF TABLES i-lean Terrierature and Humidity . . . . . . . . Evergreen Survival . . . . . . . . . . . evergreen (Group I) Fresh ;~Iei;_.,ht; Dr;,r Height; Needle Leaf Thickness . . . . . . . . Foliage Plant (Group II) Fresh Weight; Dry Height; Plant Height; Leaf Area; Leaf Thickness PI; GE 16 20 22 30 LIST OF FIGURES FIGURE. PAGE 1 GrOWth Chambel‘s O O Q C C O O O O . O O C O 0 . 13 2 Group I Water Requirements . . . . . . . . . . l? 3 Group II Water Requirements . . . . . . . . . . 18 h-12 Growth Comparisons in Low and High Humidity Paxus cuspidata cagitata . . . . . . . . . 23 Taxus cuspidata findersoni . . . . . . . . 2h Taxus media Hicksi . . . . . . . . . . . . 2S Juniperus chinensis . . . . . . . . . . . 2S Pianus nigra 0 O O O 0 O O O O O O O 0 C O 26 Scindapsus aureus. . . . . . . . . . . . . 28 Cissus rhombifolia . . . . . . . . . . . . 28 ngeromia obtusifolia variegata. . . . . . 29 Dracena godsefianna . . . . . . . . . . . 29 13-A,B Transaction of Taxus cuspidata capitata leaf. . 33 13-C,D Transection of Cissus rhombifolia leaf . . . . 33 INTRODUCTION An office or lobby without foliage silhouettes to soften sharp architectural lines and angles is a rare sight. The slow growth and fre- quent death of many of these plants during the winter months in centrally heated homes may be caused by the dry atmosphere in which they are grow- ing. It is natural to suppose that plants which are trOpical in origin would grow more luxuriantly in a saturated atmosphere. This limitation of growth assumes greater significance with the ever eXpanding pOpular- ity of decorative plants. With this in mind, an investigation was undertaken of the effects of high and low humidity on certain selected ornamental plants. Small evergreens, native to temperate regions, have rarely been used as house plants. Their acceptance by the consuming public would be greatly enhanced if it could be shown that they could withstand an atmos- phere of low humidity. Naturally, mmch depends upon the ultimate appear- ance and type of growth made, but an Opportunity to provide another outlet for nursery products prompted the inclusion of these plants in the present experiment. SURVEY OF PERTINENT LITERATURE Macrosc0pic Effects Nightingale and Mitchell (l93h) observed that nitrogen deficient tomato plants eXposed for nine days to a relative humidity of 953 elon- gated three to four cms. more than plants grown in 35$ relative humidity. Plants grown in high humidity were also darker green with succulent ex- panded apical leaves whereas no change in appearance was observed in those grown in low humidity. Plants given adequate nutrition responded in like manner; those in high humidity exhibiting a 12 to 15 cm. in- crease in stem length compared to five to seven cms. in low humidity. They were darker and more evenly green as well as being extremely succulent. Plants in low humidity were woody, stiff, and mottled green in appearance. They found the same relationship for two year apple seedlings. In fact, the high humidity was associated with a greater number of broad but thin, dark green leaves. Again, they reported for the apple a cessation of all terminal growth of current stems in low humidity. Newcombe and Bowerman (1918) reported similar findings in that seedlings grew taller and produced larger, more numerous leaves in a small stagnant bell jar than in a larger chamber with circulating air. In 1917, Hanson mentioned that possibly humid air caused leaf size in- creases, greater root deve10pment, and that leaves grew at right angles to the stem in moist air. Anatomical Effects MacroscOpic differences have been correlated with definite anatomi- cal changes (Hanson, 1917; Nightingale and Mitchell, l93h). Hanson referred to observations that increases in cuticle thickness, stomata number, amount of sclerenchyma, woody tissue, and palisade cells accom- panied growth in dry air. He believed that reductions in storage tissue, fibrovascular bundles, and intercellular Spaces as well as in- creases in amount of chlorOphyll and number of stomata per leaf were characteristic of plants in highly saturated atmospheres. Furthermore, these chlorOphyll cells were more isodiametric along with pronounced epidermal cell wall waviness. Nightingale and Mitchell (l93h) also made note of the unusually high concentration of chlorophyll present in the greater number of chloroplasts found in leaves of plants exposed to high humidity. Thinness of cuticle and cell walls in addition to loosely compacted palisade and epongy mesophyll with large intercellular Spaces were also observed. On the other hand, plants from low humidity ex- hibited comparatively thick cuticle, relatively thick xylem, and compact small celled palisade and epongy meSOphyll with greatly reduced inter- cellular Spaces. These differences might perhaps be best accounted for by a quo- tation from Pool (1923) who asserted that "of all the histological features of the leaf, the chlorenchyma is probably the most plastic, or most readily modified by environmental variations, so that some of the commonest and most striking differences between meSOphytic and xerOphytic leaves are to be found in the relative deve10pment of palisade and sponge and in the relative prOportion of lacuner volume in the chlorenchyma as a whole." Pool then proceeded to a comparison of meSOphytic and xerOphytic leaf anatomy which was in many reSpects similar to the above mentioned differences between plants grown in high and low humidity. Yet, xerOphytic plants growing in arid habitats were more prone to foliar water deficits due to low soil moisture and ab- sorption as well as excess tranSpiration (Shields, 1950). The conse- quent water shortage resulted in limited stretching growth, cell surface, epidermal cell enlargement, and laterally expanding Spongy meSOphyll. This was reflected in thicker cell walls, increased number of stomata per unit area, more prominent development of palisade, and denser network of veins. These reduced tissue pr0portions and struc- tural modifications were the result of internal water deficits and were influenced solely by rate of tranSpiration and water supply as Opposed to absolute water loss (Shields, 1950). Physiological Effects TranSpiration rate differencials were also of great significance in the growth of adequately watered plants in low and high humidity. According to Nightingale and Mitchell (19314), the contrasting growth reSponses due to humidity differences were the results of greatly accelerated transPiration accompanied by internal changes. The effect that foliar anatomy exerted on rate of tranSpiration was evident from the work of Turrell (1936) who found a close relationship between tranSpiration losses and amount of internal leaf surface area. The high tranSpiration rate of xerOphytic leaves was thus explained by their more compact structure with numerous small air Spaces and usually more palisade layers. The extensive internal evaporative surface was accounted for primarily by the greater development of palisade type of mesophyll which exposed 1.6 to 3.5 times as much surface, per unit volume, as the