TUBGIDITT 73 



out pure water, this occurrence must be due to one of two 

 conditions : (1) the protoplasm may contract with great force, 

 thus overcoming the osmotic pressure of the contained 

 solutes and causing the solvent to pass outward through 

 the membrane, or (2) a relative decrease in the internal 

 pressure may occur resulting either from an active precipita- 

 tion or condensation of some of the solutes of the sap, or from 

 an absolute rise in the external osmotic pressure. Accord- 

 ing to supposition (1), the osmotic pressure within the cell 

 remains unchanged, but is in part overcome by the mechani- 

 cal pressure of the contracting protoplasmic membrane. 

 According to supposition (2), the internal osmotic pressure is 

 relatively reduced, and the protoplasm does not exert any 

 appreciable pressure itself, but is forced inward through the 

 solvent by the osmotic pressure of the solutes outside the cell 

 and by the elastic force of the restraining cellulose wall. 

 It is probable that this last supposition expresses the truth in 

 many cases where an alteration in turgidity is observed. 

 The former supposition is not tenable at all ; the protoplast 

 would burst long before concentration of the sap solution 

 could be brought about by pressure. 



If a cell gives out a solution, the cause of this must be a 

 change in the permeability of the protoplasm, such that it 

 now allows the outward passage of solutes to which it was 

 formerly impermeable. The liquid exuded in guttation is 

 known 1 to be, not pure water, but a portion of the cell sap. 

 In the last particular this sort of shrinkage of the vacuole 

 differs from true plasmolysis, for in that we have the extrac- 

 tion of pure water. However, the apparent effect upon the 

 cell is the same ; if the volume of the vacuole is in any way 

 decreased, the protoplasmic sac will contract from its own 

 elasticity and surface tension, if for no other reason. 



1 G. BONNIEE, " Recherches exp6rimentales sur la miellee," Rev. gen. hot., Vol. 

 VIII (1896), pp. 1-22; DANDENO has shown that guttation droplets contain both 

 organic and inorganic solutes. See Trans. Canad. Inst., Vol. VII (1901), pp. 238-350. 



