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 origin in 

 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 

 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, 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 

 as are undoubtedly withdrawn from the seats of active life, 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 



