280 



LEAVES 



The vegetable proteins are of many kinds and they vary greatly 

 in physical and chemical properties. They occur as crystals, 



granules, or in solution 

 the vacuoles of the 



in 



protoplasm, or in inti- 

 mate association with 

 the protoplasm. They 

 are present to some ex- 

 tent in all plant cells, 

 but are more prominent 

 as storage products in 

 seeds, where they are 

 usually associated with 

 starch and fats. Some- 

 times, as in the aleurone 

 layer of the cereals, 

 there is little else but 

 (Fig. 251.} 



'01 



FIG. 251. Cross section through grain of 

 wheat (Triticum vulgar e); p, pericarp; t, testa; 

 al, aleurone layer containing numerous protein proteins, 

 grains ; TO, nucleus; am, starch grains. Enlarged The Legumes store con- 

 240 times. After Strasburger. siderable quantities of 



proteins, and for this reason some of them, especially the Beans 

 and Peas, are very desirable 

 for food. (Fig. 252.} 



Proteins differ chiefly from 

 the carbohydrates and fats 

 in that they contain nitro- 

 gen. They are known as 

 nitrogenous foods. In 

 addition to nitrogen they 

 usually contain sulphur and 

 sometimes phosphorus; but 

 nitrogen is the chief mineral 

 constituent. The proteins 

 are extremely complex, as the 

 formula CreoHns^isO^Ss 

 for one of them indicates. 



The steps in the process 

 by which the photosynthetic 

 sugar and the mineral elements are formed into proteins are not well 

 known; but it seems clear that the elements of the sugar are first 



FIG. 252. Section from a cotyledon 

 of a Pea, showing a few cells; i, intercellular 

 space; am, starch grains; al, aleurone 

 grains; TO, nucleus. Enlarged 240 times. 

 After Strasburger. 



