1260 



THE PIGMENT FACTOR 



CHAP. 32 



of two Chlorella suspensions containing the same number of cells of identi- 

 cal volume, but with a different content of chlorophyll in each cell. The 

 behavior will differ for two suspensions with identical chlorophyll content 

 in their cells, but with a different number of cells per unit volume — a case 

 illustrated by fig. 28.20(1), and, for a concentration ratio of 1:2, by fig. 

 32. IB. The broken curve in this figure begins with the same slope as the 

 lower solid curve (presuming the absence of a "sieve effect") ; but at 

 higher intensities, the ratio [P (dense suspension)/P (dilute suspension)] 

 increases to two, while the ratio [P (suspension of strongly pigmented cells)/ 

 P (suspension of weakly pigmented cells) ] decreases to unity. Whenever the 



J or [Chl] J 



Fig. 32.1. (A) Light curves ( — ) and chlorophyll curves (~) of photosynthesis for two 

 values of a parameter such as [CO.] or temperature. (B) Light curves of photosynthesis; 

 solid curves, two equally dense cell suspensions, one twice as rich in chlorophyll as the 

 other; broken curve, chlorophyll-rich suspension, diluted to one half the original 

 density. 



observed effect of the [Chl] factor on photosynthesis agrees qualitatively 

 Avith the picture presented by the solid curves in figure 32. IB, it can be 

 attributed tentatively to change in light absorption. In this case, the 

 effect should gradually disappear with increasing intensity of illumination. 

 In some experiments, however, a strong dependence of the rate of photo- 

 synthesis on the concentration of chlorophyll was found also in intense light; 

 in these cases, the chlorophyll concentration must have influenced photo- 

 synthesis not, or not only, by affecting the light absorption, but also in some 

 other way. 



In interpreting observations of this kind, one has to keep in mind that 

 there is no simple way of varying the chlorophyll concentration in living 

 cells. We are restricted in such studies to the use of natural specimens 

 with different contents of the pigment, and of plants in which the pigment 

 content has been affected by abnormal conditions of culture and growth, 

 e. g., deficiency of light, or lack of certain nutrient elements. All these 

 treatments are apt to cause manifold changes in the composition and 

 structure of the cells, and not merely variations in chlorophyll content. 