spongy type (Turrell, 1936). However, Pool (1923) attributed low xerOphytic tranSpiration rate to the evaporative obstacle from the interior of the leaf presented by the compact chlorenchyma. Yet, there was no doubt that a lowering of relative humidity was accom- panied by increased tranSpiration (Kisselbach, 1916). But there was a limit to this correlation as reported by Bialoglowski (1935). He found that transpiration and humidity presented a linear relationship pro- vided they were within a certain range. Below 60% relative humidity or 68°F., pronounced retardation in rate of water loss was observed. This was supported by the earlier work of Henderson (1926) who maintained that the effect of changes in humidity on tranSpiration was not ascer- tainable below 60% relative humidity because changes occurred near the wilting point and that these changes affected the rate of evaporation from cell walls. Thut (1938), on the other hand, provided data to prove that transpiration losses presented an inverse linear function over the entire relative humidity range. He inferred that as relative humidity rose, transpiration declined to a point where plant leaves in high humidity were actually absorbing water from the surrounding air instead of losing it. In fact, Breazeale, McGeorge, and Breazeale (1950) found that enough water was absorbed by the leaves of tomato plants in saturated air and tranSported to the roots to raise the soil moisture above field capacity. The authors thereby concluded that pressure developed by absorbing roots was not as great as that main- tained in foliar absorption. Thut's (1938) explanation depended upon the validity of the absorption lag in roots as recorded by Kramer (1938). This difficulty in obtaining and tranSporting adequate water for trans- piration created a deficit which was transmitted to the leaves accounting for absorption in humid air. The resulting increased water content of the leaf in high humidity was of no little significance. The water balance of the leaf as a whole was reflected in the turgor changes assumed to be associated with guard cell moVements and the consequent size of stomatal apertures (Wilson, 19h8). An increase in the moisture content of the leaf would therefore bring about wider Opening of the stomatal apertures. Yet, the effect Of low relative humidity on stomatal aperture size was slight except at high temperatures when the stomata became more nearly closed (Wilson, l9h8). This result was observed by Nightingale and Mitchell (l93h) who reported that the stomata of tomato and apple plants grown at 70°F. in high humidity (95%) were much more open than those of plants grown in low humidity. But according to Muenscher (1915) and Knight (1917), there was no observable relationship between tranSpiration rate and number or size Of stomata per unit of leaf surface. Size of stomatal Opening also had no apparent effect on the carbon fixation of leaves (Mitchell, 1936). Knight even main- tained that small changes in leaf water content did not influence the Opening or closing of stomatal apertures. It was concluded that the water content Of the leaf was more significant than the degree of stomatal Opening (Livingston and Brown, 1912; Henderson, 1926). High water content thereby tended to produce a high tranSpiration rate and low tranSpiration accompanied low water content (Knight, 1917). Granting that leaf water content did exert a significant effect on tranSpiration, there was no assurance that leaves in high humidity actually absorbed water from the surrounding air under all conditions. Briggs and Shantz (1915) and Henderson (1926) indicated that leaves in a completely saturated atmOSphere still tranSpired. Henderson (1926) found that leaves in 100% relative humidity were Of a higher temperature than that Of the surrounding air. This would indicate that a hypotheti- cal relative humidity of above 100? would be needed to completely stop tranSpiration. However, leaf temperature was determined by the evapora- tive power of the air at relative humidities less than 100% (Henderson, 1926). Thus, the leaf was often cooler than surrounding air in diffuse light but with the complete spectrum supplied by sunlight to counter- balance evaporative reapiratory heat loss, leaf temperatures would undoubtedly be higher than the surrounding air temperature. This would account for Curtis' (1936) conclusion that small differences of a few degrees between leaf and air temperature may exert a great influence on transpiration. Clum (1926) reported no definite relationship between tranSpiration rate and differences between leaf and air temperature. It was felt by Yarwood and Hazen (19hh) that this might be due to high and variable radiation on the test surfaces. This would be similar to natural light conditions during the day. Ehlers (1915) did find that even diffuse light will result in from 0.500. to 2.000. higher leaf temperatures. The same effect was fOund in winter when he reported that evergreen conifer leaves maintained temperatures from 200. to 10°C. higher than the surrounding air. This differential was increased to a 10.3100. difference in still air and decreased to 8.8300. with a slight breeze. MOreland (1937) found, moreover, that the effect Of relative humidity on leaf temperature was not as evident as that exerted by air movement. His experiments showed that sugar cane leaves in sunlight were 5°C. to 7°C. warmer than surrounding still air. The difference was slight, however, (1°C. to 2°C.) in strong breezes. Conversely, the leaf was cooler than the air in shade. These small differences were not changed to any great extent by humidity differentials except that greater differences between leaf and air temperature in sunlight were noticed at higher relative humidities. This same effect in diffused light was reported by Henderson (1926). He found that raising the humidity from 70% to 80% resulted in a 0.500. rise in leaf tempera- ture. He further stated that an ivy leaf reached atmoSpheric temperature at around 95% relative humidity with gentle air movement, diffuse light, and 68°F. air temperature. Nutritional Effects The contrasting growth reaponscs due to transpiration differences caused by low and high humidity were undoubtedly accompanied by internal changes (Nightingale and Mitchell, 1931:). Their findings indicated high dry matter content for plants grown in low humidity as evidenced anatomically by thick xylem and chemically by heavy starch and other carbohydrate depo€¥ions. This increase in dry matter could be caused ‘by'rapid loss Of water as suggested by Nightingale and Mitchell. It ‘was believed that carbohydrate increase and moisture loss are generally associated with the condensation of simple amino acids to protein. This would explain the comparatively high concentration of "total elaborated nitrogen in the form of complex, relatively immobile, dehydrated protein" found by Nightingale and Mitchell in the tissues of slow growing, non- succulent plants grown in low humidity. In fact, the condensation of soluble organic nitrogen to relatively complex protein fractions