Plant Physiol. (1971) 48, 203-207

Energy Budgets and Temperatures of Nyctinastic Leaves on

Freezing Nights1Received for publication February 3, 1971

CHRISTA R. SCHWINTZER2Department of Botany, University of Michigan, Ann Arbor, Michigan 48104

ABSTRACT

Temperatures of exposed horizontal and vertical soybeanleaves (Glycine max [L.] Merr. var. Chippewa) were measuredon calm, clear nights with temperatures near freezing. Averageleaf-air temperature differences for 5 nights were -1.5 C and-1.0 C for horizontal and vertical leaves respectively. Thehorizontal leaves were cooler than the vertical leaves. Themean of all observed horizontal-vertical leaf temperature dif-ferences was -0.5 C with a maximum average for 1 night of-0.8 C, while maximum differences theoretically attainable insimilar leaves were calculated to be -1.7 C. No differences wereobserved in the extent of frost damage in horizontal and verti-cal leaves. The apparent reduction in frost damage in verticalleaves observed by Charles Darwin was probably caused by hismethod of using corks to hold the horizontal leaves and not byleaf orientation. Theoretical considerations and the exper-imental results indicate that nyctinastic leaf movements prob-ably do not provide significant protection from frost forany plants.

Nyctinastic leaf movements are daily movements resulting invertical orientation of the leaf blades at night and horizontalorientation during the day. They are widely distributed bothtaxonomically and geographically (3, 12) and were extensivelystudied by early plant physiologists (1, 3, 8, 11, 14). Darwin (3)performed experiments in which horizontal leaves sufferedconsiderable frost damage on clear nights while vertical leaveson the same plant showed little. He concluded that on clearnights vertical leaves are warmer than horizontal leaves and thatthe vertical orientation protects the leaves from frost damageduring light frosts.

In the present study the effect of nocturnal leaf orientation onleaf energy balance, leaf temperature, and the extent of frostdamage during light frosts is examined both experimentally andtheoretically.

MATERIALS AND METHODS

Experimental Sites. The experimental sites were located atthe University of Michigan Botanical Garden near Ann Arbor,Michigan. Most measurements were made in a large, nearly flat

'Part of a dissertation submitted to the University of Michiganin partial fulfillment of the requirements for the degree of Doctorof Philosophy.

2 Present address: Missouri Botanical Garden, 2315 Tower GroveAvenue, St. Louis, Mo. 63110.

field at a site approximately 15 m from the top of a slight rise.Additional measurements were made at the center of a nearlycircular depression and beside a small creek at a site partiallyshaded by deciduous trees. All three sites were covered with amixture of grasses and broad-leaved herbs.

Measurements. Incoming radiation and net radiation weremeasured with polyethylene-shielded hemispherical radiometersand net radiometers respectively. Dew and frost were removedfrom the surfaces of the windows with a soft tissue 5 min beforeeach measurement when necessary. Leaf and air temperatureswere measured with 36 gauge, soldered, copper-constantanthermocouples. Wind speeds near the plants were measured witha small, very sensitive, 4 cup anemometer. Average wind speedswere obtained by counting the number of turns per 5 min. Detailsof the construction, characteristics, and calibration of all in-struments are given by Schwintzer (12). The amount of watercondensed on the leaf surfaces during the experimental periodwas determined by weighing, blotting, and reweighing leaf sam-ples collected at the end of the experimental period. The differ-ence in weight was taken as the total amount of water condensedon the leaf between sunset and the end of the experimentalperiod.

