ASSIMILATION OF CARBON BY PLANTS 



163 



of different kinds of sugars. Exact quantitative determinations 

 show that usually not over 30 to 50 per cent of the products of 

 assimilation are deposited in the form of starch, the rest being 

 sugars and other compounds. Still less starch is formed in the 

 leaves of wheat. In many plants, e.g., the onion, tulip. Arum, 

 etc., the process of assimilation terminates with the formation 

 of sugars. Such plants are called ''sugarformers" in distinction 

 to the usual starch formers. The accumulation of starch must 

 be regarded as a secondary process, the purpose of which is to 

 withdraw as soon as possible the soluble sugars from the sphere 

 of the primary assimilative reaction, since the accumulation of 



Fig. 47. — Prints from negatives on leaves made by means of a starch test (from 



Molisch) . 



soluble end products always checks the progress of the reaction 

 (Sapozhnikov effect). The accumulation of starch has no direct 

 connection with the process of carbon assimilation. Experiments 

 have shown that it may go on in leaves at the expense of sugar 

 obtained from without, e.g., when leaves are placed in a sugar 

 solution and kept in a dark room. 



The accretion of organic substance, sugars or starch, in the 

 process of assimilation represents but one side of this most 

 important process. These substances may be burned with 

 liberation of heat energy, as in the case of burning wood or coal. 

 When vegetable products are used for food, they are oxidized 

 within the body through respiration, and energy is obtained from 

 them, which is used in maintaining the temperature of the body 

 at a certain level (about 37°C.), as well as in work performed. 

 Assimilation is not only a process for the accumulation of organic 



