90 DIFFUSION AND OSMOTIC PRESSURE 



plasmatic membranes or which penetrate them very slowly, 

 the concentration (and hence the pressure) being greater 

 within the cell than in the external solution. The internal 

 concentration is probably kept up by the chemical activity 

 of the protoplasm itself, substances of the nature of soluble 

 carbohydrates or organic acids being formed and secreted 

 into the vacuole. How such a movement of solutes against 

 the direction of their own diffusion tension can occur is not 

 yet explained. Perhaps they change their nature after leav- 

 ing the protoplasmic layer and entering the vacuole. Turgor 

 pressure may vary in different cells through wide limits 

 (from less than two to more than a hundred atmospheres) 

 and in the same cell the variation during different periods 

 of growth may be almost as great. Turgidity is influenced 

 by variations in the amount of water at hand and also by 

 various conditions which affect the permeability of the 

 protoplasm directly. 