Procedure. Soybeans, Glycine max (L.) Merr. var. Chippewa,were grown two per pot in 10-cm square pots in a greenhouseunder natural day lengths. Plants were used for experimentsafter flower buds developed and before the leaves senesced.The energy budgets of horizontal and vertical leaves were

studied on calm, clear nights with temperatures near 0 C. Onthe preceding day, pairs of pots were selected containing plantsmatched with respect to age, leaf size, leaf number, and height.The leaflets of one remained free to assume their natural, ap-proximately vertical position at night while the leaflets of theother were fixed in a single plane with 30 gauge (0.25 mm diam-eter) varnished copper wire as shown in Figure 1. The leaveswere supported in a horizontal position by one or more strandsof wire fastened to wooden rods placed in the corners of eachpot. The wires did not affect leaf energy balance significantlysince they conducted negligible heat, did not interfere with thefree movement of air, and did not shade a significant portion ofthe leaf from radiation. Their presence probably produced nophysiological changes in the leaves since the leaves immediatelyassumed the normal vertical orientation if the wires were removedduring the night. All plants were watered thoroughly in the earlyafternoon and moved to the experimental sites, where theyremained until the next morning.The energy exchange of exposed horizontal and vertical leaves

was examined in detail in the open field on October 20 and 30.On both days 16 plants in eight pots were arranged into two rowsof four each. Plants with horizontal and vertical leaves werealternated as far as possible. On October 20, an additional sixleaves designated "horizontal pinned," were fixed in a horizontalposition with Darwin's (3) method. The leaflets were pinned to

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Plant Physiol. Vol. 48, 1971

FIG. 1. Method of fixing individual soybean leaflets in a hori.zontal position.

large corks with fine insect pins leaving an air space of about 3mm between each leaflet and cork. The corks were supported bywooden sticks. Leaf and air temperatures and net radiation abovethe vegetation were also measured on three additional nights andat the other two sites.

Theoretical. The energy budget of a leaf at night may be writtenas follows:

R1+L,E+C=O (1)

where R, = net heat exchange by radiation (cal cm-2 min7-),L, = latent heat of vaporization (cal g-1), E = rate of evapora-tion or condensation (g cm-2 min'-), and C = sensible heatflux by convection (cal cm-2 min'-). All terms are per unit leafarea (an area of 1 cm2 corresponds to two leaf surfaces of 1 cm2each). An addition of energy to the leaf is considered positive,and a loss is considered negative. Metabolic production of heat,sensible heat flux by conduction, and short term storage ofsensible heat have been omitted since they are negligibly small innonsucculent leaves in contact with air (12). Latent heat offusion released to the leaf by freezing of water in and on theleaf has also been omitted since it does not contribute undermost conditions, namely at all leaf temperatures (T1) greaterthan 0 C and at T1 < 0 C after freezing is complete and is prob-ably negligible at most other times (12).

Sensible heat flux by convection is calculated as follows:

C = k,(V/D)05(Ta - Tz) (2)

where V is the wind speed (cm sec'l), D is the leaf dimension inthe direction of the wind (cm), T1 and T0 are the leaf and airtemperatures respectively and ki is an empirically determinedconvection coefficient (5).

In fully exposed leaves at the top of the canopy on clear nights,RI is related to the net radiation at the surface of the vegetation(R,) as follows:

(3)

where k2 is a constant depending on the orientation of the leaf.The net radiation at the leaf (RI) is the average of the net radia-tion at the upper leaf surface (the surface facing away fromthe canopy) and the lower leaf surface. The net radiation at the

lower surface of both horizontal and vertical leaves is very closeto zero since all objects "seen" by this surface have temperaturessimilar to those of the leaf. The lower surfaces of horizontalleaves face the canopy interior while the lower surfaces of verticalleaves or leaflets of almost all nyctinastic species are close to-gether and face each other. The net radiation at the upper sur-face of an exposed horizontal leaf equals R,. Consequently,RL- (R, + 0)/2 0.50 R, . Since horizontal leaves are seldomperfectly horizontal this is only an approximation, but the errorsare small. A change of 150 from the horizontal reduces the netradiation at the upper surface only 4% (9). The net radiation onan exposed vertical surface is approximately 36¼/, of that on ahorizontal surface (9), hence the net radiation at the uppersurface of an exposed vertical leaf equals 0.36 R,, and conse-quently R, - (0.36 R, + 0)/2 0.18 Rc. A change of 15°from the vertical increases R, about 55c.Maximum attainable leaf-air temperature differences (T1 -

