52 PROTEINS 



with dilute potash causes both bounding membrane and crys- 

 talloid to swell and disappear. 



The protein -granules of the Pea, Bean, etc., are often regarded 

 as small aleurone grains devoid of crystalloid and globoid. 

 Similar structures occur in abundance in the so-called aleurone 

 layer found at the periphery of the endosperm of Grasses (Wheat, 

 Maize, etc.), just within the coat of the grain (Fig. 24). When 

 the latter is detached, the protein -containing layer generally 

 comes away with it, hence the greater nourishing properties of 

 wholemeal bread as compared with that manufactured from 

 white flour. For the same reason peeled potatoes are not so 

 nutritious as those boiled in their skins, since the outer layers 

 of the tuber contain protein-crystalloids (cf. above). 



Proteins, owing to their complex molecules, are not readily 



FIG. 24. Aleurone layer (A.L.) of Wheat as seen in a transverse section. 

 The starch-containing cells (S.C.) lie immediately beneath. 



diffusible, and consequently become converted into simpler, 

 freely-diffusing compounds before they are transferred to parts 

 where growth is occurring (e.g. in a germinating seed). This 

 conversion is brought about by so-called proteolytic enzymes (e.g. 

 pepsin, erepsin), which are in general similar to those occurring 

 in the digestive tracts of animals, although some uncertainty 

 exists as to the exact nature of those present in plants. The 

 chief compounds produced, by the action of these enzymes on 

 proteins, are firstly peptones and subsequently amino-acids. 

 The former still exhibit the general characteristics of proteins, 

 although they diffuse more easily, while the latter no longer 

 possess protein -properties. Amino-acids are exceedingly com- 

 mon in growing and other parts of the plant (e.g. leucin in the 

 buds of the Horse Chestnut, tyrosin in seedlings of the Lupine, 

 etc.). 



