294 EFFECT OF CHEMICAL AGENTS [Ch. XI 



to LoEW, constitutes the essential living substance. For exam- 

 ple, oxygen performs this office in LoBw's ('96, p. 39) hypoth- 

 esis of the formation of albumen in plants from the nitroge- 

 nous products which result from the action of an enzyme upon 

 the reserve proteids, i.e. leucine. Thus 



leucine formaldehyd 



C6Hi3]Sr02 -I- 7 = 2 CO2 -f H2O -I- 4 CH2O + NH3. 



formaldehyd asparagin 



4 CH26 + 2 NHa-l- O2 = CiHglSraOa-l- 3 H,0. 



Now since, as Loew makes probable, asparagin is a stage in 

 the production of albumen, the free oxygen molecule may be 

 essential to the synthesis of the living substance. 



The facts indicate that the plastic and thermogenic functions 

 of foods are inextricably intermingled — that both are exhib- 

 ited in the mutations of the living substance as well as in the 

 respiratory processes. Thus, on the one hand, assimilation is 

 accompanied not only by endothermic (energy-storing) but 

 also by exothermic (energy-releasing) processes ; while, on the 

 other hand, the partial oxidation of the food proteids may be 

 a necessary step towards assimilation. It is because the net 

 result is the storing or the release of energy that we may speak 

 of any complex of processes as enthodermic or exothermic, and 

 certain foods as plastic or thermogenic. 



The source of energy in organisms is not, however, solely 

 food. Energy may likewise be derived directly from energy 

 in the environment. This source is of greatest importance in 

 the case of chlorophyllaceous organisms, but it is probably not 

 of importance for them alone. For the heat and light of the 

 environment aid, as we have seen (pp. 166-171, 222-225), 

 various metabolic processes in all kinds of protoplasm. 



1. The Materials of which Organisms are Composed. — To 

 determine the sorts of materials which plastic food must supply 

 to the body, it will be instructive to consider the proportional 

 composition of the body out of water and dry substance, both 

 organic and inorganic. Such data, gathered from various 

 animals and plants, are given in the following table : — 