accounted for the develOping terminal buds Of apple seedlings grown in low humidity, according to Nightingale and Mitchell. CIrbohydrate accumulation in low humidity was also substantiated by the further work of Mitchell (1936) who maintained that photosynthesis was not affected to any great extent by low humidity. He based this assertion on the sustained rate of carbon fixation by squash, wax bean, cabbage, geranium, primula, and tomato leaves even in low humidity Of 15 to 20 hours duration. MOreover, he cited other workers whose findings he had found to support his own (Kisselbach, 1916; Nightingale, 1933). The influence of an external nitrOgen supply on half of the plants evidently exerted a stronger beneficial effect on growth than high humidity in that fertilized plants in low humidity grew two to three cms. more than unfertilized plants in high humidity (Nightingale and Mitchell, l93h). The plants which received the complete nitrogen feeding and subsequently grew more exhibited a depletion in sugar and starch because of the synthesis of less complex, water soluble, organic proteins made necessary and possible by fertilization. This was evidently followed by proteolysis and increase in growth since hydrolysis of proteins generally comes after decreases in dry weight. Nightingale and Mitchell (193h) cited: Nightingale and Robbins, 1928; 10 Nightingale and Schermerhorn, 1928; Nightingale, Schermerhorn, and Robbins, 1928, 1930; Pearsall and hwing, 1929, and Nightingale, 1933, as evidence of complete agreement with this hypothesis. Other Effects It has been indicated that the conversion of soluble organic ni- trogen to relatively complex protein fractions with concomitant build— up of carbohydrates aOCOunted for the developing terminal buds of apple seedlings grown in low humidity. Indeed, the presence of abundant car- bohydrates has been shown to be a factor resulting in plant reproduction (Kraus and Kraybill, 1918; Nightingale, Schermerhorn, and Robbins, 1928, 1930). It has been mentioned by Nightingale and Mitchell that a hastening of flowering and fruiting is attributed to low humidity., High humidity produced by mechanical misting systems has been utilized with great success in the prOpagation of difficult to root cuttings (Fisher, 19hl; Gardner, 19h1; Stoutemeyer, 19h2), and the production of better quality Better Times roses (Kohl, 1955). Although callusing of apple cuttings was inhibited by tissue desiccation as humidity was decreased, only slight callusing resulted from 100% rela-l tive humidity unless damp peat or Sphagnum was used as a storage medium (Shippy, 1930). Fully saturated atmOSpheres were also important in the storage Of various horticultural creps such as narcissus (Hume, 1937), apricots, plums, peaches, apples, grapes (illen and Pentzer, 1935), and potatoes (loomis, 1927), the latter in combination with high temperatures and damp moss. High humidity was further recommended for holding cut flowers, especially carnations, which lasted from two to three times as 11 long in relztive h midities over 80% (Hitchcock and Zimmerman, 1929). On the other hend, papaya fruits were severely injured in relative humidities greater than 60%, but were not visibly affected by lower humidities (Jones, 1939). Pollen (Pinus strobus and E. resinosa) longevity has also been found to be influenced by SOj relative humidity, practicrlly nc germina- tion being observed in OJ to 10% relztive humidity (Duffield and Snow, 19h1). Humidity had at best cnly a minor effect on the critical daylength of Xanthium and other plants (Long, 193h). 12 PROCEDURE seven or more stem cuttings of Group I: Taxus cuspidata capitata, media Hicksi, Juniperus chiaensis and U) Taxus cuspidata Anderscni, Taxu Pinus nigra seedlings were included in the experiment; as well, mature plants of Group II: Scindapsus aureus, Cissus rhombifolia, Peperomia obtusifolia variegata and Dracena godsefianna were selected for observa- tion. Rooted stem cuttings of the conifers(3roup I) were potted in four inch pots on January 18, 1957. They were pruned uniformly to with- in five inches of the soil level and remained in the greenhouse for three weeks when they were inserted into the experimental chambers (Figure l). Twenty of each Specie3 of broad leaf plants (Group II) growing in two and one-half inch pvtS were placed in the chambers on January 5, 1957. The experimental chambers (Figure 1) consisted of a wooden frame supporting three shelves. The whole structure was sprayed white. Each of the four chambers had a capacity for about forty plants. Two of the chambers were enclosed with tranSparent "Saran-drap" to conserve humidity. The other two were left uncovered. The plants were numbered and placed on the shelves so that each Species occupied the same rela- tive position in each of the treatment chambers. The chambers were located on a table in a laboratory facing an east window. The tempera- ture of the surrounding air was not altered in any manner. There was slight reduction in the amount of light received because of the con- densasion of moisture in the closed ch mbers. Constant temperature and l3 GiiOa‘Jl‘H CELL-1‘3 g) .3 (“Sc"‘LE‘ 1" " “i’E-‘l‘il'iislately 8") I4“ ’/ 22 High Humidity - covered. with tranSparent "Saran-Urap" Low Humidity - left open humidity records were made with two "Hygro-Thermogra”hs”l, one for each treatment, and since there were four chambers, the same instrument was rotated weekly between the two chambers of each treatment. The amount of water supplied to every Species was recorded as were length of new erminals produced by the conifers (Group I) and the production of new leaves by foliage plants (Group II). At the termination of the experiment on June 15, 1957, fresh weight, dry weight, height of plants, leaf area, and leaf thickness were re- corded and comparisons were made for the plants in each treatment. Representative samples for anatomical investigation were collected on May 1 and again on May 15. Mid portions of newest needle leaves over 2 mm. long comprised samples from Group I plants. Group II samples consisted of 1 mm. squares from the edges of the newest mature leaves near the apex. Samples were killed and fixed in FAA (5 n1., formaldehyde; 5 ml., glacial acetic acid; 90 ml., 703 ethyl alcohol), dehydrated, embedded in paraffin and transverse sections were cut 10 micra thick. The prepara- tions were stained in saffranin-fast green. l Eriez Instrument Division, Bendix Aviation Corp., Baltimore, Md. DISCUSSION OF 3.113 ULTS Humidity It was suggested in the Introduction that poor growth of plants during the winter months might be due to their growth in the dry atmos- phere of centrally heated homes. The significance of the experiment thus depends upon the similarity of experimental atmospheric conditions to those of a home. Pertinent data (Table I) indicates that average temperatures around 75 F. with a relative humiditv of 3h% were main- tained in the uncovered chambers. hcccrding to results published by Phillips (l9h0), 2h§ of the homes heated to 700F., where observations were made, nad indoor relative humidities between 303 