260 



Plant Physiology 



larly in the seed, so that the storage forms are better 

 known. The aleurone grains of the endosperm of cereals 



are familiar protein 

 bodies, and in this 

 case they are found 

 abundantly in the 

 outer layer of the 

 endosperm, while 

 starch is more abun- 

 dant within. 



FIG. 66. Endosperm cell (A) of Ricinus in 



water ; aleurone grains (B) in olive oil ; wneat Consists OI a 

 protein crystal (k) and globoids_(0). [After variable mixture of 

 Strasburger.] . -, 



proteins. Flour may 



contain about 10 per cent of this material and about 

 70 per cent of starch. The gluten is readily separated 

 from the starch in a proximate manner by kneading the 

 flour under water in a thin cotton bag. In this manner 

 the flour is filtered out and the gummy nitrogenous sub- 

 stance remains in the bag. Hard wheats are particularly 

 valuable in the manufacture of products like macaroni 

 (also in bread-making) as a result of the relatively high 

 content and composition of the gluten. 



148. Classes of proteins. A system of classification 

 of the proteins in keeping with that of other chemical sub- 

 stances (the molecular structure of which is better known) 

 is now impracticable. Classification is based largely upon 

 solubility under certain standard conditions. Three 

 principal groups of proteins are recognized, namely, (1) 

 simple, (2) conjugate, and (3) derived. 



The simple vegetable proteids include albumins, some 



