86 DIFFUSION AND OSMOTIC PRESSURE 



From the evidence at hand it may therefore be concluded 

 that there is great variability among different plants with 

 regard to the particular substances called into requisition 

 to maintain the turgor pressure. There seems to be a gen- 

 eral tendency for these to be of an organic nature, and to 

 possess rather complex molecules. 1 



VI. THE MAINTENANCE OF TURGIDITY IN SPITE OF PERMEA- 

 BILITY TO CERTAIN SOLUTES 



It might be supposed that the fact of greater or less per- 

 meability of the protoplasm to various solutes would lessen 

 the value of the osmotic explanation of the phenomenon 

 of turgidity. This, however, does not necessarily follow. 

 While certain substances are diffusing in and out of a cell, 

 its turgidity may be maintained by the presence within the 

 vacuole of some other osmotic substance or substances to 

 which the protoplast is impermeable, or very slightly 

 permeable. It is probable that this is what occurs in living 

 plant cells. These effective osmotic substances are usually 

 of the nature of carbohydrates, plant acids, and mineral 

 salts. They are probably secreted into the vacuole by the 

 activity of the surrounding protoplasm. How this can occur 

 is not yet known. The process must involve movement of 



1 Cf. PFEFFER-EWAKT, Physiology of Plants, Cambridge, 1900, p. 141. 