Ta) and hence maximum horizontal-vertical leaf temperaturedifferences (Tlh - T1,) are observed when the net radiation atthe surface of the vegetation (R,,) has its maximum negativevalue, at maximum leaf size (D), minimum wind speed (V), andin the absence of water condensation on the leaf (LAE). WhenL,E = 0, the energy lost by the leaf by net radiation equals theenergy gained by convection and T, - Ta may be obtained fromequations 2 and 3 for any R,., D, V, and Ta . An estimate of thepossible range of R,. on clear, calm nights with light frost wasobtained from values taken from Geiger's chart (7) for air tem-peratures of 0 C at 2 m and surface temperatures of -3 C. Thevalues range from -0.093 cal cr-2min-1 at relative humidity(r.h.) = 100% to -0.127 cal cny2min-1 at r.h. = 40%. Thecorresponding values of R1 are -0.046 and -0.063 cal cm-2min-for horizontal leaves and -0.017 and -0.023 cal cm-2minq-for vertical leaves. Calculated maximum Tlh - Ti, are shown inTable I. The wind speed was taken as 10 cm sec'l since this isthe lowest rate of air movement likely to be encountered innature (5). The average dimension of the leaf was taken as 5 cmand 10 cm because in most nyctinastic species D < 5 cm whilein a few D > 10 cm.

Experimental. The energy budgets of vertical and horizontalleaves on October 20 and 30 are shown in Table II. Measuredvalues are means of all measurements taken during the timeintervals indicated. Mean values for a period of up to 2 hr maybe used in calculating nighttime energy budgets because con-ditions are relatively constant with time late at night on clear,calm nights. Convective heat transfer, C, was calculated using

Table I. Calculated Leaf-Air Temperature Differentces for Hori-zontal and Vertical Leaves anid Horizontal-Vertical Leaf

Temperatutre DifferenicesCalculations were made on clear nights with air temperatures

of 0 C and slight air movement (V = 10 cm sec-').Leaf Horizontal Leaf Nertical

Relative __H_umi,dity L'

Rl Tlh-TTa RI Tiv-Ta Tlh-Tiv

C cm cal CM-2 cal cm-2/0 ~ tinImin-'100 5 -0.046 -2.0 C -0.017 -0.7 C -1.3 C

10 -4.6 C -1.7 C -2.9 C40 5 -0.063 -2.7 C -0.023 -1.0 C -1.7 C

10 -6.3 C -2.3 C -4.0 C

1 L = average dimension of the leaf; R, = average net radia-tion of the leaf calculated from equation 3; Ta , Tlh , and TlV aretemperatures of the air, horizontal, and vertical leaves respec-tively; leaf-air temperature differences calculated from equation 2.

204

SCHWINTZER

R I-- k2Rv,

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Plant Physiol. Vol. 48, 1971 TEMPERATURES OF NYCTINASTIC LEAVES

Table II. Entergy Budgets of Exposed Horizonital aiid Vertical Soybeani Leaves

All values except temperatures are in cal cm-2 min-1. Measurements were made on calm, clear nights in an open field near Ann Arbor,Michigan, in October 1968.

Radiation IncidentTimne LeafI T27 Cs LE BancOctober EST Orientation T_ RaRp Re6 RI7 C | E' Balace

As (upper B4 (lowersurface) surface)

20 0100-0130 Horizontal -0.7 C 1.6C 0.38 0.46 0.40 -0.43 -0.03 0.05 0.00 0.0220 010-130 Vertical 0.1 C 1.6C 0.42 0.46 0.42 -0.43 -0.01 0.03 0.00 0.0230 2200-2320 Horizontal -5.7 C -4.5 C 0.35 0.42 0.37 -0.40 -0.03 0.05 0.00 0.0230 2200-2320 Vertical -5.3 C -4.5 C 0.39 0.42 0.38 -0.40 -0.02 0.03 0.00 0.01

