392 



PHYSIOLOGY 



of wheat is apparently a part 

 of the network of protoplasm 

 in which the starch grains are 

 imbedded. The best known 

 storage forms appear in vacu- 

 oles of the endosperm in seeds. 

 The proteins accumulate in the 

 small vacuoles, and upon the 

 loss of water, characteristic of 

 maturation for a resting period, 

 become more and more con- 

 centrated, until finally they 

 solidify, forming the " aleu- 

 rone " or protein grains. These 

 Fig. 663. — Outer portion of a cross section a re very commonly associated 



' m 



QO't 



of a wheat grain: h, various integuments of the 

 ovary and seed, forming the husk; a, cells of 

 "aleuronc layer" of endosperm, loaded with 

 protein grains; b, starch-bearing cells. — After 

 Cobb. 



with reserve starch, either in 

 the same cells, as in the pea 

 and bean, or the protein grains 

 are characteristic of certain 

 cells, as in wheat and other cereals, where they abound in the outer 

 layer of the endosperm (fig. 663). In large grains some proteins may 

 crystallize out, as in the castor bean (fig. 664) and the Brazil nut, but 

 oftener they remain apparently homogeneous. 

 Amides. — Amides occur in such quantities, 

 especially in some sappy reservoirs, that they 

 may be considered as stored food. There 

 they may form 40-70 per cent of the nitroge- 

 nous materials. 



Alkaloids. — Some recent studies of cacao ("cocoa " ) 

 and coffee make it probable that their alkaloids (see 

 p. 415), which are of a different type from most, may 

 be a form of surplus nitrogenous food, since they come 

 again into use. They constitute a very compact 

 source of available nitrogen. 



Combination of food. — It must not be sup- 

 posed that the foods above named accumulate 

 independently. On the contrary, they always 

 occur associated, though one form is likely to 

 be dominant. Rarely, if ever, are they so re- 



FlG. 664. — Cell from en- 

 dosperm of castor bean 

 {Ricinus communis'): p, />, 

 protein grains, made up 

 of amorphous proteins, crys- 

 talline proteins (c) (" erys- 

 talloids")i and globular 

 compounds of -proteins with 

 calcium and magnesium, the 

 globoids (g). — Adapted. 



