ASSIMILATION OF CARBON BY PLANTS 179 



as potential energy of the accumulated organic compounds, and 

 what part is utilized in other processes or given off to the environ- 

 ment. Determinations of the radiant energy that falls on the 

 leaf and that passes through it show that on the average about 

 75 per cent of it is absorbed, while 25 per cent, especially the 

 green and red rays, are either reflected or transmitted. From 

 this it does not follow, however, that all of the absorbed energy 

 is utilized by the leaf in photosynthesis. The amount of energy 

 that is actually used may be determined in various w^ays. The 

 total quantity of matter accumulated by the leaf during a certain 

 interval of time may be determined and compared with the 

 amount of energy absorbed during the same time. The method 

 of using the two halves of a leaf as checks, described previously 

 in Art. 38, may be conveniently used for this purpose. The 

 increase in dry weight per unit leaf area will show, as has been 

 noted, the amount of dry matter accumulated. And if the heat 

 of combustion is known or determined, it is possible to calculate 

 the percentage of energy stored. A number of valuable studies 

 have been made according to this method by Krasheninnikov. 

 Another method is to evaluate the amount of carbon dioxide 

 decomposed by the leaf during a certain period without injuring 

 the leaf. Since the amount of energy required for this process 

 is known, it is easy to compare it with the total, amount of radiant 

 energy absorbed by the leaf during a certain period. This pro- 

 cedure was used by the English scientists Brown and Escombe. 

 The third method, employed by Purievitsch, is based on the fact 

 that a leaf decomposing carbon dioxide absorbs a greater quantity 

 of radiant energy than a similar leaf in an atmosphere free from 

 carbon dioxide. The difference between these values will show 

 the amount of energy utilized in photosynthesis. All these 

 determinations give concordant Results. The coefficient of 

 utilization of radiant energy by the plant in photosynthesis has 

 a comparatively low value. Usually from 1 to 5 per cent, and 

 only in exceptional cases is as much as 10 per cent, of the total 

 energy absorbed, used in photosynthesis. The remaining 90 to 

 99 per cent is transformed into heat and is utilized for the 

 evaporation of water in transpiration. 



Recent determinations made by Warburg with single-celled 

 algae suspended in water have given a considerably higher 

 coefficient of utilization of radiant energy absorbed by the chloro- 