I Measured leaf temperature.2 Measured air temperature.3 Measured radiation incident on the upper leaf surface.4Calculated radiation incident on the lower leaf surface = blackbody radiation at Ta.5Radiation absorbed by the leaf = 0.95 [(A + B)/21 where 0.95 is the long wave absorptivity of the leaf (6).6 Radiation emitted by the leaf = 0.95 (blackbody radiation at T,).7Net radiation at the leaf = R. + R,.8Convection calculated with equation 2.9 Latent heat released by condensation calculated with L4 = 595 cal g-I at 0 C (15) and measured E.

the mean leaf dimension obtained October 30 (D = 4.8 cm) andthe mean square root of the wind speed for each of the timeintervals, since C varies directly with V0I5 (V0 5 = 3.04 and 5.68cm0-5 sec70-5 for October 20 and 30, respectively). The averageheat transfer by condensation, L,E was assumed to be the sameon October 20 as on October 30 when values of 0.0049 and 0.0031cal cm-2min-1 were obtained for horizontal and vertical leavesrespectively. These values are less than half as large as the lastsignificant figure in the terms for radiation absorbed and radiationemitted.Both horizontal and vertical leaves were colder than the sur-

rounding air on October 20 and 30 (Fig. 2). On October 30 theaverage leaf-air temperature difference at 40 cm was -1.2 Cfor horizontal leaves and -0.8 C for vertical leaves. The hori-zontal leaves were about 0.4 C colder on the average than thevertical leaves. All temperatures decreased between 2000 and2320 hr, with temperature changes of the air and the two groupsof leaves following each other closely but not exactly. Even thoughthe average temperature of the horizontal leaves was consistentlylower than that of the vertical leaves, there was a great deal ofvariability in the temperatures of the individual leaves in eachgroup and much overlap between the two groups. For example,at 2000 hr the mean temperature of nine horizontal leaves was-4.0 C with a standard deviation of 0.64 C and individual valuesranging from -2.7 C to -5.0 C while the mean temperatureof ten vertical leaves was -3.4 C with a standard deviation of0.52 C and individual values ranging from -2.5 C to -4.3 C.On October 20 there was considerable variability among individ-ual leaves in each group and limited overlap between horizontaland vertical leaves while there was no overlap between either ofthese groups and horizontal pinned leaves. The horizontal leaveswere 0.8 C cooler on the average than the vertical leaves while thehorizontal-pinned leaves were 2.2 C cooler than the verticalleaves (Fig. 2). The cooling rates of horizontal and vertical leaveswere very similar from 0100 to 0130 hr while the cooling rate ofthe horizontal-pinned leaves followed the other two groups from0100 to 0115 hr but was much lower from 0115 to 0130 hr.None of the plants survived the night of October 30 while all

survived October 20. The surviving plants were returned to thegreenhouse and all corks and wires were removed. Some plantsappeared slightly wilted but none showed obvious frost damage,even though all were covered with frost during the night. Twenty-four hours later none of the "vertical" and "horizontal" leaves

showed any visible evidence of frost damage, whereas one of thesix "horizontal-pinned" leaves had developed necrotic spots onabout one-third of its surface.The effects of leaf orientation on leaf temperatures, cooling

rates, and net radiation on 5 nights and at three sites are sum-marized in Table III. Mean values are given for each night be-cause net radiation and hence leaf-air temperature differences andcooling rates are relatively constant late at night on clear, calmnights. Observed leaf-air temperature differences for exposedhorizontal and vertical leaves were small and varied considerablyfrom night to night and with the experimental site. The meanvalues were -1.5 and -1.0 C in horizontal and vertical leaves,respectively. Measured horizontal-vertical leaf temperaturedifferences had a mean value of -0.5 C. Cooling rates were less

5

4

3

~o

C) 110C,

54 0:1

h

o -2O4.-3

-4

-5

0100 0115 0130

time (EST)

FIG. 2. Air temperatures and average temperatures of exposedhorizontal and vertical soybean leaves in an open field near AnnArbor, Michigan on October 20, 1968: A: Air at 115 cm (averageof 2); A: air at 40 cm (average of 2); 0: leaf horizontal (averageof 2); 0: leaf vertical (average of 3); (: leaf horizontal pinned to

cork (average of 3) according to Darwin's method.

