258 VEGETABLE PHYSIOLOGY 



CHAPTEE XVII 



METABOLISM 



WE have seen that the object of all the processes of con- 

 struction and digestion that we have examined so far has 

 been to present to the protoplasm materials which it can 

 incorporate into its own substance. If we consider the 

 processes which take place in a vegetable cell or protoplast, 

 we find that they can be divided into those which minister 

 to this construction or building up of the living substance, 

 and those which are connected with its breaking down. The 

 latter accompany or immediately follow the former, and the 

 two together may be considered as the manifestation of the 

 life of the protoplasm. The whole round of changes in which 

 the living substance is concerned is generally spoken of as 

 its metabolism. So many of the reactions as culminate in 

 the construction of protoplasm are described as anabolic, 

 while the changes which it initiates, or which are concerned 

 in its decomposition, are termed katabolic. 



We have been occupied mainly so far in discussing the 

 anabolism of the protoplasts. The substances we have 

 traced to the cells in which growth and repair are vigorous 

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

 of organic -nitrogenous substances, either proteins them- 

 selves or the products of their decomposition, or substances 

 constructed from simple materials with a view to the 

 formation of proteins, such as asparagin or leucin. In the 

 anabolic processes the protoplasm is continually recon- 

 structing itself at the expense of such nutritive substances, 

 which indeed constitute its food in the strict sense of the 

 term. What is true of such cells as are actively growing 

 and multiplying, which are found, as we have seen, in the 



