CHLOROPHYLL AND THE CHLOROPLASTS 369 



shade leaves have a relatively higher chlorophyll-content than leaves grown 

 in direct sunlight. This relationship may not hold, however, on the basis 

 of leaf surface, for shade leaves are often very thin. The ratio of the 



two chlorophyll components, designated by Willstatter and Stoll ^^ as Qt-» 



undergoes no appreciable change during the course of the day. 



The carotinoid-content ranges from 0.07 to 0.2 per cent of the dry 

 weight and 0.03 to 0.07 gram per square meter of leaf surface. The 



c 



ratio of carotin to xanthophyll, Q— , exhibits no great variations; it is on 



the average 0.60 with a maximum variation of ± 0.1 ; to each mole of 



carotin there are 1.5 moles of xanthophyll. In general leaves contain 



more chlorophyll than carotinoids, though the ratio of the chlorophyll to 



a + b 

 the yellow pigments, Q , may exhibit considerable variation, from 0.15 



in yellow autumn lea\es to 4.68 in shade leaves. It should be noted that 

 the carotin and xanthophyll-content of leaves does not change greatly 

 during the course of their development, and that the yellow color of 

 autumn leaves is, in part at least, due to water soluble yellow pigments, 

 of unknown chemical composition. 



e. The Role of Chlorophyll in Photosynthesis. 



That the presence of chlorophyll is essential for photosynthesis was 

 recognized by the earliest investigators of the phenomenon, Senebier, 

 Ingen-Housz and de Saussure. This fact has been repeatedly verified and 

 there exists no evidence which could raise much doubt regarding the 

 necessity of chlorophyll for photosynthesis. Certain bacteria perhaps 

 constitute an exception to this, though the nature of the entire respiratory 

 functions as well as the manner of carbon dioxide reduction in these 

 organisms is probably different from that of chlorophyllous plants. The 

 well-established facts are these : that photosynthesis has been observ-ed to 

 occur only in cells containing chlorophyll, that in albino portions of a 

 leaf no photosynthesis occurs. The latter portion of the argument should 

 perhaps be accepted with some caution, for it is possible that the stroma 

 of the chloroplasts of albino cells may differ from that of chlorophyll- 

 bearing cells in other respects besides chlorophyll-content. It is true that 

 the chloroplasts of albino and chlorotic cells are capable of forming starch 

 from sugar, but this fact does not entirely meet the objection raised to 

 the argument. The problem cannot be studied by the use of etiolated chloro- 

 plasts, because in these chlorophyll is formed very rapidly on exposure 

 to light. Many theories have been suggested to describe the function of 

 chlorophyll in photosynthesis, some of these are not worth discussing 

 and few of them have been tested experimentally. In fact, the problem 



" Willstatter and Stoll, "Untersuchungen iiber Chlorophyll," p. 109. 



