32 TEXTBOOK OF PLANT PHYSIOLOGY 



greater quantity of radiant energy than a similar leaf in an atmos- 

 phere free of carbon dioxide; the difference between these values 

 will show the amount of energy utilized in photosynthesis. All 

 these determinations show, on the whole, concordant results, 

 namely, that the coefficient of utilization of radiant energy by the 

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

 and only in exceptional cases as high as 10 per cent of the whole 

 absorbed energy is required in photosynthesis. The remaining 

 90 to 99 per cent passes over directly into heat energy and is utilized 

 for the evaporation of water in the process of transpiration. 



Recent determinations made by Warburg, with single-celled 

 algae suspended in water, have given a considerably higher coef- 

 ficient of utilization of radiant energy absorbed by the chloro- 

 plasts. With feeble illumination, a coefficient of 50 or even 60 

 per cent was obtained. This important fact shows that the 

 chloroplast is a very perfect photochemical mechanism. If under 

 natural conditions the leaf of a plant uses a much smaller percentage 

 of the absorbed energy than its chloroplasts would be able to use, 

 this must be ascribed to secondary causes, preventing the mechan- 

 ism from working to full efficiency. 



That the plant utilizes the photochemical capacity of the 

 chlorophyll but insufficiently is shown by another interesting fact. 

 The amount of carbon dioxide decomposed per unit time and 

 per unit weight of chlorophyll is variable. In plants with dark- 

 green leaves, this amount, termed " assimilation number," is 

 comparatively high. Thus, for instance, in one of his experiments 

 with etiolated bean leaves which were exposed to light and had 

 acquired gradually a dark-green color, Willstatter observed, on 

 the first day, an assimilation energy per unit surface of 40 mg. ; 

 on the third day, 96 mg.; and on the fifth, 104 mg. Hence, 

 while the green color was being produced, assimilation increased 

 2.5 times. But the amount of chlorophyll increased during the 

 same period 16 times, while the assimilation number was falling 

 continually from 133 on the first day to 24 on the third and to 13 

 on the fifth. 



Similarly, if we take different varieties of plants of the same 

 species, distinguished from one another by darker or paler leaf 

 color, it must not be expected that the varieties with darker 

 leaves will assimilate more energetically than those with pale 

 leaves, for in the pale-green leaves the assimilation number is 



