THE ROOT 55 
therefore take place without osmosis, that is, in the absence 
of a membrane, as, for example, when we sweeten our tea or 
coffee by allowing sugar to diffuse through it. Many mem- 
branes offer little resistance to the osmotic movement of 
crystallizable substances. Such membranes are said to be 
permeable. Membranes which are not permeable to the dis- 
solved solids, are called semi-permeable, since they allow the 
diffusion of water but not of the substances in solution. 
Living protoplasm is of this class. It is only very slightly 
permeable to many substances toward which, when dead, it 
acts as a permeable membrane. 
58. Absorption in living and dead cells. — There is one 
great difference between the action of the artificial cell used 
in the foregoing experiments and that of the cells of which 
a living body is built up. The living cell always has at least 
two membranes. One of these, the cell wall, is readily per- 
meable, while the other, the protoplasm, is semi-permeable 
—that is, substances in solution usually diffuse more or less 
slowly, while water diffuses rapidly. Hence in the living cell 
the protoplasm exercises a power of absorption independent 
of the cell wall, sometimes rejecting substances admitted by 
the latter, sometimes retaining others to which it is perme- 
able, as shown in Exp. 40. In the boiled beet the protoplasm 
had been killed and the red coloring matter passed through 
it unhindered, while in the living one it was held back 
by the protoplasmic lining, which is thus seen to control the 
absorptive properties of the cell. 
59. Plasmolysis. — Cells can be killed or injured in other 
ways than by heat; for example, by cold, by poisons, by 
starvation, and by overfeeding through the use of too much 
fertilizer or too rich a one. In this last case, the soil water 
becomes impregnated with soluble matter from the manure, 
which may render it denser than the sap in the roots. When 
this happens, it will cause the osmotic flow to set outward 
and thus deplete the cell of its water; whence we have 
the paradox that a cell, or even a whole plant, may be starved 
