104 HOW CROPS GROW. 



its action on starch that strikingly distinguished it from 

 the ordinary proteids. 



Pepsin is that ferment of the so-called gastric juice of 

 the animal stomach which enables this organ to dissolve 

 and "peptonize" the albuminoids of the food. It may 

 be extracted from the inner coating of the stomach by 

 glycerine or very dilute hydrochloric acid, and is precip- 

 itable from these solutions by strong alcohol. Pepsin 

 requires the presence of a free acid to dissolve the albu- 

 minoids ; in neutral or alkaline solution it has no "di- 

 gestive power." 



Trypsin is a ferment formed in the pancreas and exist- 

 ing in the pancreatic juice which, in mammalian animals, 

 during the digestion of food, is poured into the upper 

 intestine, where it continues and completes the solution 

 of albuminoids begun by the gastric juice. Trypsin acts 

 jn neutral but most effectively in alkaline solutions ; its 

 operation is arrested by free acids. The results of its 

 action differ in some respects from those of pepsin. 



Papain. The milky juice of the Brazilian plant Car- 

 tea papaya, or melon-tree, contains this ferment, which, 

 like trypsin, is freely soluble in water, rapidly dissolves 

 albuminoids, best in neutral or alkaline solutions, convert- 

 ing them into proteoses and peptones. Papain itself, as 

 obtained by Wurtz & Bouchut, has the properties and 

 composition that characterize the proteoses. 



Ferments appear to perform very important functions 

 in the vegetable as well as in the animal organism, and 

 have to be referred to frequently as occasioning the con- 

 version of insoluble into soluble substances, and of com- 

 plex into simpler bodies. 



Composition of the Albuminoids. There are va- 

 rious reasons why the exact composition of some of the 

 bodies just described is still a subject of uncertainty. They 

 are, in the first place, naturally mixed or associated with 

 other matters from which it is very difficult to separate 



