CHAP, ii Photosynthesis 321 



synthesis shall not be allowed to take place. So far back as 

 1868 Boussingault showed that the power of appropriating 

 carbon dioxide gradually diminishes in a branch removed 

 from a plant, before any ill effects of the separation are 

 manifested. A similar observation was made by Saposch- 

 nikoff in 1890, when he noted that photosynthetic products 

 cannot increase beyond a certain amount in a leaf of Vitis. 



The ultimate application of the energy absorbed by the 

 leaf was inquired into by several observers. In 1868 

 Ed. Becquerel made the first reliable experiments on the 

 point and computed that in the case of the sunflower 

 about one two hundred and fiftieth part of the available 

 solar energy is stored by the plant in the potential form. 

 Two years later Timiriazeff estimated it at i per cent. 

 N. J. C. Miiller, using a more exact method in 1876, came to 

 the conclusion that under the most favourable conditions 

 it reaches 5 per cent. He exposed a pyroheliometer to 

 the sun's rays, side by side with a leaf in a condition to 

 effect photosynthesis. Some experiments made by Detlefsen 

 in 1888 are of considerable interest. He placed a thermo- 

 pyle immediately behind a leaf, which was exposed to 

 sunlight, and registered the difference of temperature which 

 it showed when photosynthesis was prevented by absence 

 of carbon dioxide and when it took place in air con- 

 taining 10 per cent, of that gas. He found that in the 

 former case there was a slight rise of temperature, but 

 only enough to indicate that the leaf had used a fraction of 

 the incident light, the amount varying from -3 to i-i per cent. 



Detlef sen's method was ingenious, but the possible sources 

 of error caused his results to be received with a certain 

 reserve. 



In 1894 Timiriazeff, using a similar arrangement to that 

 employed by Miiller, computed the amount used to be as 

 much as 3-5 per cent, of the total. 



Very careful estimations of the radiant energy absorbed 



GREEN V 



