236 PLANT PHYSIOLOGY 



which is acceptable must explain why the hydrostatic pressure 

 developed varies with the number of molecules or particles of 

 solute present. Each of the above theories does this. If more water- 

 attracting particles are present, more water will be held and the 

 pressure will vary accordingly. If, on the other hand, the trap 

 theory is accepted, here also the more particles present, the 

 more would be in the way to hinder the outward passage of the 

 water and the greater would be the hydrostatic pressure developed. 



However, in the light of the diffusion hypothesis favored above, 

 it may not be necessary to assume either of these two hypotheses. 

 If diffusion can explain the entrance of the solvent, and if the 

 water diffuses in because there are more molecules outside than 

 inside, then no further hypotheses may be needed. The more 

 sugar inside, the less water, and the more must flow in before an 

 equilibrium results. This will also explain why the pressure 

 varies with the number of molecules of solute present. 



Osmosis and Plants. — To return to the plant, we need now 

 only to apply these principles to the conditions found there. 

 Every cell is bounded by a differentially permeable membrane 

 which is generally more permeable to water than to solutes. This 

 membrane is not the cell wall, which plays only a minor role in 

 these phenomena, but the plasma membrane. Other membranes 

 are present in the cell, such as the tonoplast, the nuclear membrane, 

 the membranes bounding the plastids, etc., all of which exhibit 

 these same properties, which are summed up in the term differen- 

 tially permeable. These membranes, however, all differ in one 

 important essential from the parchment, bladder, etc., already 

 described, viz., in the fact that they are alive and that their per- 

 meability changes. Some of the factors which determine these 

 changes in permeabilit}^ have already been mentioned (Chap. XI). 



Every substance which enters the plant must enter in solution 

 and obey the laws of diffusion. If the plasma membrane is per- 

 meable to KC1 and, if there is more KC1 in the soil water than 

 inside the root hairs, this salt will diffuse in. Because of the 

 formation inside the plant of osmotic substances such as sugars, 

 etc., there is generally a higher osmotic solution pressure inside 

 the cell than outside, with the result that water enters. There is 

 thus maintained a condition of hydrostatic pressure inside the cell 

 known as turgor. For this to be manifested there must be (1) a 

 difference in the osmotic pressure within and without the cells and 



