METABOLISM 277 
Among the non-nitrogenous bye-products may be mem- 
tioned the great variety of vegetable acids. Conspicuous 
among these are tartaric, malic, citric, and acetic acids. 
They are usually regarded as arising in the course of the 
katabolic processes, but it is at least possible that some 
of them may be formed in the elaboration of food from 
the raw materials absorbed, having thus their origm im 
anabolism. 
The bye-products include also a variety of aromatic sub- 
stances. Mention has already been made of tannin, and 
its position discussed. In addition we may include phloro- 
glucin and a variety of aromatic acids, such as benzoic, 
salicylic, &c., but the nature of the processes which give 
rise to them is not well ascertained. 
Certain decomposition products of cellulose may also be 
mentioned here. The lignin and suberin which are char- 
acteristic of woody and corky cell-walls arise in this way. 
During their formation, which takes place in the substance 
of the cell-wall, they can be removed by appropriate solvents, 
leaving the remainder of the cellulose skeleton which they 
have been gradually replacing. These differ from most of the 
substances described in that they can be produced in the 
walls of cells that have lost their protoplasm, so that their 
formation is not directly dependent on metabolism. 
We have again the odorous substances, and the colour- 
ing matters other than those already mentioned. Many 
colourmg matters are products of the decomposition of 
chlorophyll, especially certain of those to which the autumnal 
tints of leaves are due. One of this group, xanthophyll, is 
a bright yellow pigment which is always associated with 
the chlorophyll, though in varying amount. 
We have finally in connection with the metabolic pro- 
cesses to touch upon the excretions of plants. The term 
must be used in a wide sense to include all such substances 
ag are undoubtedly withdrawn from the seats of active life, 
whether thrown off from the plant-body or not. The 
excreta which are completely eliminated are few; under 
normal conditions only the carbon dioxide and water which 
