284 VEGETABLE PHYSIOLOGY 



of complexity, some nitrogenous and others not. Among 

 the former may be mentioned the great group of the 

 alkaloids, which have not so far been found able to 

 ^minister" to the nutrition or growth of the plant, though 

 their nitrogen is in organic combination. If a plant is 

 supplied with them, but with no other form of combined 

 nitrogen, it is rapidly starved. Many physiologists consider 

 this group to belong rather to the definite excretions of the 

 plant than even to its bye-products. They are always 

 deposited in regions which are situated well away from the 

 seats of active life, such as the bark of trees, the pericarps 

 of fruits, &c.' It is apparently very difficult to draw 

 a distinct line of separation between excretions and bye- 

 products, just as it is to distinguish clearly between the 

 latter and secretions. 



The amidated fatty acids, as we have seen, generally 

 occur in direct relation to nutrition. We have examined 

 the part played by leucin and asparagin in proteid con- 

 struction and metabolism. Several other related substances 

 are met with in various plants, but how far they are avail- 

 able for nutrition and how far they are merely bye-products 

 is uncertain. Such substances are xanthin and glycin, 

 which can be extracted from various cells. The latex of 

 plants frequently contains many of these substances. Caou- 

 tchouc is also a frequent constituent of latex. 



Among the non-nitrogenous bye-products may be men- 

 tioned the great variety of vegetable acids. Conspicuous 

 among these are tartaric, malic, citric, and acetic acids. 

 They are usually regarded as arising~m the course~oTthe 

 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 

 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 



