TUEGIDITY 87 



solute particles from a lower concentration to a higher, 

 against their own diffusion tension. It may be that these 

 substances are freed from the protoplasm in a certain form, 

 and that, after entering the vacuole, they polymerize or 

 change their nature in some way, according to special con- 

 ditions there existing. The accumulation of many substances 

 within the vacuole (e. #., anilin dyes 1 and caffein) is surely 

 due to a chemical reaction after the substance has passed the 

 protoplasmic layers. 



VII. KELATION OF TUEGIDITY TO VITAL ACTIVITY 



Only because of the existence of the phenomenon of tur- 

 gidity has the plant organism been able to develop into what 

 it is. In several ways turgidity is absolutely essential, and 

 in many others advantageous, to vital activity as it is now 

 exhibited in plants. 



a) The retention of form. By means of turgor pressure 

 the delicate fluid or semi-fluid plasma of the cell is kept 

 pressed out against the cellulose wall, and the plasmic mem- 

 branes are thus kept in a uniform state of tension, upon which 

 condition some of their physical properties doubtless depend ; 

 as has been seen, for instance, great variations in turgor may 

 so affect the protoplast as utterly to change its permeability 

 to certain solutes. 



b) Mechanical support. All parenchymatous tissues and 

 nearly all filamentous and uni-cellular forms owe most of 

 whatever rigidity they possess to the stress set up between 

 the internal osmotic pressure and the elastic force of the 

 stretched cell walls. A heavy weight might be supported 

 upon a pile of inflated footballs if they were properly placed, 

 but if the individual balls were leaky they would no longer 

 be available for such a purpose. In a similar manner the 

 plasmolysis of any thin-walled tissue is accompanied by a 



i Ibid., pp. 119 ff. 