and h0% when outside temperatures ranged between 200F. and 29oF. With lOOF. increments in outside temperature, between 300F. and over 600F., reapectively, the percentages were: 363, hhfi, 3h%, and 39%. It may be argued that reduced humidity would have been more desirable in order to better duplicate atmOSpheric conditions of heated homes in winter. Yet, Phillips' figures and charts did show quite a percentage Spread at any particular outside temperature with no one relative humidity range predominating at all outside temperature ranges. Watering The differences in amount of water required by plants in the two treatments is striking (Figures 2, 3). It was apparent that plants kept in drier atmospheres needed larger amounts of water more frequently. TA8LE I Kean Temperature Kean Humidity 16 (in degrees Fahrenheit) (in percent) Uirht Day Ni~ht Day .ggpm—8am) (8am—8pm) (8nm—8am) (8am-8pm) Humidity Humidity Humidity Humidity Time in Weeks Low Hish Low Hiéh Low Hich Low High Jen. 25-Feb. l 7K.5 78.5 78.5 75.0 20.5 76.5 21.5 69.0 IP31). l-Febo 8 7305 7300 73.0 724.5 2000 9800 19.0 9300 Feb. B—Feb. 15 72.0 78.0 73.5 70.5 32.0 98.0 28.0 98.0 Feb. 15-Feb. 22 66.5 67.0 70.0 70.0 21.0 83.0 25.0 82.5 Feb. 22-Lar. 1 09.0 70.0 75.0 76.5 36.5 98.0 36.0 98.0 ‘11:.0 8—:.Ll‘o 15 7000 LL'OO 73.5 0100 3300 9800 Zjoo 95.0 Lax. lS—har. 22 68.5 72.5 73.0 78.0 31.0 98.0 26.0 98.0 Mar. 2h-Apr. 1 69.5 71.0 70.5 80.5 28.0 86.0 28.0 81.0 Apr. l—Apr. 8 68.5 71.0 70.5 71.0 38.0 98.0 37.0 98. Apr. 8—Apr. 15 70.0 70.0 81.0 85.0 32.0 98.0 33.0 98.0 Apr. 15—Apr. 22 77.0 77.0 78.5 85.0 38.0 98.0 61.0 98.0 Apr. 22.Apr. 29 76.0 73.0 83.0 87.5 h7.0 93.0 88.0 89.5 Apr. 29-xay 6 77.5 80.0 73.5 86.0 28.5 93.0 30.0 89.0 3an 6—I.Iay 13 79.5 77.0 78.5 90.5 38.0 87.0 63.0 76.0 May 13-May 20 71.0 73.0 77.5 79.5 85.0 93.0 1.1.5 91.0 May 20-May 27 75.5 77.0 79.0 86.0 68.0 80.0 87.0 77.0 may 27-June 3 75.0 77.5 80.0 85.0 h3.§ 96.0 h1.5 97.5 June 3-June 10 77.5 78.0 77.0 86.0 38.5 96.0 38.0 88.0 June 10—June 17 8h.0 83.0 86.0 90.0 53.0 93.0 h9.0 96.0 AVERAGE 73.1 7h.1 76.2 80.h 38.7 92.9 38.0 90.9 DAY 4 NIGHT AVERAGES Temperature Humidity Low Humidity 7h.’”0 F 36 33. High humidity 77.200 F. 91.90; —-°v_v F‘ EVERGREEN NI‘TER REQUIREMENTS 1? (fverage of 90 plants) a , 0 T H 7 1 Low Humidity Total 1.30 L. deter/Plant 20 1 HI- 4 F IO 40 High Humidity Total .33 L, Meter/Plznt Average Amount of water in Mls./P1ant 0 1 201 Fl 10 F 015' JD 1; -1‘ Figure 3 . - . 18 FOILXGE PIANT WATER RfiQmifl'flLPITS (Average of 1.0 plants) Low Humidity 1‘<’»“.z.l 1.30 L. inter/81: nt _ 7 .1 - l l 7 30 W n F I l 1 n 1 fl " '1 W 1 20f r‘ F) T F) W '7 T H) r- - —‘ .T)" _ W +3 1 F E, _ _ _ TH +— 2 3 . __ U; (0 _ "‘ ._ r2 .5 3.. 8 :‘z" o c... O .p a High Humidity $330 Total .32 L. Lister/P7133 (d r1 8 :> 4: 20-) _ H H 1 fl . n 7 F" /o 7 PH! w r F m . n ) Hrfl 1 H /5’ 30 30 15' 30 [5 MARCH APRIL may .TUNE 19 Much of this water requirement mas due to evaporative water less from the tOp of the soil as well as through the sides of the porous clay pots. The evaporation rate was undoubtedly hastened in the exposed chambers by air movement and low humidity which created strung vapor pressure deficits in the surrounding atmosphere. In fact, it was difficult to prevent the plants from drying out during the course of the experiment and occasionally they underwent brief periods of soil moisture shortage. The covered chambers, on the other hand, were pro- tected from these desiccating influences. Cnndensetion of water drop- lets on the transparent cover was often observed, especially in the mornings. Another factor contributing to the greatly reduced water re- quirements of plants grown in high humidity might be foliar water intake. This reversal of nztural 00nditions in which the leaves become the principal water absorbing organ of the plant in highly saturated atmos- pheres has previously been noted (fhut, 1936; Breazeale, Mcdeorge, and Breazeale, 1950). This effect was so great that maturation, flowering and fruit set of tomato plants has been observed with no other source of water than that absorbed through the leaves from a fog or an atmose phere of 1003 relative humidity (Breazeale, theerge, and Bree eale). Growth deference to the survival Record (Table II) shows that a majority of evergreens could not grow in low relative humidity. This was best illustrated by the death of every Taxus cuspidata Andersoni stem cut- ting in a dry atmosphere whereas all but two survived in a saturated one. The same tendency was exhibited by Texas cuspidata capitata and 20 TABLE II SJRVIVAL RECORD Group I Plants Ihwfier Dead of Low High ' rile .Tiri.1' .2 ti- i ' Fla +s ‘1 City *Iiid ty m 5. ., ‘ 1. lanes eugjidata ca itata 7 s l Tarus cuspidata Andersoni CO (33 N Taxus media Hicksi 9 2 3 Juniporus chinensis 10 S - Pinus nigra ll 10 9 Group II Plants Ihnier Dead of Low Hig Plants Humidity Hunidit r Scindapsus aureus 10 — - Cissus rhoflbifolia lO - - Peperomia Ohtusifolia varicsata LO — - m Dracena :odsefianra 10 - - 21 Juniper ohinensis. Pinus nigra seedlings and out tings of Taxus media .flifikfii were evidently not even benefited by high humidity, the pine being the least adapt:.o lo to either OHViTOHHLnt (T511e III). However, Cause of oeath in high humigity was pathological rather than piysiolOLi- cal in that evidences of oerary infection by ”hytlium follow d by A secondary Fuscrium and Jerticiilium iniections were observed. It was therefore likely that high huxidity not only encouraged the entry of Phythium but also facilitated the gerxn inatiox n of secondary invader Spores. Other workers have TBpartEd the growth of hold on bulbs in storage (Rune, 1937) and on p aoka gee of stored deciduous tree fruits after a month or two in hi “,h humidity (Allen and Pentzer, 1735). Death of plants in low humid ”t3 Lght possibly oe due to the same primary arent but era giz ation of the root systems disclosed no signifi- cant reduction in development. Instead, the stems were extremely dry and br'ttle and a physiological basis for death is most plausible since death was rapid and no fungal mycelia were evident. Unfortunately, not enough plan,s were included in the experiment to make a statistioe 1 analysis possible but it is worth noting that death expectanay for Taxus cuspidata capitata and Juniperus chinersis in low humidity was quite near to being stzmt sic: llys nifioant. L mar 1 mi g poteztial for th. evergreens (Group I) was discouraging. Even in high humidity, terminals produced very long feathery new growth. Similar results to a lesser extent were obtained from surviving plants in low humidity. Growth comparisons are provided in graphic form by Figures 2 through 8. They indicate definite growth differences only for Taxus cuSpidata capitata. But roger less of any growth differential in 22 TELJE III FRESH WEIGHT; RY WE IGIIT:I.1;.;3DIE LEAF THICKNESS Low Humidity High Iiumidity {T e?[; (I 43 ° W 'tt- . ' ,C: b - \ I E: 4: '3 (:3 t.) i1 +3 ch ‘3 '5‘ C1 afinv *3 (0 {‘1 (D ,{1 v U) ,3 v (.4 U) 0.) v i ) 93 U) A ;- H g)” gum F: -m' G) (‘J u} :1) 53 O :4 (IS (1) O t.) m 21H .. , :- so r4 ;: :4 :3 £4) E: cu r4 :4 E) E’ (.3 .