264 VEGETABLE PHYSIOLOGY 
CHAPTER 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, 
