522 



NATURE 



[March 30, 1905 



the whole ground of physical science, and are in many 

 instances of the greatest importance, but generally have 

 reference to definite researches undertaken by individuals, 

 not calling for wide cooperation. A list of papers, prepared 

 possibly to pave the way for future applications, is added, in 

 which are discussed the conditions of solar research at 

 Mount Wilson, by Prof. Hale : the southern observatory 

 project, by Prof. Boss ; fundamental problems of geology, 

 by T. C. Chamberlin ; plans for obtaining subterranean 

 temperatures, by G. K. Gilbert ; magnetic survey of the 

 Pacific Ocean, by L. A. Bauer ; and geological research in 

 Eastern Asia, by B. Willis. 



THE RECEPTION AND UTILISATIOh OF 

 ENERGY BY A GREEN LEAF> 

 'T'HE subject of my lecture is derived from the series of 

 •*• papers laid before the society to-day by my colleagues 

 and myself, dealing with some of the physiological pro- 

 cesses of green leaves. In giving an account of some of 

 these investigations I shall dwell mainly on their relation 

 to the energetics of the leaf, and shall endeavour to show 

 how the leaf behaves under various conditions when re- 

 garded from the point of view of the exchange of energy 

 between itself and its surroundings. 



One of the problems which we attempted to solve was 

 to draw up a " revenue and e.xpenditure account " of 

 energy for a green leaf, showing the proportion of the 

 incident energy absorbed, the amount of this absorbed 

 energy which is used up for the internal work of the leaf, 

 and the proportion which is dissipated by re-radiation and 

 the losses due to the convective and conductive properties 

 of the surrounding air under varying wind-velocities. 



Of these various factors, the one I have last mentioned, 

 which presupposes a knowledge of the thermal emissivity 

 of the leaf-surface, presented by far the greatest difficulty ; 

 but during the past year Dr. 'W. E. Wilson and I have 

 been able to devise a suitable method for determining the 

 thermal emissivity of a leaf-surface in absolute units, so 

 that our story is now fairly complete. 



The discussion of the thermal relations of a leaf to its 

 surroundings will be simplified if we first consider the case 

 of a leaf when it is shielded from solar radiation. We 

 will assume that a detached leaf, freely supplied with 

 water, is placed in an enclosure the wails of which are 

 non-reflective and are maintained, along with the enclosed 

 air, at a perfectly uniform temperature /. We will further 

 assume that the air is saturated with water-vapour. 



Under these conditions the system would remain in 

 thermal equilibrium if it were not for the respiratorv pro- 

 cesses going on within the leaf-cells. These are exothermic 

 in ^}^f"_ fina' result, so that the state of complete thermal 

 equilibrium can only be attained when the temperature 

 of the leaf has risen to a point (', somewhat higher than 

 t. The magnitude of the difference t' —t, representing the 

 maximal thermometric disturbance between the leaf and 

 its surroundings, will depend on three main factors : — 



(i) On the rate of evolution of the heat of respiration. 



(2) On the rate at which this heat is dissipated by the 

 thermal emissivity of the leaf-surface, and, 



(3) On the magnitude of the slight rise of partial 

 pressure of the water-vapour in the interspaces of the leaf, 

 which gives rise to a certain amount of diffusion of water- 

 vapour through the stomata. 



The rate of evolution of the heat of respiration can be 

 deduced with sufficient exactness from the amount of 

 carbon dioxide liberated per unit area of the leaf-lamina in 

 unit of time, since there is evidence that the carbon 

 dioxide proceeds from the oxidation of a carbohydrate with 

 a heai of combustion which cannot be far removed from 

 3760 calories per gram. Taking the concrete example of 

 a leaf of the sunflower respiring at the rate of 070 c.c. 

 of carbon dioxide per square decimetre per hour, it can 

 be shown that the heat of respiration in this case amounts 

 to about 000582 calorie per square centimetre of leaf- 

 lamina per minute. From the known weight of a square 

 centimetre of the leaf-lamina, and its specific heat, this 



1 The Bakerian lecture, delivered at the Royal !jo:icly, March 23, by 



IJr. Horace T. Brown, F.R.S. 