- .4 '0 o o c9 c5 :4 U o "-1 p 8 {a Q o; L! ;.-1‘ a) $4 :2; o c.) -r-i .1. 5*;. ,s.z 9"11 .5 e 2+:> <2+s\» ~4 J r *1 .494 0-4" L)“: r4i“ . ‘1 r“: Taxus ouspidata o " . P O capitata 2.37 .77 lol.2; 2.hS .oh 320.00 Ta} us cuspidata AI ders oni Taxus media Hicksi Jul i‘OBI‘US chinexsis Pinus nigra 2.70 1.01 .75 1.95 253.75 3&3.7S 187.50 2.30 2.03 b.7o .93 1065 hS0.00 250.00 «3 n4 idata ca itata b igure h Tmmsou unoposfiaawa ca pcmaa\mawsfisnmp soz HH¢ mo mesons ommpo>< 2h »ni v x’ Figure 5 s cu"oidata Anders if V C11\'-L ' . mampotaaafiz an pcaai\ m mCflEhmH 302 Had I Mo £PBOHS mwmhm>~ 25 Figure 6 Taxus media Hicksi mm mumpaeaaaa: ea peeam\mam nu sashes 302 HH< mo Apache owwum>< 26 Figure 8 a Pinus oEHHA HS 0 pgfimagmsts 3% HO npsono ommnm>4 IO /' / / ’9’ 27 the two treatments, the type of growth was weak and the stems and leaves were pale green. The plants presented little interesting stem structure or lateral branch diversion. Aside from lack of visual appeal, poor growth and death under low humidity conditions would pre- clude the sale of any indoor plant, no matter how attractive, during the winter months. Consequently, it would seem that the value of small evergreen cuttings and seedlings would not be evident until the plants were more nature. The broad leaved plants (Group II) reSponded well to the con- ditions of high humidity. The 1003 survival rate (Table II) in either treatment was not unusual since they were sold commercially for indoor growth in winter and originally come from trepical habitats. While most of these plants reSponded to the corditions of high humidity (Figures 9 through 12), Peperomia exhibited almost complete absence of new leaf production (Figure 11) in high humidity although the plants were taller with larger and thicker leaves (Table IV). However, in every instance except Dracena, plants grown in high humidity not only made more linear growth but also produced larger, more numerous, and darker green foliage. However, PEEEEEE produced more larger leaves of almost the sane thiokress in low humidity (Figure 12; Table IV). Yet, this plant was originally found by Godsoff in Upper Guinea which is definitely trOpical in habitat (Bailey, 19h9). thhongh anatomical investigation disclosed no significant structural differences, the stiff, leathery texture of leaves from.both high and low humidity'might be a factor resulting in similar growth. Habitat could possibly be the factor accounting for the lack of 28 I E N _ / a- e S u m 9 a a I a. m a F m .C S 5 I u. m A I H C R H M 5 I 95Hm\mo>w3 Mo 89552 emanate“ 29 Figure 11 ngeromia obtusifolia variegata APR "- MRRIH 00 7. la N.maa N.Hma m.mmw w.moH m.~oN m.mom N.@~H m.m;H o.oem m.mnm 0.42m 0.03m o.mmoa o.mmw o.mmm o.wmo 0.0mm o.oow o.m:oa Amoco Asmpm pumaasp Ammo: Ammoa pmmaawp peeing: OpV mo mop Opv omega“; 09V mo omen opv I a H.m m.e m.m o.m a.m a.m m.ma m.w NQN NON OOH No Q. Q. N“. mew. HON 40H m.HH 4.HH m.m 4.m N.m m.m H.0m N.ma m.mm w.NH aflfi Ed eufl is .mfm aEH aha 28g afifi ES mwewOflamp mHHOMHnEonn mecmfimmmoow. mflaomflm:»oo II muonsm mowpmmmm mammao memomum flaonmomm mammepmwom .mHaH umsmeHemHa\ Amuofle adv mmmzxowga HwQH .m>4 .Eov woa¢ awoq .m>¢ moooofi Gav panama seaflm .mp4 A.mfio ea .e>4v panama and A. 96 CH . 953 pnnflou nmohm . i. .. n _- r. a . a . . mmflrmmobrw as fir”... Gama“ .hHmHH. 5.3m. .Ew 3... “Cum .fiNUHH? 3.03%. >H mHmde 31 difference between growth of Hepatica (Table IV) in low and high humidity since it is a perennial native to temperate forests. It was evident that most plants grown indoors during winter would benefit by some means of increasing humidity in centrally heated homes. The possi- bility of fungal infestation would not be great provided there was movement of air. Anatomy In transection, the Tgxgs leaf was elliptical in shape (Figure 13). A single collateral endarch vascular bundle traversed the central bundle region, the xylem tracheids being oriented toward the adaxial side. An obscure bundle sheath separated the vascular and mes0phyll tissue. The leaf was enclosed by a light cuticle. The epidermis of leaves of plants from high humidity (Figure 13, A) consisted of a single row of fairly large rather thin walled cells. The cells varied in size and shape. Many large Spherical cells which re- sembled bullifcrm cells were observed, eSpecially at the bundle region of the abaxial surface. ConSpicuous deposits of deeply staining granu- lar material resembling tannin were common in all epidermal cells except the bulliform type cells. The sunken xeric type stomata were protected by arching subsidiary cells over the guard cells. Subtending the epi- dermis was a single layer of hypodermis. The cells had abundant protOplasts with large nuclei and numerous chlorOplasts but were without evident wall thickenings. The mesophyll cells were parenchymatous and more or less irregular in size and shape, varying from Spherical to polyhedral. ChlorOplasts and nuclei were prominent as were deeply 32 staining granular inclusions which resembled those found in the epidermis. Intercellular spaces were large and extensive. As is characteristic of the genus (DeBary, lE8h), no dermal glands, resin canals, or other internal secretory resevoirs were observed. The vein of the leaf was surrounded by an irregular layer of cells which had peripheral walls of greater length than the innermost walls. The fibrovascular strand consisted of a one to several celled serpentine row of extremely thick walled, angular tracheids which adjoined a two to three celled layer of thin walled phloem cells on its lower periphery. These phloem cells were squarish and hexagonal in shape, being located in the approximate center of the vascular bundle and the leaf. Cyto- plzsm was dense with very prominent nuclei, some of which occupied most of the cell lumen. Immediately subtending the phloem was a three to five celled layer of collenchyma cells. A single layer of epidermal cells protected leaves of plants from low humidity (Figure 13, B). A rather heavy cuticle covered most of the cells. Pyramidal cap—like peripheral cell walls were characteris- tic as in leaves from high humidity. Elongated hexagonal cells were prevalent on the adaxial epidermal surface with larger bulliform type Spherical cells at the bundle region and tips of the leaf. These cells were also present at the abaxial bundle region although they were smaller and more squarish. Tannin like deposits were observed in all epidermal cells except the bulliform type cells. A definite hypodermal layer of smaller hexagonal cells subtended the abaxial epidermal layer. Cells of the adaxial hypodermis were elongated parallel to the leaf blade surface and so were more rectangular than overlying epidermal cells 33 ‘ Azania-r? 7:sz .1 ( wPenmfjttfi. -:V . ‘ 'fif Transection WWW . tyne»... . .3“: pOQton a , ‘1", Ah of Taxus cuspidata capitata leaves " pissus rhombifolia VT. 11'" -.\.".'l :. . , 1' . , I! .‘IOIi-o. (E; '1'. ,5 _a-."/ _;;; ‘- _ ‘ "3‘ i. ,- r1" - l , . - D‘ L‘uL'l 3" Figure 13 I! " fl . . . \. '- * ‘ . T13». . u - . ‘ 9 ‘ l ' m “ ""‘ k v’ "- '4' "- s; . we wwfimr : .. .- .. 3.3”} wag; 3 “w ‘ .