16 TEXTBOOK OF PLANT PHYSIOLOGY 



products, sugars, from the sphere of the primary assimilative 

 reaction, since the accumulation of soluble end products always 

 checks the progress of the reaction. 



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 outside, 

 for instance, when leaves are placed in a sugar solution and kept 

 in a dark room. 



The first product of assimilation therefore is not starch but 

 sugar, most probably some hexose. Hence, the reaction of 

 assimilation may be written as follows: 



6C0 2 + 6H2O -> C 6 Hi 2 G + 60 2 



This reaction, of course, is but a scheme, because the process of 

 transformation of carbon dioxide and water into carbohydrates 

 passes through several intermediate reactions. These reactions 

 have not been sufficiently determined as yet. The theories refer- 

 ring to them will be examined further on, after the internal and 

 external conditions of assimilation have been studied. 



The accumulation of organic substance, sugar or starch, in the 

 process of assimilation, represents but one side of this most 

 important process. The organic substance obtained may be 

 subjected to combustion, in which a certain amount of energy is 

 eliminated. Thus energy is obtained from the substance accumu- 

 lated by plants in heating stoves with wood. In using vegetable 

 products for food, they are oxidized within the body in the process 

 of respiration and energy is obtained from them. This energy is 

 used in maintaining the temperature of the body at a certain 

 level (about 37° C), as well as in work done. It should be 

 said, therefore, that assimilation is not only a process of accu- 

 mulation of organic matter, but a process of accumulation 

 of potential energy in the form of combinations of carbon and 

 hydrogen capable of being oxidized. This potential energy is 

 accumulated from the radiant energy falling on the leaf, which is 

 caught by the chlorophyll and transformed from radiant into chemi- 

 cal energy. The thermodynamics of the assimilation process, 

 which plays such an important role in the general transformation 

 of energy on the surface of the earth, will be discussed with more 

 detail in Art. 8. An idea of the potential energy accumulated by 

 the assimilation process can be given by the immense reserves 



