METABOLISM 271 
them may be formed in the elaboration of food from the 
raw materials absorbed, having thus their origin in 
anabolism. 
The bye-products include also a variety of aromatic 
substances. Mention has already been made of tannin, 
and its position discussed. In addition we may include 
phloroglucin 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 
characteristic 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 beremoved by appropriate 
solvents, leaving 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 
colouring 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, 
szanthophyll, 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 sub- 
stances as are undoubtedly withdrawn from the seats of 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 are products of respiration can be specified. Under 
abnormal conditions volatile compounds of ammonia or 
ammonia itself may be added to these. But there are 
