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 soUds, 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. AVhen 

 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 



