
Fig. 296. Cells near the outer part of 
the cotyledon of peanut 
The large globules are oil; the white 
bodies, starch grains; the small dark 
granules, protein ; and the knoblike thick- 
eningson the walls, hemicellulose. (x 225) 
THE FRUIT AND THE SEED | 293 
from milk. The proteins al- 
ways contain carbon, hydro- 
gen, oxygen, and nitrogen, 
and many contain sulphur. 
Other substances, such as 
phosphorus, are also found 
in some proteins. Proteins 
in the form of solid gran- 
ules are frequently found in 
plants as food-storage mate- 
rial (Figs. 295-298). 
Plant materials as human 
food. The sugars, starches, 
fats, and proteins which are 
stored in plants are also used 
as food by animals, including 
man. The fats and carbohy- 
drates serve as sources of en- 
ergy, while proteins are both sources of energy and body-building 
materials. Vitamins, which are manufactured by plants and 
not by animals, are also necessary for the metabolism of animals, 
although they do not appear 
to furnish energy or to take 
part in the building of tis- 
sues. Vitamins are abundant 
in leaves. 
DIGESTION 
Food stored as starch, cellu- 
lose, fatty oil, or protein is not 
soluble in water, and so must 
be converted into a soluble 
form before it can be used by 
plants. This is accomplished 
by substances known as en- 
zymes, Which belong to the 

Fig. 297. Cells of lumbang nut 
The granules are protein. Note how the 
oil fills the cells. (x 265) 
