270 VEGETABLE PHYSIOLOGY 



tion of prussic acid and benzole aldehyde. Some plants 

 have been shown to be capable of utilising the former of 

 these, toxic as it is to many forms of animal life. 



The bye-products include bodies of very varying degrees 

 of complexity, some nitrogenous and others not. Among 

 the former may be mentioned the great group of the 

 alkaloids, many of 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. In certain cases in which 

 relatively large quantities of them are stored in seeds, they 

 have been observed to diminish in amount during germi- 

 nation. They may have a nutritive value in these cases. 

 Many physiologists consider this group to belong rather to 

 the definite excretions of the plant than even to its bye- 

 products. They are usually 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 protein 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. 

 Caoutchouc 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 in the course of the 

 katabolic processes, but it is at least possible that some of 



