TUKGIDITY 61 



within the vacuole, either of which phenomena could not take 

 place without penetration. If plasmolysis does not occur 

 even at high concentrations, we have evidence that the pro- 

 toplasmic sac is not only penetrable to the substance used, 

 but that this substance has no marked immediate toxic 

 action. 



Cane sugar, glucose, KNO3, and NaCl are usually found 

 to produce permanent plasmolysis. Plant cells placed in 

 concentrated solutions of these substances do not, as a rule, 

 regain their original turgid condition as long as they remain 

 therein ; no perceptible penetration occurs. However, there 

 are many cells whose protoplasts are more or less permeable 

 to these compounds, and there are all gradations between 

 absolute impermeability and rather slow permeability. One 

 extreme of this series is Massart's Bacterium termo,' which 

 was not plasmolyzed at all in strong solutions of cane sugar 

 and KNO3. 



But in most cases plasmolysis is the first result of irri- 

 gating the cells with the test solution, and it is only after 

 the lapse of some time that the first effect disappears. The 

 gradual inward diffusion of the external osmotic substance, 

 or, in some cases, the gradual secretion of an osmotic substance 

 within the cells, finally brings about an equalization of the 

 internal and external pressures. Then the original internal 

 pressure, produced by the solutes within the vacuole, 

 becomes again effective in producing turgidity. De Vries^ 

 found that the tonoplasts of various plant cells were gener- 

 ally freely penetrated by acids and alkalies, but that salts 

 passed these membranes much more slowly. However, 

 many cells were found which, after being plasmolyzed in a 

 solution of KNO3 or NaCl, gradually returned to their origi- 



1 Massaet, "Sensibility et adaption des organismes h la concentration des 

 solutions salines," Arch, de biol., Vol. IX (1899),pp. 515-70. 



2H. De Veibs, " Plasmolytische Studien fiber die Wand der Vacuolen," Jahrb. 

 f. wiss. Bot., Vol XVI (1885), pp. 465-598. 



