Respiration, Aeration, and Fermentation 285 



chiefly through foods associated with the protoplasm, it 

 takes an active part in the chemical changes of the cell, 

 as a result of which the ultimate excrete products CO 2 and 

 H 2 O may be formed. 



(3) Complex organic molecules are decomposed ; thus 

 simpler products are produced, and kinetic energy is 

 released. A part of this energy is in some manner utilized 

 by the plant in growth and other activities, while another 

 part, set free as heat, has little or no obvious value. 



(4) Of the final excrete products of this series of changes, 

 the most significant is carbon dioxid, and it is eliminated 

 from the tissues by the general diffusion mechanism in- 

 volved in the entrance of gases. 



164. Oxygen promotes catabolic processes. .The 

 studies which have been made upon the decomposition 

 and hydrolysis of protein and other foods and the iden- 

 tification of oxidizing and hydrolyzing enzymes as 

 of widespread occurrence in cells point clearly to decom- 

 position as the essential nature of the process. By some 

 the decomposition of the protoplasm (cf . Barnes and others) 

 is regarded as most important, while others view the pro- 

 cess as essentially, or at least in part, a decomposition of 

 foods, including carbohydrates and fats. Neither view is 

 at present entirely satisfactory, but it is not possible here 

 to review the evidence. It is certain, however, that the 

 presence of free oxygen involves ultimately the decom- 

 position of less material and a more economic energy- 

 release. Moreover, normal respiration, and consequently 

 the growth of most plants, is promptly checked by even 

 the temporary exclusion of free oxygen. 



165. The ratio of O 2 absorption to CO 2 production. By 



