148 



PHOTOSYNTHESIS 



These facts point to the existence of some other internal factor and give 

 weight to the theory that the rate of photosynthesis, under constant and 

 favorable external conditions is determined not only by the chlorophyll- 

 content but also by another internal factor. The latter apparently is more 

 rapid in development than chlorophyll, resulting in a high Pc during the 

 first days of the life of the leaf. After full development of the leaf and 



TABLE 25 



Rate of Photosynthesis, Chlorophyll-Content and Photosynthetic Number, 

 AT 25°, 5 Per Cent CO2, About 48,000 Lux, Determined by 



WiLLSTATTER AND StOLL. 



increase in the chlorophyll-content this other factor is relatively less active 

 than the chlorophyll factor. 



Very similar results vi^ere obtained by Willstatter and Stoll in a study 

 of the photosynthetic activity of leaves of diiTerent ages. They compared 

 the activity of a light-green leaf from the end of a branch with that of a 

 dark-green leaf from the base of the same branch. Some examples of 

 the differences between old and young (but almost full grown) leaves are 

 given in Table 26. 



The results of Willstatter and Stoll of the photosynthetic activity of 

 leaves in different stages of development show that although the chloro- 



