524 



NA TURE 



[March 30, 1905 



to a less extent of the endothermic process of photo- 

 synthesis, and on the other hand by the losses due to 

 thermal emissivity, which even in still air are consider- 

 able, and may assume large dimensions if the air is in 

 movement. 



First, as regards the energy used in internal work, that 

 portion which produces the vaporisation of water, and 

 which I will denote by W, is determinable from the weight 

 of water lost by a given area of the leaf in a given time, 

 and from the known latent heat of water-vapour. On the 

 other hand, the amount of the absorbed energy which is 

 used up in the photosynthetic process, and which I will 

 denote by in, is deducible from the actual amount of 

 carbon dioxide which enters the leaf, on the legitimate 

 assumption that the synthesised product is a carbohydrate, 

 the heat of formation of which is approximately known. 



The generalised form of the thermal equation of a leaf 

 which is receiving solar radiation, and has acquired 

 a state of thermal equilibrium, may therefore be repre- 

 sented by Ra = (\V + -m) + r. 



When Ro is greater than W-)-«i, that is to say, when 

 the energy absorbed by the leaf in a given time Is more 

 than sufficient to perform the whole of the internal work, 

 r Is a positive quantity, and represents in absolute units 

 the sum of the losses due to radiation and convective cool- 

 ing, and it is the only portion of R which can produce 

 a rise of temperature in the leaf. 



Provided we know the thermal emissivity of the par- 

 ticular leaf which we are using, the actual rise of tempera- 

 ture of the leaf-lamina above Its surroundings can be de- 

 termined from r; for if e Is taken to represent the 

 emissivity, then the temperature difference between the 

 leaf and its environment, that is to say, i' —i, will be 

 r/je. 



On the other hand, when Ra is less than W-nu, that 

 Is to say, when the absorbed radiant energy is insufficient 

 to perform the whole of the internal work, r is a negative 

 quantity, and the excess amount of energy requisite to 

 perform the internal work must be drawn from the 

 surroundings of the leaf ; in otjier words, when thermal 

 equilibrium is established the temperature of the leaf under 

 these conditions must be below that of its surroundings. 

 Here again, however, the thermometric difference expressed 

 by (- (' will be r/2e. 



The true measure of the photosynthetic work effected by 

 suitable radiation is, strictly speaking, not given exactly 

 by the amount of atmospheric carbon dioxide absorbed by 

 the leaf, but by this amount plus the small amount of 

 carbon dioxide which would have been evolved by re- 

 spiration if photosynthesis had been in abeyance. This is 

 a correction which has to be taken Into account in certain 

 special cases, but it does not affect the generalised thermal 

 equation I have given, since the heat of respiration is 

 opposite in sign to that of the heat of re-formation of the 

 carbohydrate, and these values, representing a concurrent 

 gain and loss of energy by the leaf, must exactly balance 

 each other if the carbohydrates standing at the two ends 

 of the reversed process are identical, and if they are not 

 Identical the difference in their thermal relations must be 

 so small as to be inappreciable. 



Before proceeding to show how these general views can 

 be applied to the construction of a revenue and expenditure 

 account of energy for a leaf, I must briefly refer to the 

 mode in which the various factors have been determined. 

 We have already considered the manner in which the 

 thermal emissivity of the leaf e is determined, a value 

 which Is all-important in considering the temperature of 

 the leaf, and I have also sufficiently indicated how we 

 can determine the work of transpiration, and consequently 

 the value of W. 



R, the intensity of the solar radiation falling on the 

 leaf-surface, was measured by means of a specially con- 

 structed Callendar's radiometer, the coils of which were 

 enclosed In a flat rectangular rase mounted on an adjust- 

 able stand so that the orientation of the receiving surfaces 

 could be made to correspond with that of the leaf under 

 experiment. The radiometer was connected with a 

 f'allendar's recorder furnished with a planimeter which 

 automatically integrated the curve recorded on the drum. 



