THE NATURE OF PHOTOSYXTHESIS 145 



a quantitative analysis of the influence of the various factors afifecting 

 photosynthesis under field conditions. 



A great advance in the knowledge of the chlorophyll factor in photo- 

 synthesis is due to the researches of Willstatter and Stoll/^^ These 

 workers, on the basis of their thorough investigations of the chemistry 

 of chlorophyll and other plant pigments devised reliable quantitative 

 methods for the determination of chlorophyll, a contribution of the first 

 importance toward the solution of these problems. A description of these 

 methods will be found in Chapter 7 of this book. 



According to Willstatter and Stoll chlorophyll is composed of two 

 components designated as a and h : 



Chlorophyll a: [MgN4C3oH3oO ICO.CHs . CO0C20H39 

 Chlorophyll /' : [MgN^CaoH^sOojCOoCHs . CO0C00H39. 



Accompanying the chlorophyll pigments in the chloroplasts there are 

 two yellow pigments, carotinoid c and x : 



Carotin : C40H56, Xanthophyll : C4oHg602. 



The chlorophyll-content of leaves ranges as follows : 



0.15-0.35 gram per 100 grams fresh weight. 

 0.6-1.2 gram per 100 grams dry weight. 

 0.3-0.7 gram per 1 sq. m. leaf surface. 



The yellow pigments range from 0.07 to 0.20 per cent of the dry 

 weight or about 0.0 to 0.07 gram per sq. m. of leaf surface. The ratio of 



c 



the two yellow components, O — , is fairly constant, equal on an average 



.X. 



to 0.60. The ratio of the chlorophvH components, O — , also varies but 



~ b 



slightly and is equal to 2.9. 



In this work of Willstatter and Stoll there are two points of particular 

 importance bearing on the question of the relation of chlorophyll-content 

 to photosynthetic rate. These are: 1, that the chlorophyll-content does 

 not change during the course of photosynthesis, and 2, that the ratio of 

 chlorophyll a and b also remains nearly constant during photosynthesis. 



The second point is about as significant as the first, for in most of the 

 subsequent analyses the two components are determined together. If 

 during photosynthesis the ratio of the two components showed a great 

 variation, it would not be permissible to compare mixtures composed of 

 the two. Willstatter and Stoll studied the relation of photosynthesis to 

 chlorophyll-content of leaves in different stages of development and in 

 leaves which exhibit rather extreme conditions such as yellow varieties, 



"* Willstatter and Stoll, "Untersuchungen iiber die Assimilation der Kohlensaure," 

 Berlin, 1918. 