-- . v’ . V "’fi'flf'a _wm. . ”i”Pauikhl:"‘"*”*s _..ansss‘. ' ,ab . » ~ s ‘ ~ “1.3;...31 .r u. .= 1'. is» .. "‘898.."'p' .I ‘I . " 1! " in high humidity " low humidity " high humidity " low humidity 33 as in leaves from high humidity. No schlerenchyma or wall thickenings of any kind were observed. ProtOplasts were densely stained with many chloroplasts and prominent nuclei. Deposits resembling tannin were common. The remainder of the meSOphyll was uniform with cells of varied sizes and shapes, ranging from Spherical to polyhedral. Very prominent nuclei and extremely abundant chlorOplasts were evident. Intercellular (1 .paces were fairly extensive but with densely compacted cells at the central bundle region and tips of the leaf. A fairly well defined bundle sheath surrounded the vascular bundle region. The large cells contained prominent nuclei and irregular starch grains. The vein of the leaf consisted of the central vascular elements surrounded by laterally and adaxially extending parenchymatous trans- fusion tissue. Tne Cissus leaf in transaction was hifacial or dorsiventral with distinct palisade and spongy mesophyll develOpment, protected by a uniseriate epidermis. Plants from high humidity were protected by a single epidermal layer (Figure 13, C). A thick cuticle completely covered cells on the adaxial surface while abaxial cells had thickened outer walls with less deposition on radial and inner walls. The rounded hexagonal epidermal cells possessed sharp angles only on inner adaxial cell walls and to a much lesser extent on peripheral walls of abaxial cells. The general shape of guard cells was oval in transection with thickened walls above the stomatal pore and between the ppre and substomatal chamber. These projections (horn~like structures) were evident on the upper and lower sides of the walls facing the stomatal aperture (i.e., the front 31; walls). Thin hinges in the wall occurred on the entire back wall and at the mid-point of the front wall. The single row of thin walled palisade parenchyma cells were elongated at right angles to the leaf surface and were more or less conical with tapered side walls. Walls adjoining spongy mesOphyll were shorter and straighter than the sharply serrated upper walls adjoining aeaxial epidermal cells. The palisade arrangement allowed room for fairly large lacunar air Spaces. Nrmerous large spherical chlorOplasts were usually oriented near the side walls. Thin walled spongy meSOphyll cells varied in size and shape. Ovoid to Spherical chlorOplasts were more numerous in cells adjacent to palisade parenchyma. Intercellular Spaces often extended from one epidermal surface to the other. Leaves of plants from low humidity possessed a uniseriate layer of rectangular epidermal cells (Figure 13, D). Their long axes were oriented at right angles to the leaf blade surface. Cells of the adax- ial epidermal surface were longer and Lsually narrower than the abaxial cells. The palisade neSOphyll consisted of two layers of cells. Long, thin, deeply staining palisade cells Subtended the adaxial epidermis. Cells of the lower row were reduced in length. The palisade layers were densely compacted as were the four to five rows of squarish to spherical {0 pongy neSOphyll cells. These rows were oriented obliquely parallel to the leaf blade surface. Chloroplasts were numerous but more prevalent near the epidermal surfaces. Intercellular Spaces were not observed. 35 APrLICATIDN OF RESULTS Anatomical Agreement Cissus leaves from high humidity had a greater number of larger chlorOplasts both in the palisade and Spongy mesophyll than plants grown in low humidity (Figure 13, C and D). Much greater photosynthe- sizing capacity was therefore to be expected, eSpecially considering that plants from high humidity usually produced larger and more numer- ous leaves (Table IV). This would tend to offset any reduction in quality and intensity of light, either from the plastic covering or from condensation of moisture on it. Moreover, loosely compacted palisade and Spongy meSOphyll with large intercellular Spaces were clearly characteristic of Cissus as well as the uniform meSOphyll of laying leaves grown in high humidity (Figure 13, A and 0). Conversely, plants from low humidity exhibited small celled, very closely compacted meSOphyll with correspondingly greatly reduced intercellular Spaces (Figure 13, B and D). Anatomical Difference There were, however, significant deviations in anatomical effects from those reported by others. Most outstanding were the greater thickness of leaves grown in high humidity'with consequent dry weight increases in most cases (Tables III and IV) and the absence or slight development of a thick epidermal cuticle on Cissus and Taxus leaves from plants grown in low humidity (Figure 13, B and D). In fact, to O“ Cissus leaves from high humidity were covere with a thick cuticle on both surfaces (Figure 13, C). Lnother difference was the lack of call thickening in cells of plants from low humidity. For example, sclerenchyma was lacking in the hypodermal layer of Taxus cuspidata capitata leaves in low humidity (Figure 13, B). Determination of the number of stomata per unit of leaf surface area was not attempted. TranSpiration If more compact chlorenchyma did present an effective obstacle to transpiration (Pool, 1923), reduced tranSpiration was to be expected. On the other hand, should his compactness result in greater internal evaporative leaf surface area, more tranSpiration would be anticipated, according to Turrell (1936). Moreover, leaves tranSpired at a greater rate and even into a saturated atmosphere in sunlight (Briggs and Shantz, 1915; Henderson, 1926). This was due to absorption of infra-red and ultra-violet radiation in addition to heat energy from visible sun- light by the leaves, as well as their heat of reapirasion, which resulted in higher leaf than surrounding air temperatures. It is further evident from Table I that day temperature of the high humidity chambers averaged about hOF. higher than in low humidity. This reflected the heating effect of the tranSparent cover and in combination with high humidity probably also led to higher leaf temperatures and resulting greater differences between leaf and air temperatures in sunlight (Moreland, 1937). high humidity in diffuse light would have a similar effect (henderson, 1926). These differences would be accentuated in the still air of the enclosed atmcswleze cf “be ligh humidity clambers (Vhlers, 1915; Loreland, l?37), a con ’tien IhiCh pres rably would lead to an increased tanSplrétiCn rate as a result of consequent grez ter va r1- zation of water within the leaf at least during the day. This effect may be reversed and redlce trans piration in diff«s ed light or dark oss when leaf temperature a lowered as a consequence of heat radiation to space. However, either deter inant of trrnspiration could be modified by the absorption lag created by the resistance to wet er movement in the living cells of the root (Kramer, 1938), a lag which supposedly accounted for the intake of water vapor from a saturated atmosphere (Thut, 1938). Consequently, if