ABSORPTION AND TRANSMISSION OF WATER 97 



sac, upon being stretched beyond a certain limit, changes its 

 nature in some way so as to become permeable to the con- 

 tained solutes. If this were the case, turgidity would rise to 

 the critical point, and then, when the change in permeability 

 took place, there would result an exudation of cell sap through 

 the plasmic membrane, the contraction of the previously 

 stretched cellulose wall forcing the solution through the 

 now permeable layers. This exudation might be equal 

 in all directions, or might take place mainly in the 

 direction of the water pore, as if the portion of" the protoplast 

 lying next the modified air cavity were the only part to 

 become permeable at the assumed critical point. It seems 

 probable that, if such an occurrence takes place at all, the 

 change is brought about uniformly, and that the exudation 

 from the vacuole is equal in all directions. But, since the 

 adjacent tissues of the leaf are engorged with water, a marked 

 flow could take place only in the free direction, namely, out- 

 ward into the cavity of the pore itself. 



After the cellulose walls had ceased to contract, external 

 pressure would be removed from the protoplast, the flow of 

 liquid would cease, the internal pressure would have fallen 

 below that at the assumed critical point, and it is not at all 

 inconceivable that under these conditions the protoplasm 

 might return to its original condition of semi-permeability 

 toward the contained solution. If this were so, absorption 

 from the surrounding tissues would again take place, and 

 turgidity would gradually return until the critical point was 

 again reached, when the process of external discharge would 

 again occur. Evaporation from the surface of the exuded 

 droplet, which must become rapid as soon as the latter is 

 pressed into the air space, would concentrate the solution, 

 and thus osmotically prevent resorption of the extruded water. 

 More than that, with the increasing concentration it would act 

 as a plasmolyzing solution to extract still more water from 