The constants for the instrument were determined for 

 us by Prof. Callendar, and the planimeter readings were 



NO. 1848, VOL. 71] 



readily convertible into water-gram-units of energy in- 

 cident on unit area of surface in unit of time, thus giving 

 a mean value of R. 



The proportions of the radiant energy of sunlight re- 

 spectively absorbed and transmitted by the leaf-lamina 

 were determined with the same instrument by observing 

 in steady sunlight the amount of radiation which reaches 

 the radiometer with and without the interposition of the 

 leaf. This gives a measure of the coeflficient of absorp- 

 tion of the leaf a with a close approach to accuracy, if we 

 neglect the amount of reflected radiation, which is very 

 small in cases of perpendicular incidence. U 



The coefficient of absorption, as might be expected, varies ' 

 considerably with leaves of different species of plants, as 

 shown in the following table, and there are also small 

 individual differences in leaves of the same plant. 

 Coefficients of Absorption (a) and Transmission (i— a) of 

 the Radiant Encrt;y of Sunlight for Leaves. 



Coefficient ot CoefiicieDt of 



Plant absorption transmission 



(a, U-a) 



Helianthus annuus 0686 0-314 



Polygonum Weyrichii 0-647 °'353 



Polygonum Sachalincnse ... o-6gi 0'309 



Petasites officinalis 0728 0272 



Silpliium terebrinthaceum ... 0-699 0'39' 



.Arctium majus 0-728 0-272 



Vcrbascum olympicum ... 0-758 0-242 



Senecio grandifolius 0.774 0226 



In the generalised thermal equation the value w, repre- 

 senting the amount of energy expended in photosynthesis, 

 measures the effective internal work of a useful and con- 

 structive kind, for the due performance of which the leaf 

 may be said to exist, and the relation which this bears 

 to the total energy flowing into the leaf gives an estimate 

 of the true economic coefficient when the leaf is regarded 

 as a thermodynamic engine. 



In the five or six years during which these researches 

 occupied Mr. Escombe and myself at the Jodrell Labora- 

 atory, a large share of our attention was given to deter- 

 mining the best means of estimating the rate of photo- 

 synthesis in green leaves exposed to sunlight in air con- 

 taining the normal amount of carbon dioxide. 



.\t the time we commenced our experiments the only 

 practical method was a gravimetric one introduced by 

 Sachs, by which the amount of material assimilated by a 

 leaf in a given time is deduced from variations in the 

 dry weight of known areas of the leaf-lamina. Unfor- 

 tunately, we found that the errors to which this method 

 is liable tend on the whole too much in one direction, 

 and their sum, which frequently exceeds the value we are 

 trying to estimate. Is swept into the final result. 



The method which we finally adopted was one based 

 on the measurement of the intake of carbon dioxide at a 

 partial pressure somewhere near that at which it exists 

 in normal air, i.e. 3/10,000 of an atmosphere. 



It is evident that such experiments must be conducted 

 on a relatively large scale, both as regards the area of 

 leaf-surface exposed and the volume of air passed over it. 



(The nature and disposition of the apparatus were shown 

 In a diagram on the scrim.) 



The leaf, which. If desired, may still remain attached to 

 its plant, is enclosed in a glazed case through which a 

 stream of air is drawn by a water-pump, the 

 volume of the air being measured by a suitable 

 meter. Between the leaf-case and the meter there 

 is a Reiset's absorption-lube filled with a solution of 

 caustic soda, which ensures the complete absorption of the 

 carbon dioxide remaining after the air has passed through 

 the case. 



.\ duplication of the meter and absorption apparatus 

 allows of a simultaneous determination of the carbon 

 dioxide In the air before it passes over the leaf, and the 

 difference between these values measures the carbon dioxide 

 taken up by the leaf. Tnis Is referred to unit area of the 

 leaf by measuring, by means of a planimeter, the area of 

 the photographic impression of the lamina on sensitised 

 paper. 



A very delicate method was used for titrating the 

 absorbed carbon dioxide in the alkali, and when all proper 

 precautions are taken the errors of experiment are small. 



