284 ORGANIC AGRICULTURAL CHEMISTRY 



in the cell contents ^ in cell sap, in plant juices and as reserve food 

 in seeds, roots, etc. In these different localities we find proteins 

 existing in three different states or forms: (i) in solution in 

 the cell sap or plant juices ; (2) in crystalline form in the cell 

 contents or in reserve organs; (3) in amorphous solid bodies 

 as reserve food in seeds. The two solid forms, crystalline and 

 amorphous, in which they occur as reserve food in seeds, roots, 

 etc., may be either pure or mixed, i.e. partly crystalline and 

 partly amorphous. The solid protein bodies resulting are 

 known as aleuron grains. They are found in a certain layer in 

 the seed known as the aleuron layer, as in castor oil bean, wheat 

 grain, etc. The structure of these aleuron grains is often 

 complex, but they consist usually of solid protein inclosed in a 

 less soluble protein membrane and the whole body has a dis- 

 tinctly granular appearance. Several different proteins may be 

 present in one aleuron grain, but the most common one is a 

 globulin. In most seeds the aleuron layer is directly beneath 

 the outer seed coat. 



Occurrence. — Proteins occur principally in the seeds of 

 plants and are most abundant in the seeds of legumes and 

 cereals. Crops yielding these seeds are thus of especial im- 

 portance as food for domestic animals, for while carbohydrates 

 and fats furnish the greater part of the energy value of foods 

 it is from plant protein alone that herbivorous animals must 

 build their own body protein. 



In discussing the chemical nature of proteins we stated that 

 they yield amino-acids on hydrolysis. The principal amino- 

 acids obtained from vegetable proteins differ from those that 

 are obtained from proteins of animal origin. Thus, glutaminic 

 acid, proline and arginine are obtained in larger amounts from 

 vegetable proteins. Also alcohol soluble proteins, e.g. gliadin 

 t of wheat, are found only in plants. On the other hand, some 

 amino-acids are obtained only from animal proteins. These 

 differences in the two classes of proteins evidently affect their 

 food value, but at present we do not know enough in this regard 

 to warrant further discussion. 