205

- A~~A

-. 0

0~~0

:i

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Plant Physiol. Vol. 48, 1971

Table III. Summary of Average Temperature Differences between Leaves antd the Surrounzdinzg Air; Average Coolilng Rates of Leaves;anid Average Net Radiationi above the Vegetationz

Measurements were made on clear, calm nights with temperatures near 0 C near Ann Arbor, Michigan.

Date Time EST eSiteLees' Measure- T' -Ta Tit 5 - Ta Tlh - T Cote RadiationmentS2 Rt aito

deg C hr-1 cal cmn-2 min-'5,1-5/2/68 2000-0430 Open field 1 8 -2.5 C -1.7 C -0.8 C 0.9 -0.085/5-5/6/68 2100-0400 Open field 2 6 -2.6 C -1.9 C -0.7 C 0.4 -0.105'5-5'6/'68 2130-0430 Creek side 1 4 -0.8C -0.7C -0.1 C 0.4 -0.075/'22/68 0100-0530 Open field 12 5 -1.1 C -0.6 C -0.5 C 0.2 -0.075/22,/68 0200-0500 Creek side 2 4 -0.6C -0.2 C -0.4C 0.4 -0.055/22/68 0215-0430 Kettlehole 2 6 -1.3 C -0.8 C -0.5 C 0.810,/20/68 0100-0130 Open field 3 3 -2.3 C -1.5 C -0.8 C 2.4 -0.0610 30/68 2000-2320 Openfield 10 6 -1.2C -0.8C -0.4C 0.7 -0.06

1 Leaflets measured in each orientation.2 Measurements per leaflet.3 Temperature of horizontal leaves.4 Air temperature at the level of the leaves.5 Temperature of vertical leaves.

than 1 C hr-" on all nights but one, varied from night to night,and varied with the site. Net radiation was small and negativewith a mean value of -0.07 cal cm-2 min-'. No differences wereobserved in the extent of frost damage in horizontal and verticalleaves.

DISCUSSIONDarwin's (3) hypothesis that vertical leaves should be warmer

than horizontal leaves on clear nights is supported by both thetheoretical and the experimental results. The significance ofthe horizontal-vertical leaf temperature differences depends ontheir size and the conditions under which they develop. The verti-cal orientation is an advantage only if it prevents cold and frostdamage under conditions in which horizontal leaves are damagedor killed. A leaf is killed by frost if it reaches a certain critical tem-perature (the killing temperature). Above this temperature theleaf survives even in the presence of frost. Under most conditions,i.e., on alU frost-free nights and on all nights with frost when airtemperatures are at or below the killing temperature, both hori-zontal and vertical leaves either survive or die. At any given timeduring the night even very small temperature differences maypermit the vertical but not the horizontal leaves to survive. How-ever, plants in nature must survive a whole night with steadilydeclining temperatures until shortly after dawn when tempera-tures begin to increase again. A temperature difference that pro-tects the vertical leaf at one time provides no protection at a latertime if the cooling rate of the whole air mass and all the leaves isgreat enough so that at the later time all have reached killing tem-peratures. The time interval between the attainment of killingtemperatures by horizontal and vertical leaves is determined bythe horizontal-vertical leaf temperature difference (Tlh - Tl,)and the cooling rate. Even the maximum theoretical Tlh - Ti, of-1.7 C for D = 5 cm and -4.0 C for D = 10 cm (Table 1) willnot provide significant frost protection to vertical leaves un-less the cooling rates are low enough so that the time interval be-tween attainment of the killing temperature by horizontal andvertical leaves is long enough that the vertical leaves survive untildawn. The time interval is relatively short for most cooling ratesobserved in the field. For example, at cooling rates of 0.5 C/hror greater it is less than 4 hr for D = 5 cm and less than 8 hr forD = 10 cm. Consequently, attainment of even maximum Tlh -

