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 
in the vacuoles of the 
protoplasm, or in inti- 
Fic. 251. — Cross section through grain of 
wheat (Triticum vulgare); p, pericarp; t, testa; 
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 
al, aleurone layer containing numerous protein proteins. (Fig. 251.) 
grains; 7, 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 CyoHi3iNeisOQossSs 
for one of them indicates. 
The steps in the process 
by which the photosynthetic 
Fic. 252. — Section from a cotyledon 
ofa Pea, showing a few cells; 7, intercellular 
space; am, starch grains; al, aleurone 
grains; n, nucleus. Enlarged 240 times. 
After Strasburger. 
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 
