THE PROCESS OF ASSIMILATION 137 



formed. In all seeds in which the food is not stored in the 

 first assimilating organ assimilation lags behind chloro- 

 phyll formation. This is the case in the French bean, 

 where the food store is in the cotyledon and the first assimi- 

 lating organ is a foliage leaf ; in the castor oil plant, where 

 the food is stored in an endosperm and the first assimilating 

 organ is the cotyledon ; and in the cereals, where the food is 

 stored in an endosperm and the first assimilating organ is a 

 foliage leaf. In all these cases a transfer of food must take 

 place from the storage organ to the assimilating leaf, and 

 photosynthesis is more or less delayed, Briggs points out 

 the interesting fact that, in so far as getting a start with the 

 work of assimilation is concerned, the least specialised type 

 of embryo, that in which the same organ functions as food 

 store and assimilating organ, is the most efficient. 



V^^illstatter believes that the factor which develops more 

 slov/ly than chlorophyll is an enzyme, and is concerned with 

 the later stages of the assimilating process carried on by the 

 protoplasm, as distinct from the early stages in which light 

 and chlorophyll are effective. Briggs, however, brings 

 forward evidence to prove that the factor in question affects 

 both the " light " (or chlorophyll, or photochemical) stage, 

 and the " dark " (or protoplasmic, or chemical) stage of the 

 process. He suggests that " The conception of the process 

 of photosynthesis which seems best to fit the facts is that 

 the seat of the process is the surface of the chloroplast, for 

 here the proportion of light absorbed will tend to be greater, 

 and this will be the portion of the chloroplast with which 

 the carbon dioxide will first come into contact. To give 

 more detail to the picture, we may postulate that the photo- 

 chemical phase of the process consists of an activation, by 

 means of light energy, of the molecules of the surface of the 

 chloroplast either before or after combination with carbon 

 dioxide, and that the chemical phase is an interaction of 

 such activated molecules." This " reactive surface " of 

 the chloroplast is not necessarily the actual surface area ; 

 it may be the " internal " surface of a colloid. It is the 

 development of the full reactive surface which is required 