TI, will not provide protection against frost for individual leavesunder many conditions. Maximum Tm.h - Tl, are seldom if ever

approached in the field because the necessary environmentalconditions are very rare (see below).Leaf orientation has little or no effect on leaf temperatures in

the canopy interior, because both horizontal and vertical leavesare near air temperature since they exchange radiation primarilywith each other. Therefore, the effect of leaf orientation on theplant as a whole is small even on those rare nights when exposedhorizontal leaves are killed while exposed vertical leaves are not.The possibility that the effect of leaf orientation is greater in the

low latitudes where nyctinasty probably evolved (12) or that itaffects the extent of cold damage or the rates of temperature-dependent processes must also be considered. Its effect on the extentof frost damage during winter frosts in the low latitudes is notlikely to be greater than in the higher latitudes since conditionsare similar to late spring and early fall frosts in the higher lati-tudes. Similarly, leaf orientation probably does not significantlyaffect cold damage or the rates of temperature-dependent proc-esses, since even the maximum temperature differences are rela-tively small and affect only the exposed leaves at the canopyexterior.

All measured Tlh - Ti, (Table III) were appreciably smallerthan the calculated Tlh - T1, . The measured values are expectedto be lower than the theoretical values since the latter were cal-culated for conditions producing maximum attainable Tlh-Tl,.The observed values probably were lower than the calculatedvalues for V = 10 cm sec-" and D = 5 cm (the approximate di-mension of soybean leaflets) because wind speeds generally ex-ceeded 10 cm sec-" (mean V = 12 and 32 cm sec'l for October20and 30, respectively), because high relative humidities preventedthe net radiation from becoming strongly negative, and becausethe vertical leaves were not perfectly vertical.The measured horizontal and vertical leaf temperatures showed

a large amount of variability which makes it appear questionablewhether leaf orientation really affects leaf temperature in nature.However, the difference between horizontal and vertical leaf tem-peratures, though small (Table III), is almost certainly real sincethe mean horizontal leaf temperature was consistantly lower thanthe mean vertical leaf temperature (Fig. 2). Similar results wereobtained at all three sites and on all nights (Table III, and ref. 12).The variability in measured leaf temperatures probably is largelydue to differences in the microenvironments of the individualleaves.The observed nocturnal leaf-air temperature differences and the

206 SCHWINTZER

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TEMPERATURES OF NYCTINASTIC LEAVES

observed leaf energy budget are consistent with the observationsof others. The leaf-air temperature differences are similar to thosereported by Cole (2), Mishchenko (10), and Shaw (13). More-over, both the observed nighttime soybean leaf energy budgets(Table H) and a burr oak leaf energy budget observed by Gates(4) on a clear, calm night with an air temperature of about 17 Care characterized by a small net loss of heat by radiation and ad-dition of heat by convection.The energy budgets adequately describe the heat exchange of

the leaves, since the balance in the last column in Table II is small.However, a small consistent error is indicated because the leaveswere actually cooling and not warming, as would be consistentwith a positive energy balance. Most of the error is probably dueto an increase in the measured downward radiation due to ac-cumulation of small amounts of dew and frost on the windows ofthe radiometer.The observed temperature differences (Table III) are probably

not large enough to result in a significant reduction in frost dam-age even in exposed leaves. Yet Darwin's experiments indicatethat there may be noticeable differences in the extent of frostdamage. This suggests that either conditions during Darwin's ex-periments were extremely favorable to the reduction of frostdamage, i.e., attainment of maximum possible Tlh - T, and verylow cooling rates or onset of killing temperatures in horizontalleaves near dawn, or that Darwin's results were artifacts of hismethods.

