THE ROOT 55 



take place without osmosis, for example, when we sweeten 

 our te.a or coffee by allowing sugar to diffuse through it. 

 It may also take place through the cell wall in connection 

 with the osmotic current ; or substances may diffuse into or 

 out of the cell independently of osmosis. For this reason it 

 is not safe to rely on the evidence of differently colored 

 or differently tasting liquids in making experiments, unless 

 we are sure of the osmotic properties of the solution, as the 

 contained substances may diffuse through the membranes 

 even though no interchange of liquids is going on at the 

 time. 



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 former contains no proto- 

 plasm, and the osmosis is a purely mechanical process depend- 

 ing on the nature of the liquids, or possibly on some physical 

 property of the membrane. Any substance to which the 

 membrane is permeable can pass through. 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 contents ; whence we have 

 the paradox that a cell, or even a whole plant, may be starved 



