52 BOTANY: PRINCIPLES AND PROBLEMS 



vacuole or sap-cavity, filled with water in which various sub- 

 stances, sugar usually prominent among them, are dissolved. On 

 its outer surface next the wall, and on its inner surface next the 

 sap-cavity, the cytoplasm is bounded by a delicate membrane, so 

 that we find here fulfilled all conditions necessary for osmotic 

 activity — one solution, in the sap-cavity, separated by a mem- 

 brane or membranes from another solution, which may be the 

 soil water (in the case of a root-hair) or the sap-solution of an 

 adjacent cell. 



These cytoplasmic membranes, unlike the cell-wall, offer 

 resistance to the diffusion of certain things and are thus highly 

 important in cell physiology. We find that they are character- 

 istically semi-permeable, preventing the passage of such sub- 

 stances as sugar, which are dissolved in the sap-cavity ; and we have 

 already noted that the cell is thus able to retain these valuable 

 materials within itself and to use them as a means for bringing 

 in osmotically a continuous supply of water from the soil or from 

 adjacent cells. To the essential mineral salts and to many 

 other dissolved substances, however, these membranes are 

 generally permeable, though in varying degrees, and the cell is 

 therefore readily able to absorb a supply of such substances from 

 any adjacent solution. It has been found by experiment that 

 the degree of permeability of the cell membranes is not a fixed 

 and constant one but is subject to change from moment to 

 moment in response to changes in the environment or in the 

 protoplasm itself. A cell which at one time admits a given 

 substance very readily may at another allow it to enter but 

 slowly, or may exclude it altogether. Many of the physio- 

 logical activities of the cell are probably regulated by changes 

 in the permeability of its membranes. 



The rapidity with which a substance passes through a mem- 

 brane is due not only to these differences in permeability but to 

 differences in the concentration of the solutions on the two sides 

 of the membrane. Where this difference is great (other things 

 being equal) osmotic diffusion will be more rapid than where it 

 is slight. Therefore if the concentration of a dissolved substance 

 within a cell is reduced, either by its diffusion into an adjacent 

 cell or its conversion into an insoluble form (as must occur when 

 it enters into the construction of a complex organic molecule in 

 the protoplasm) the rate at which a new supply enters the cell 

 from without is at once correspondingly increased; but the 



