272 VEGETABLE PHYSIOLOGY 



CHAPTEE XVIII 



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 con- 

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

 organic nitrogenous substances, either proteids themselves 

 or the products of their decomposition, or substances 

 constructed from simple materials with a view to the for- 

 mation of proteids, such as the amido-acids asparagin or 

 leucin. In the anabolic processes the protoplasm is con- 

 tinually reconstructing itself at the expense of such nutri- 

 tive substances, which indeed constitute its food in the 

 strict sense of the term. What is true of such cells as are 



