106 GROWTH AND WORK OF PLANTS 



the formation of food products which are later used by the plant 

 and are also used as food by animals. There is a tendency in 

 recent years on the part of some botanists to distinguish the kinds 

 of assimilation. Those kinds of assimilation which relate to the 

 making of the new life substance, or the making of structures 

 which are part of the living organism, are looked upon as true 

 assimilation. Those processes which result in making food 

 products are called synthetic assimilation. The making of sugar 

 and starch (carbohydrates) is called photo synthetic assimilation, 

 because the sun supplies the energy for the initial stages in the 

 process. The making of carbohydrates by the nitrite and nitrate 

 bacteria is called chemo synthetic assimilation, because the chemi- 

 cal process or metabolism in changing ammonia compounds to 

 nitrites and these into nitrates by these bacteria, gives them the 

 energy to fix the carbon from the CO 2 . 



177. Metabolism. Metabolism means change, or changing 

 around, and in the life processes of animals and plants refers to the 

 chemical changes taking place. The building up processes, the 

 different kinds of assimilation, are constructive metabolism (an- 

 abolism), while the breaking down processes, respiration, fermen- 

 tation, decay, etc., are destructive metabolism (katabolism). 



178. The building up of proteids in the plant. The proteid 

 substances in plants have a more complicated molecule than the 

 carbohydrates since they contain C, H, O, N, S and sometimes P. 

 These are largely formed in the leaf, but are formed in other parts 

 of the plant also, and perhaps can be formed in limited extent in 

 any living plant cell. Some plant physiologists claim that they 

 are formed only in sunlight and in the presence of chlorophyll.* 



* One reason for this belief is that in some plants it has been found 

 that the nitrates (salts of nitric acid = HNO 3 ) which are absorbed by the 

 roots accumulate in the leaves during the night, and disappear during the 

 day. During the day the nitrates unite with carbohydrates (sugar) and 

 some sulphur compounds to form proteids. Those who believe that pro- 

 teids are also formed during the night concede that they are formed more 

 actively during the day because photosynthesis is then going on, since the 

 formation of carbohydrates is then active and in their elemental condition 

 may more easily unite with the nitrates. 



