180 PLANT PHYSIOLOGY 



^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 photosynthesizes 

 a much smaller percentage of the absorbed energy than its 

 chloroplasts would be able to use, this must be ascribed to sec- 

 ondary causes that prevent the mechanism from working to 

 full efficiency, such as the intensity being too great in direct 

 sunlight for synthesis of the carbon dioxide available, or injurious 

 effects on the plastid. 



That the plant utilizes the photochemical capacity of the 

 chlorophyll 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 the '' assimilation number," 

 is relatively low, while in those with light-green leaves it is com- 

 paratively high. In one of his experiments with etiolated bean 

 leaves that 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, as among 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 

 may be higher. Willstatter observed in one of his experiments 

 that the rate of assimilation was the same in the dark as in the 

 pale variety of elms, though in the former the chlorophyll content 

 was ten times greater. A similar relation was obtained by 

 Lubimenko with shade and light plants, the first of which had 

 darker leaves. The considerable increase of the assimilation 

 number, coincident with the decrease in amount of chlorophyll, 

 shows that there is always a certain surplus of chlorophyll in the 

 leaf. 



