1903.] The. Co*mir.cd Function oj the Green Plant. 447 



Edmond Becquerel, in his already cited work 'La Lumiere, ses 

 Causes et ses Effets,' was the first to attack the question of the storage 

 of the solar energy in the green plant ; he made the first attempt to 

 obtain an approximate value for what Mr. Horace Brown has recently 

 so appropriately called the economic coefficient of the photo-chemical 

 process. Edmond Becquerel arrived at a very moderate estimate. 

 The potential energy represented by the organic matter of a culture 

 of sunflower is only 4/1000 of the available solar energy ; in a forest 

 it does not surpass 1/1000. A couple of years later, in 1871, I applied 

 the same calculation to more definite physiological data and obtained 

 about 1 per cent. In 1876 my friend Professor N. Miiller made for 

 the first time a direct experiment, the leaf which decomposes the 

 carbon dioxide and the pyrheliometer being placed side by side. 

 His estimate is perhaps somewhat too high, being 5 per cent., which 

 I think must be attributed to the fact that the pyrheliometric data 

 seemed to be too low, when compared with the numbers generally 

 accepted. In 1894 I repeated the &ame experiment by placing the leaf 

 and the pyrheliometer in exactly the same conditions, as will be 

 presently described, and obtained the value 3J per cent. It must 

 be noted that in Miiller's and my experiments the leaves were placed 

 in an artificial medium, in a gaseous mixture containing from 5 10 

 per cent, of carbonic acid gas, this mixture giving the highest 

 effect. Mr. Horace Brown has more recently made a highly interest- 

 ing and much more difficult determination of the same value, placing 

 himself in more natural conditions by using a current of atmospheric 

 air with its normal content of carbon dioxide. The result was 

 0'5 per cent., but in one case (in which the air had been enriched 

 with carbon dioxide to the extent of about 5*5 times the normal 

 amount) the efficiency of the leaf was raised from 0'5 2'0 per cent. 



Now that we are quite sure that only those rays that are absorbed 

 by chlorophyll effect the reduction of the carbon dioxide, it is evident 

 that this economic coefficient must principally depend on the degree 

 of absorption of light by the green matter of the leaf. It is useless to 

 insist how important it is to know this fraction of the total radiation 

 available to the plant. The following thermoscopic method which I 

 adopted in 1884 seems to me still the surest and simplest way of esti- 

 mating this quantity. The problem, as it presents itself in a real leaf, 

 is highly complicated, because of the scattering effect of the cells and 

 chloroplasts, very similar to that presented by the silver deposit in 

 the photographic plate so ingeniously explained by Sir William Abney. 

 But the problem may be considerably simplified, if for a real leaf we 

 substitute what may be called a liquid leaf. 



If using a steel punch we cut out of a leaf a certain area and 

 dissolve its green matter in a volume of alcohol just sufficient to fill 

 ;i glass cell of exactly the same vertical section, we obtain a liquid 



2 I 2 



