468 MANUAL OF BOTANY 



In the metabolic changes going on in the protoplasm we find, 

 however, that while its own molecules are in a constant state of 

 decomposition and reconstruction, other changes also take place 

 in the various substances which are enclosed in its meshes, into 

 which its own molecules do not enter. Processes of slow 

 oxidation and gradual reduction are taking place there continu- 

 ally, excited, however, in all probability by the changes in the 

 protoplasm itself. Even these, however, are by no means 

 simple, and the direct oxidation of either carbon or hydrogen 

 has probably no place amongst them. An instance of them 

 may be seen in the oxidation of alcohol in the cells of 

 Mycodenna aceti, a fungus which converts alcohol into acetic 

 acid. This process, into which the molecule of protoplasm can 

 apparently not enter, yet can only go on in the living cell. 

 Other similar instances could be quoted. 



The probable course of events is that the oxygen in some way 

 unites with the molecule of protoplasm, rendering it unstable 

 and initiating a series of decompositions which result in the 

 formation of many of the bodies already spoken of in the last 

 chapter. If the temperature be low, the breaking down of the 

 protoplasm proceeds l^it slowly, and reconstruction, either from 

 some of these residues or from food supplied to it from the cell 

 contents, or both, is rapid. Consequently the quantity of oxygen 

 absorbed or fixed by the protoplasm is greater than the quantity 

 of CO.. formed by its decomposition. At a higher temperature 

 decomposition is much more easily carried on and its products are 

 more numerous and simpler. The decomposition and recomposi- 

 tioD go on side by side, simpler bodies being gradually produced, 

 either by their splittmg from the protoplasm directly, or by their 

 being formed at the expense of the more complex decomposition 

 products, during processes of slow oxidation in its meshes, till 

 finally a certain production of CO., and water is arrived at. So 

 long as the protoplasm remains alive the amount of these is not 

 large, reconstruction continually taking place. When, how- 

 over, the protoplasm dies, simpler bodies such as CO., water, and 

 possibly Ammonia in addition, are produced abundantly from 

 the decomposition which attends its death. 



The CO., is thus tlie final term in a series of decomjiositions 

 of which the living substance is the seat, and which are promoted 

 by the access of oxygen. In some cases, such as that of the 

 Cactus already alluded to, this final term is not reached, no CO., 

 being exhaled. In these cases the process stops short at the 



