PHYSIOLOGICAL STUDIES 59 



a tendency to be smaller the greater the chlorophyll concen- 

 tration. Yellow varieties were found to have higher assimila- 

 tion numbers than leaves of the corresponding green 

 varieties (Table 4.1). Furthermore, if leaves of a green 

 variety were investigated during the course of a season it was 

 found that as the leaf aged the chlorophyll content increased 

 but the rate of photosynthesis increased to a lesser extent 

 (Fig. 4.9). Thus all these experiments suggested the exist- 

 ence of factors other than chlorophyll which varied from 

 plant to plant and mignt determine the rate of photosyn- 

 thesis at high light intensities. 



Internal factors other than chlorophyll 



The rate of development of such internal factors com- 

 pared with the rate of development of chlorophyll can be 

 investigated during the greening of etiolated leaves. Will- 

 statter and StoU found that during greening the assimilation 

 number fell, the first formed chlorophyll resulting in an 

 activity as great as that in some yellow varieties. Earlier 

 work by Irving and later by Briggs (1922) showed that the 

 development of the other internal factors might be more or 

 less rapid than the development of chlorophyll. Thus in 

 Phaseolus the young leaves may develop considerable chloro- 

 phyll and yet have a low rate of photosynthesis. If however 

 leaves are allowed to develop either in the dark or in the 

 absence of oxygen, so that no chlorophyll is formed, and 

 then allowed to green the photosynthetic activity per chloro- 

 phyll is much greater. During the period of growth prior to 

 greening it was supposed that an internal factor essential for 

 photosynthesis had been formed. 



Internal factors and mineral nutrition 



Chlorophyll deficient plants may be readily produced by 

 culture in media deficient in iron but such plants are likely 

 to be different not only with respect to chlorophyll but also 

 with respect to a whole variety of 'internal' factors. Thus 

 Chlorella cultures which are deficient in potassium or man- 

 ganese show a rapid increase in the rate of photosynthesis, 

 both at high and low light, when the respective elements are 



