118 HOW CROPS GROW. 



the extent of one-half per cent ; in tea it occurs in much 

 larger quantity, sometimes as high as 6 per cent. 



Theobromin resembles caffein in its characters. It 

 is found ii, tha cacao-bean, from which chocolate is man- 

 ufactured. 



Vernin, discovered recently in various plants, young 

 clover, vetches, squash-seedlings, etc., yields guanin by 

 the action of hydrochloric acid. All these bodies stand 

 in close chemical relations to each other, being complex 

 imide derivatives of dioxymalonic (mesoxalic) acid. 



The amides and amidoacids, like ammonia, are able to 

 combine directly with acids, are accordingly bases, but 

 they are weak bases, because the basic quality of their 

 ammonia is largely neutralized by the acid radicals already 

 present in them. On the other hand, amides and ami- 

 doacids often act as weak acids, for a portion of the hydro- 

 gen of the NH 2 group is easily displaced by metals. 



The amides thus in fact possess in a degree the quali- 

 ties of both the acid and of the base (ammonia) from 

 which they are derived. They also are commonly "neu- 

 tral" in the sense of having no sharp acid or alkaline 

 taste or corrosive character. 



In vegetation amides appear as intermediate stages be- 

 tween ammonium salts and albuminoids. They are, on 

 the one hand, formed in growing plants from ammo- 

 nium salts by a constructive process, and from them or 

 by their aid, probably, the albuminoids are built up. On 

 the other hand, in animal nutrition they are stages 

 through which the elements of the albuminoids pass in 

 their reversion to purely mineral matters. In germinat- 

 ing seeds and developing buds they probably combine 

 both these offices, being first formed in the germ from 

 the albuminoids of the seed, entering the young plant or 

 shoot, and in it being reconstructed into albuminoids. 

 Their free solubility in water and ability to penetrate 

 moist membranes adapt them for this movement. They 



