INFLUENCE OF THE MEDIUM 141 



Massart, 1 experimenting upon certain bacteria, found 

 that they were negatively osmotactic to solutions of many 

 different substances. The proof that the phenomenon is 

 osmotaxis and not chemotaxis lies in the fact that the organ- 

 isms were repelled always at the same osmotic concentration, 

 irrespective of the chemical nature of the solute. Rothert 

 found Treptomonas agilis positively osmotactic toward solu- 

 tions which are so concentrated that they kill the organism 

 by plasmolysis. Only such solutes are available for experi- 

 ments upon osmotaxis as are known to be unable to pene- 

 trate the protoplasm of the organism to be tested. Of course, 

 if penetration occurs, the difference in concentration within 

 and without the cell can last but a short time ; it will soon 

 be equalized by the inward diffusion of the solute. 



d) The analogy between the effects of high osmotic pres- 

 sure of the medium and those produced by other water- 

 extracting processes. Attention has already been called to 

 the fact that a lowering in temperature is often accompanied 

 by giving out of water. Thus Spirogyra filaments when 

 cooled in olive oil may be seen to give off water before freez- 

 ing. It seems probable that in this case the protoplasm 

 becomes more permeable at these low temperatures and thus 

 the solute escapes with the solvent. If this is true, we can- 

 not look upon cold plasmolysis as producing a concentration 

 of the cell sap. It would only decrease its volume. 



Of course, a part of the shrinkage in such a case could 

 be accounted for by the diminution of osmotic pressure due 

 to cooling. If the original internal pressure were p at tC., 



then it would decrease to p - ^-r^ at t'C. The pres- 

 sure of the external solution will decrease according to the 



i J. MASSART, u La sensibilit6 & la concentration chez les 6tres unicellulaires 

 marins," Bull, de Vacad. ray. de Belgique, Ser. Ill, Vol. XXII (1891), pp. 148-67 ; IDEM, 

 "Sensibilit6 et adaptation des organismes & la concentration des solutions salines," 

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