the water content of the leaf was thereby raiscd, an increase in tranSpiration in high humidity might be expected, considering the importance that some writers have attached to foliar water content and transpiration_ratc (Livingston and Brown, 1912; Knight, 1917 3 Henderson, 1226). Tl'e rate of an r 1C ss became greatly retarded below 60% relativ humidity or 68°F., according to BiaIOglowshi (1935). Assuming that all the aforementioned stinulat ing influences on transpiration were valid and that leaves in high humidity did tranSpire more, there might be a direct effect upon growth. It would depend upon the resolution of the controvorsial relationship between transpiration and the adsorption of various mixieral salts by the plant. wright (1939) devise 3d a technique that supposedly offset plant metabolism effects on mineral absorption. He concluded that there was a corresno‘d ce between in- crease in transpiration rate and increase in uptake of phOSphorus, calcium, potassium, and nitrate ions. he was supported in this 38 conclusion by several other forkers (Haas and Reed, 1927; hitchcock and Zimmerman, 19353 Freeland, 1936, 1937). On the other hrnd, errlier ‘ investigators hzve e fended the ccnve rs e proposition that different tralspiration rates have no effect upon rbs oration of mineral salts (hasselbring, 19lh3 huenscher, 19153 Ki sse elba ch, 1916). If this is true, high transpiration rate of plants in high h aidity is of no use in oxylaining growth ~. iffc rezlces. However, there is just as much justification for maintzining that the opposite is true. Thus, by tranSpiring more, pl:nts in high humidity are able to absorb more mineral salts and t2 fly sy xthGLiZG more water soluble orgrnic pro- teins, the ela boratien of which entails ca rboh drate depletion. In fact, the growth stimulrting ef1ect of increasing the amount of nitrate in nutrient solution has been attributed to the depletion of carbohy- drates in the top, resulting in more growth (Turner, 1922). Further- more, thickness of cuticle was not found to have any correlation with rate of tr:nspirction for apple fruits (Pieniazek, 19hh). This might mean that the heavy layer of depositi n on both epidermal surfaces of 0155 us leaves in high humidity (Fi gure 13, 0) would have no deterrent effect on tranSpiration. Decrease in dry weight (carbohydr: te )ercentz go) f: llowe} by h:«31ol ysis of proteins (Nightingale and Robbins, 1928; Nightingale and ochermerhorn, 19283 Nightingzle, Schermerhorn, and Robbins, 1928, 19303 Pearsall and hwing, 19293 Nightingale, 1933) and increased growth has been postulated as accountingf or rapid gro.1th in hi_gh humidity (Nightingale and hitchell, 193h). In feet, high lry hatter content has seen Cited as a CCMJOH low hu idity effect ( ightingale et al; Pesrsall 39 and Ewing; Nightingale). Yet, in the present experiment dry weight in high humidity was in one instance almost double that in low humidity (Scindapsus aureus, Table IV). It will be noticed that increased dry weight in high humidity was correlated with more leaves, taller plants, and greater leaf area and thickness, at least for all Group II plants (Table IV). When dry weight was reduced in high humidity (Dracena godsefianna), correSpondingly fewer leaves, shorter plants, and less leaf area and thickness were produced. It is difficult to rationalize how increased dry weight in high humidity may account for better growth if a decrease in dry Weight is supposedly associated with better growth of plants in high humidity . Resolution depends upon the thicker leaves of plants grown in higher humidity which indicates that more dry matter is to be expected. In fact, Pickett (1937) observed that a larger gain in total dry matter per unit area may be produced by leaves with prominent inter- cellular Spaces. Thicker leaves also mean more photosynthetic activity and production of carbohydrates. Assuming that transpiration is main- tained at a high enough rate to assure thtt a constant supply of mineral salts reaches the tOp of plants in high humidity, it is conceivable that the usual metabolic activity of the leaves (Turner, 1922) would lead to a more rapid growth rate than for plants in low humidity with fewer chlorOplasts and reduced tranSpiration. ho SUMMARY In order to determine the effects of humidity on the growth of plants, 80 tropical foliage plants (h Species) and LS young evergreen cuttings and seedlings (F Species) were grown for six months in atmos- pheres of low and high humidity. Half of the plants were placed in chambers covered with transparent plastic ("Saran-drap"). The remainder were left exposed in identical chambers. These chambers were located in a laboratory facing an east window in which temperatures and humidities averaged 7h.6OF. end 3h.h$, res- pectively. Average temperatures and relative humidities of 75.20F. and 91.9%, reSpectively, were maintained in the covered chambers. All evergreens produced thicker needle leaves in high humidity but a definite growth reaponse in terms of new growth made was observed for only one Species. A definite growth response in high humidity was obtained for most of the foliage plants (with one exception) in terms of greater pro- duction of thicker and larger new leaves and taller plants with glossy dark green foliage. Although none of the plants died in low humidity, it was evident that some means of increasing humidity in the home, at least during winter, must be provided before foliage plants can attain their maximum growth. Attention must further be directed to the greater amount of water required by plants growing in low humidity. Failure to prevent extreme soil dryness due to excessive water loss from the sides of clay pots and the soil surface, in addition to hi tranSpiration, led to tissue desiccation and death of a majority of evergreens in low humidity. Death in high humidity was probably patho- logical since fungal mycelia and Spores were observed on the needles and stems of dying plants. Surviving plants were not attractive in appearance. New growth in both high and low humidity was feathery and often insignificant. It was suggested that a marketing potential for these plants exists only after they had matured. Better growth in high humidity has been attributed to increased transpiration and mineral salt uptake inasmuch as anatomical investiga- tion disclosed no significant differences in leaf structure other than the compact cell arrangement of leaves from low humidity. This in it- self may act as a detmrrent to tranSpiration but there were more definite indications that leaves in high humidity tranSpired more. For example, greater leaf water content, difference between leaf and air temperature, or lack of surrounding air movement led to a higher rate of tranSpiration. This mi ht have increased the tr neport of minerfl salts resulting in the synthesis of proteins and depleted sugar and starch reserves in leaves of plants in high humidity. The resultant decrease in dry weight was accompanied by proteolysis and release of energy for rapid growth in high humidity. L2 BTUQNRECHED Allen, F. n. and W. T. Fentzer. 