182 



STORAGE OF FOOD AND WATER 



with starch or oil. In the garden pea and bean, for example, 

 they occur as very small grains filling up the spaces between 

 the starch grains, while in the castor bean they are much larger 

 and together with the oil fill up the fine meshes of the cyto- 

 plasm (Fig. 100). Here'and in some other oily seeds the aleurone 

 grain is made up of a proteid body, called the ground substance, 



A 



FIG. 100. To show aleurone grains. .4, cells from cotyledon of seed of garden bean; 

 n, aleurone grains; m, starch; B, cell from endosperm of castor bean; a, aleurone grain; I, 

 ground substance; k, crystalloid; j, globoid. (A, after Sachs; B, after Frank.) 



inclosing one or more proteid crystalloids, and one to several 

 mineral granules called globoid s, composed of a double phos- 

 phate of calcium and magnesium. 



Amorphous and soluble proteids occur in bulbs and tubers, 

 and in the storage tissues of ordinary stems and roots. 



There are many different kinds of proteids in plants, but 

 they are not yet well enough known to admit of complete 

 classification. 



Most proteids are digestible in the animal body, and they 

 are either soluble in water or made so by the enzymes pepsin 

 and trypsin of the animal body or similar enzymes occurring 

 in plants. Of these digestible proteids the globulins and albu- 

 moses occur most abundantly in the reserve foods of seeds: 

 the former of these coagulates on heating to 75 C., while the 

 latter does not. Albumins occur in seeds and in the general 

 cell-sap and like the globulins coagulate on heating, but unlike 

 them are soluble in pure water. Gliadin and glutenin, insoluble 

 in water but soluble in dilute alcohol, occur abundantly in the 



