6 PRACTICAL COURSE IN BOTANY 
may be illustrated by observing the difference between a 
rubber bag when quite full and when only half full of water, 
or a football when partially and when fully inflated. In 
its simplest form, however, the cell is a mere particle of 
protoplasm, which has one part, constituting the nucleus, 
a little more dense in appearance than the rest, but this 
kind is not common in vegetable structures. 
8. How food substances get into the cells. — As there 
are no openings in the cell walls, the only way substances 
can get into a cell or out of it is by soaking through the 
inclosing membrane, as will be explained in a later chapter. 
Since starch, oil, and proteins, the most important foods 
stored in seeds, are none of them soluble in the cell sap, it is 
clear that they could not have got into the cells in their 
present state, but must have undergone some change by 
which they were rendered capable of passing through the 
cell wall. 
9. Digestion.— The process by which this change is 
brought about is known as digestion, from its similarity to 
the same function in animals. Not only are foods, in the 
state in which we find them stored in the seed, incapable 
of passing through the cell wall, but the protoplasm, the 
living part of the cell, has no power to assimilate and to 
utilize these substances as food until they have been re- 
duced to a soluble form in which they can be diffused freely 
from cell to cell through any part of the plant. By diffusion 
is meant the gradual spread of soluble substances through 
the containing medium, as when a lump of sugar or salt, 
dropped into a glass of water, dissolves and slowly diffuses 
through the contents, imparting a sweet or salty taste to the 
whole. 
During the process of digestion the different kinds of 
food are acted upon and made soluble by certain chemical 
ferments, which are secreted in plants for the purpose. The 
digestion of starch, the most abundant of plant foods, is 
effected by diastase, a common ferment obtained from ger- 