Careful examination of Darwin's work (3) shows that both hismethod of holding the leaves in a horizontal position and hischoice of short exposure times favored the production of maxi-mum differences between the amount of frost damage suffered byhorizontal and vertical leaves. The plants were exposed to night-time conditions for periods ranging from 30 min to 5 hr. Leavingthe plants outside for only part of the night considerably increasesthe chances that the horizontal but not the vertical leaves willattain killing temperatures. The use of corks to hold leaves hori-zontal substantially increases horizontal leaf-air temperaturedifferences (Fig. 2). The cork reduces convection by reducing therate of air flow past the lower surface of the leaf, especially if theleaf is pinned close to the cork. One of the horizontal leavespinned to a cork was permanently damaged by frost on October20, while none of the other leaves was permanently damaged. Thisis the only case in the whole series of experiments in which someof the leaves were damaged when others were not. Darwin himselfobserved that leaves pinned close to the cork suffered more frostdamage than the leaves pinned 1 to 1.5 cm above the cork. Theseobservations strongly suggest that Darwin's results are artifactsof his procedure and consequently can no longer be used as evi-

dence that vertical leaves may suffer significantly less frost damagethan horizontal leaves.Note Added in Proof. C. L. Wong (Ph.D. thesis, 1967) exam-

ined nocturnal energy budgets, extent of frost damage, andleaf temperatures in isolated horizontal and vertical primarybean leaves (Phaseolus vulgaris L.). He observed a maximalTlh - T1. of -1.7 C in leaves with D = 9 cm on cold, clearnights. The larger temperature differences are to be expectedsince the leaves were almost twice as large as those used in thepresent study. Wong also obtained differences in the extent offrost damage in horizontal and vertical leaves. However, mostof the leaves were exposed for 2 hr or less and none were ex-posed for the whole night. (C. L. Wong, 1967. Energy exchangeof plants in relation to their physical environment. Ph.D. thesis.University of New South Wales, Kensington, Australia.)

Acknowledgment-I thank Dr. Conrad S. Yocum for valuable advice and helpon all phases of this work.

LITERATURE CITED

1. BLNNING, E. 1959. Tagesperiodische Bewegungen. Encycl. Plant Physiol. 17/1:579-656.

2. COLE, J. S. 1967. Some effects of the environment on the temperature of tobaccoleaves in field plots in Rhodesia. J. Exp. Bot. 18: 254-268.

3. DARWIN, C. 1881. The Power of Movement in Plants. Reprinted 1966. DaCapoPress, New York.

4. GATES, D. M. 1964. Leaf temperature and transpiration. Agron. J. 56: 273-277.5. GATES, D. M. 1968. Transpiration and leaf temperature. Annu. Rev. Plant

Physiol. 19: 211-238.6. GATES, D. M. AND W. TANTRAPORN. 1952. The reflectivity of deciduous trees

and herbaceous plants in the infrared to 25 microns. Science 115: 613-616.7. GEIGER, R. 1966. The Climate Near the Ground. (translated from German

Scripta Technica, Inc.) Harvard University Press, Cambridge.8. GOEBEL, K. 1920. Die Entfaltungsbewegungen der Pflanzen und deren teleolo-

gische Deutung. Gustav Fischer, Jena.9. KONDRAT'YEV, K. YA. 1965. Radiative Heat Exchange in the Atmosphere.

(translated from Russian, 0. Tedder; translating ed., C. D. Walshaw) Per-gamon Press, New York.

10. MISHCHENKO, Z. A. 1966. The heat balance and temperature of plants. Trudy,Main Geophysical Observatory 190: 41-56. (translated from Russian, A.Nurklik) In: Meteorological Transl. No. 13 Canada-Department TransportMeteorological Branch, 1968.

11. PFEFFER, W. 1875. Die periodischen Bewegungen der Blattorgane. WilhelmEngelmann, Leipzig.

12. ScHwiNTZER, C. 1969. Adaptive significance of nyetinastic and related leafmovements. Ph.D. dissertation. University of Michigan, Ann Arbor.

13. SHAW, R. H. 1954. Leaf and air temperature under freezing conditions. PlantPhysiol. 29: 102-104.

14. STAHL, E. 1897. Ueber den Pflanzenschlaf und verwandte Erscheinungen. Bot.Zeit. 55: 71-109.

15. WEAST, R. C. AND S. M. SELBY, eds. 1967. Handbook of Chemistry andPhysics, 48th ed. Chemical Rubber Co., Cleveland.

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