THE CELL fi 
greatly interfering with the passage of water through it. Cells 
that are on the outside of plants and exposed to dry air fre- 
quently have their outside walls impregnated with a waxy 
substance, cutin. Water does not readily pass through such 
cell walls; consequently the walls tend to protect the cells 
from excessive loss of water due to evaporation. A layer of 
cutin is frequently found on the outside of cells that are 
exposed to dry air. Such a layer is known as a cuticle. Stems 
of plants are frequently protected from high rates of evapora- 
tion by the development of cork. The walls of cork cells also 
are impregnated with a waxy substance, suberin, which is 
impervious to water. 
_ A cell can exist without a cell wall, and in some of the 
simpler plants, at certain times, the protoplasm escapes from 
the cell and surrounds itself with a new cell wall. 
Animal cells do not have hard cell walls, like plants, but 
have a soft covering; in some cases they may be naked. For 
this reason the flesh of animals is soft, while the form of large 
animals is due to a considerable extent to their bones, and not 
to cell walls, as in plants. 
Protoplasm. Within the cell wall is found the protoplasm, 
or living part of the cell. The gray material in Figs. 10 and 
11 is protoplasm. Protoplasm is usually a viscous liquid or a 
jelly. There is no sharp dividing line between these two states, 
and the same protoplasm may change from one to the other. 
The liquid state is associated with a more active condition. 
Protoplasm has a slightly grayish color, or it may have a yel- 
lowish tinge due to included food particles. It is usually rather 
transparent. Included within the protoplasm are numerous 
minute granules, many of which are food particles. 
Protoplasm is essentially a colloidal dispersion of proteins in 
water. Active protoplasm usually contains more than 90 per 
cent of water. In some cases, as in seeds, the amount of water 
may be much smaller, and the protoplasm becomes relatively 
hard. In such cases, however, the protoplasm loses most of 
its activity and becomes active only when additional water is 