1935. Studies on the effect of humidi— ty in the cold storage of fruits. Proc. A er. Soc. Mort. Sci. 33:215—223. Wailey, L. H. 19h9. Vanual of Cultivated Plants. The fiacmillan Co. Few York. Pialo~lowski, J. 1935. Effect of humidity on tranSpiration of rooted lemon cuttings under controlled conditions. Proc. Amer. Soc. Hort. Sci. 33:166-169. Brezeale, E. L., W. T. ”cCeorge and J. F. Brezeale. 1950. hoisture ab- sorption by plants from an atmosphere of high humidity. Plant Physiology. 25:Ll3—hl9. Briggs, L. J. and H. L. Shantz. 1915. An automatic transpiration scale of large capacity for use with freely eXposed plants. Jour. Agr. Res. 5:117-133. Clum, H. H. 1926. The effect of tranSpiration and environmental factors on leaf temperatures. I. TranSpiration. rer. Jour. Bot. 13: 19h—2 16 . Curtis, 0. F. 1936. Comparative effects of altering leaf tenperatures and air humidities on vapor pressure gradients. Plant Physiology. 11:599—603. DeBary, A. lflah. Comparative Anatogy‘gf the Vegetative Organs 2f the Phanerogaus and Ferns. Clarendofi Press. London Duffield, J. W. and A. G. Snow, Jr. 19hl. Pollen longevity of Pinus strobus and Pinus resinosa as controlled by humidity and tempera— ture. Arrer. Jour. Bot.78:l75—177. Ehlers, J. H. 1915. The temperature of leaves of Pinus in winter. Amer. Jour. Bot. 2:32-70. Fisher, G. H. 19h1. Difficult cuttings respond to use of overhead mist spray. Florist's Rev. 88:13-1h. Freeland, R. O. 1936. Effect of transpiration upon the absorption and distribution of mineral salts in plants. Aner. Jour. Bot. 23: 355-362- h} Freeland, R. O. 1937. Effect of trans iration upon the absorption of mineral salts. Amer. Jour. Bot. 2 :373-37h. Gardner, E. J. l9h1. Propagation under mist. Aver. Nurseryman. 73 (9):§-7. Hanson, H. C. 1917. leaf structure as related to environment. Amer. Jour. Bot. 14:533—560. Haas, A. R. C. and H. 3. Reed. 1927. Relation of desiccating winds to fluctuations in ash content of citrus leaves and phenorena of mottle-leaf. Bot. Caz. 83:161-172. Hassclbring, H. l91h. The effect of shading on tranSpiration and assimilation of the tObacoo plant in Cuba. Bot. Caz. 57:257-286. henderson, F. Y. 1926. On the effect of light and other conditions upon the rate of water—loss from the mesophyll. Ann. Bot. hO: 507-533- Hitchcock, A. E. and P. W. Zinrernan. 1929. Effect of chemicals, temperature, and humidity on the lasting qualities of cut flowers. Amer. Jour. Bot. 16:1433—hh0. . 1935. Absorption and movement of synthetic growth substances from soil as indicated by the res- ponses of aerial parts. Contrib. Boyce Thompson Inst. 7zhh7eh76. Hume, E. P. 1937. Humidity studies on narcissus bulb storage. Proc. Amer. Soc. Hort. Sci. 35:850-853. Jones,'W. W. 1939. The influence of relative humidity on the respira- tion of papaya at high temperatures. Proo. Amer. Soc. Hort. Sci. 37:119-123. Kisselbach, T. A. 1916. Transpiration as a factor in crop production. Nebraska Agr. Exp. Sta. Res. Bull.' 6:170—18h. Knight, R. C. 1917. The interrelations of stomatal aperture, leaf water-content, and transpiration rate. Ann. Bot. 31:221—2h0. Kohl, H. 0., Jr. 1955. Intermittent mist as a cultural practice for roses grown for out flower production. Proc. Amer. Soc. Hort. Sci. 67:53h-538. Kramer, P. J. 1938. Root resistance as a cause of the absorption lag. Amer. Jour. Bot. 25:110-113. Kraus, E. J. and H. R. Kraybill. 1918. Vegetation and reproduction 23th special reference to the tomato. Oregon Agr. Exp. Sta. Bull. 9. Livingston, B. E. and W. H. Brown. 1912. The relation of the march of tranSpiration to variations of water content of foliage leaves. Bot. Gaz. 23:309. Long, E. M. 1939. Photoperiodic induction as influenced by envionmen- tal factors. Bot. Gaz. 101:168-188. Loomis, W. E. 1927. Temperature and other factors affecting the rest period of potato tubers. Plant Physiology. 2:287-302. hitchell, J. W. 1936. Effect of atmospheric humidity on rate of carbon fixation by plants. Bot. Gaz. 98:87-10b. Moreland, C. F. 1937. leaf temperatures of sugar cane. Plant Physiology. 12:989-995. Muenscher, W. L. C. 1915. A study of the relation of tranSpiration to the size and number of stomata. Amer. Jour. Bot. 2:1187—5014. - 1922. Effect of transpiration on the absorption of salts by plants. Amer. Jour. Bot. 9:311-330. Newcombe, F. C. and E. T. Bowerman. 1918. Behavior of plants in un- ventilated chambers. Amer. Jour. Bot. 5:28h-29h. Nightingale, G. T. 1933. Effects of temperature on metabolism in potato. Bot. Gaz. 95:35-58. — and N. R. Robbins. 1928. Some phases of nitrogen retabolism in polyanthus narcissus (N. tazetta). New Jersey'Agr. Exp. Sta. B11110 2472 o ‘— and L. G. Schermerhorn. 1928. Nitrate assimilation by asparagus in the absence of light. New Jersey.Agr. Exp. Sta. Bull. 1476. Nightingale, G.‘T., I" G. Schermerhorn and W. R. Rdbbins. 1928. The growth status of tomato as correlated with organic nitrogen and carbohydrates in roots, stems, and leaves. New Jersey Arr. Exp. Sta. Bull. h61. —— . 1930. Some effects of potassium deficiency on the histological structure and nitrogenous and carbohydrate constitutuents of plants. New Jersey'Agr. Exp. Sta. Bull. h99. and J. V. Mitchell. 193h. Effects of humidity on metabolism in tomato and apple. Plant Physiology. 9:217—236. Pearsall, W. H. and J. Ewing. 1929. The relation of nitrogen retabolism to plant succulence. Ann. Bot. 143:27-3h. Phillips, T. D. 19h0. A survey of humidities in residences. U. 8. Dept. of Commerce. Washington, D.C. Report RFSSé. Pickett, H. F. 1937. The relationship between the internal structure and photosynthetic behavior of apple leaves. Kane. Agr. Exp. Sta. Tech. Bull. h2. Pieniazek, S. A. l9hh. Physical characters of the skin in relation to apple fruit transpiration. Plant Physiology. 19:529-536. Pool, R. J. 1923. Xerophytism and comparative leaf anatomy in rela— tion to transpiring power. Bot. Gaz. 76:22192h0. Shields, L. R. 19FO. leaf xeromorphy as related to physiolcsical and structural influences. Lot. Rev. 16:399-hL7. Shippy, N. B. 1930. Influence of environment on the callusing of apple cuttings and grafts. Aver. Jour. Bot.’ 17:290—326. Stoutemeyer, V. T. 19h2. Humidification and the rooting of green— wood cuttings of difficult plants. Proc. Arer. Soc. Hort. Sci. Thut H. F. 1938. Relative hunidit variations affectinc transpira- ) z a . tion. Ar‘er. Jour. Bot. 2V:589-59S. Turner, T. H. 1922. Studies of the mechanism of the physiological effects of certain mineral salts in altering the ratio of top growth to root growth in seed plants. Amer. Jour. Bot. 9zh15—Lh5. Turrell, F. T. 1936. The area of the internal.eXposed surface of dicotyledon leaves. Amer. Jour. Bot. 23:255-26h. Jilson, C. C. l9h8. The effect of some environmental factors on the movements of guard cells. Plant Physiolocy. 23:5—37. Wrisht, K. 3. 1939. Transpiration and the absorption of mineral salts. Plant Physioloéy. 1h:l71~17h. Yarwood, C. E. and fl. 3. Hagen. 19hh. The relative humidity at leaf 5} lll‘f’dCGS o A1133 0 JO‘U" o 1301:: o 31:129—13K’. \2 r"! {M has”; {jSE GM! ‘_Date Due Demco-293 IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 10293143 21105