G 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 paj't, 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- 