NO. 1848, VOL. 71] 



spontaneous liberation of energy within the leaf might 

 conceivably raise its temperature through o°-033 C. per * 

 minute, provided there were no simultaneous losses due to 

 radiation, conduction and convection of the surrounding 

 air, and internal vaporisation of water. .All these sources 

 of loss, of course, become operative immediately the 

 temperature of the leaf exceeds that of its surroundings. 

 We shall see presently that the thermal emissivity of this 

 leaf in still air is 0015 calorie per square centimetre of 

 leaf-surface per minute, for a difference of temperature of 

 1° C. between the leaf and its surroundings, so that the 

 temperature of the leaf, under the conditions postulated, 

 cannot exceed that of its surroundings by more than 



000582/2 xooi5 = o°oi9 C. 



But this is assuming that transpiration has been in abey- 

 ance, which is certainly not the case, so that this small 

 temperature difference of o'-oig C. will be still further 

 reduced. 



The main point which 1 wish to bring out here is that 

 the thermometric disturbances due to the processes of re- 

 spiration are very small, so small, in fact, that they may 

 be neglected in considering the large disturbances induced 

 by other causes. 



Let us now suppose our leaf to be placed under the 

 same conditions as before, but in air which is not fully 

 saturated with aqueous vapour for the temperature (. 



The conditions are manifestly unstable owing to the 

 excess of the partial pressure of the water-vapour in the 

 saturated air of the interspaces of the leaf over that of 

 the vapour in the unsaturated air outside. 



The diffusion-potential thus set up will result in water- 

 vapour passing outwards through the stomata, and the 

 temperature of the leaf will fall. This fall will continue 

 until the gradient of temperature between the surroundings 

 and the leaf is sufficiently steep to allow energy to flow 

 into the leaf from without at a rate just sufficient to pro- 

 duce the work of vaporisation, at which point a steady 

 thermal state will be established which will remain con- 

 stant so long as other conditions are unaltered. The leaf 

 will then have assumed a temperature (', which in this 

 case w'ill be lower than that of its surroundings. 



Now it is manifest that when this steady thermal con- 

 dition has been attained, the amount of water vaporised 

 per unit of area of the leaf in unit of time must be a 

 measure of the energy flowing into the leaf for the 

 gradient of temperature represented by t — t', and pro- 

 vided we determine the amount of water lost by the leaf, 

 and the temperature difference between the leaf and its 

 surroundings under the steady conditions, we have all th» 

 data necessary for finding the coefficient of thermal 

 emissivity of the leaf-surface in absolute units, that is to 

 say, the rate at which a leaf-surface will emit or absorb 

 energy from its surroundings in still air for a difference 

 of temperature of 1° C. 



Following out this idea, Dr. Wilson and I have success- 

 fully determined the constants of thermal emissivity for 

 leaves of different kinds, both under " still-air " conditions 

 and in air-currents of determinate velocity. The results 

 are interesting from several points of view, since amongst 

 other things they enable us to estimate the rate at which 

 the excess of solar radiant energy falling on a leaf is 

 dissipated by mere contact with the air moving at any 

 ordinary wind-velocity, and they also give us, under certain 

 conditions, a means of deducing the actual rate of trans- 

 piration from mere observations of temperature-differences. 



Before proceeding to show more in detail the manner in 

 which the thermal emissivity of a leaf is determined, we 

 will turn for a moment to the magnitude of the difference 

 of temperature between a leaf and its surroundings which 

 may be expected from a given rate of transpiration. We 

 will assume that the leaf of a sunflower, transpiring into 

 tht unsaturated air of the enclosure, when the steady 

 thermal condition is attained, is losing water at the rate 

 o' "S gram per square decimetre per hour, or 00000833 

 gram per square centimetre per ininute. 



The heat required to vaporise this amount of water at 

 20° C. is 00000833x502(1 = 004938 calorie, which, on the 

 theory of exchanges, must represent the amount of energy 

 entering and leaving a square centimetre of the leaf- 

 lamina per minute. The thermal emissivity of this leaf 



