52 FOUNDATIONS OF BOTANY 
is water containing solid substances dissolved in it, the 
greater flow of liquid will be away from the pure water 
into the solution, and the stronger or denser the latter, the 
more unequal will be the flow. This principle is well illus- 
trated by the egg-osmosis experiment. Another important 
principle is that substances which readily crystallize and 
are easily soluble, like salt or sugar, pass rapidly through 
membranes, while jelly-like substances, like white of egg, 
can hardly pass through them at all. 
64. Study of Osmotic Action of Living Protoplasm; 
Plasmolysis. — The obvious parts of most living and grow- 
ing plant-cells are a cell-wall, which is a skin or enclosure 
made of cellulose, and the living, active cell-contents or 
protoplasm. Every one is familiar with cellulose in vari- 
ous forms, one of the best examples being that afforded by 
clean cotton. It is a tough, white or colorless substance, 
chemically rather inactive. Protoplasm is a substance which 
differs greatly in its appearance and properties under differ- 
ent circumstances. It is of a very complex nature, so far as 
its chemical composition is concerned, belonging to the group 
of proteids and therefore containing not only the elements 
carbon, hydrogen, and oxygen, common to most organic 
substances, but nitrogen in addition. The protoplasm in 
a living cell often consists of several kinds of material; for 
instance, a rather opaque portion called the nucleus, and a 
more or less liquid portion lining the cell-walls and extend- 
ing inward in strands to the nucleus (Fig. 126). Often, in 
living and active cells, the spaces left between strands and 
lining are filled with a watery liquid called the cell-sap. 
The action of the protoplasm in controlling osmosis is 
well shown by the process known as plasmolysis. 
