460 MANUAL OF BOTANY 



CHAPTEK X. 



THE CATABOLIC PROCESSES. 



We have seen that the object of all the processes ot construc- 

 tion and digestion that we have examined so far has been to 

 present to the protoplasm materials which it can incorporate 

 into its own substance. The bodies which we have traced to it 

 consist, in far the greatest part, of some form of sugar and of 

 organic nitrogenous bodies, either proteids or the complex pro- 

 ducts of their decomposition, such as the amides. The proto- 

 X3lasm of the cell is continually^ reconstructing itself at the ex- 

 pense of such nutritive substances, wdiich indeed constitute 

 its food in the strict sense of the term. But there is also 

 going on, side by side with this process of reconstruction, a 

 decomposition of the substance of the protoplasm, involving a 

 splitting off from its complex molecule of various bodies of 

 great variety but of less complexity than the living substance 

 itself. These often, in the first instance, include such carbo- 

 hj^lrate and nitrogenous residues as it made use of in building 

 itself up. These can again be used in reconstruction of the 

 protoplasm or can be further broken down into simpler bodies 

 still. So long as the protoplasm is living, it is continually in a 

 state of change or chemical activity, undergoing reconstruction 

 and decomposition continually. 



Besides this power of initiating chemical changes in which 

 it takes itself a prominent part, it is also the seat of a large 

 number of processes of both oxidation and reduction which are 

 continually going on in its meshes at the expense of the various 

 materials which are found there, either from being transported 

 from other cells or from being formed in the processes of the 

 self-decomposition of the protoplasm. 



The formation of simpler from more complex bodies by 

 either of these methods constitutes what has been called the 

 catabolism of the plants. It may sometimes go on to the 

 extent of producing such simple bodies as CO., and water, which 

 are given off from the organism. In most cases, however, the 

 catabolic changes are not so far-reaching, and there remains in 

 the plant a great accumulation of organic substance such as 

 woody or corky tissue. The relatively small extent of the 

 catabolism as compared with the anabolism finds its expression 

 in the enormous bulk which many trees and other plants attain. 



