UNKNOWN FACTORS 



45 



mon fact that the reserve food supply is not contained within 

 the organs of the seedling which are the first to show photo- 

 synthesis, suggest that the translocation of food materials 

 to the assimilating structure may be a causal factor in the lag 

 described ; it is, however, very doubtful if it be the only factor. 

 The accompanying table gives some of Briggs's results, in 

 which the carbon assimilation is represented in cubic centi- 

 meters of oxygen evolved per hour per 50 sq. cm. of surface 

 in a light intensity of 9000 lux. 



Helianthus . 



Ricinus 



Zea 



Dastur * suggests that the variation in the assimilation 

 numbers obtained by Willstatter and Stoll is due to loss of 

 water, a factor of great importance in photosynthesis. Further, 

 the irregularities observed in autumn leaves and also in normal 

 leaves may be explained by the difficulty in selecting leaves of 

 the same photosynthetic power, for this power is not neces- 

 sarily the same in leaves of apparently the same age or col- 

 lected at the same distance from the apices of the shoots, 

 notwithstanding the fact that they may appear equally healthy 

 and fresh. 



The Unknown Factor. — The fact that the temperature 

 relations of carbon assimilation are those of a chemical rather 

 than a photochemical reaction indicates the presence of an 

 internal factor, independent of the chlorophyll and associated 

 rather with the protoplasm, which controls the rate of carbon 

 assimilation. Irving concludes that this factor controls the 



* Dastur : " Ann. Bot.," 1925, 39, 769. 



