NOTE ON THE INFLUENCE OF SURFACE- 

 EVAPORATION UPON THE DISTRI- 

 BUTION OF INFUSORIA. 



T. BRAILSFORD ROBERTSON. 



(From the Rudolph Spreckels Physiological Laboratory of the University of Cali- 

 fornia. ) 



During the course of my experiments on the chemotaxis of 

 Paramccdum and Colpodiinn L I was struck by the fact that in 

 certain media the infusoria showed a remarkable attraction to the 

 edge of the fluid under the cover-glass these media were aV/25 

 methyl alcohol, TV/so CaCl 2 , ti/ 5,000 NaOH and N/ 5,000 KOH. 

 This is contrary to the usual behavior of the organisms, in other 

 saline media, in sugar solutions and in the culture medium they 

 avoid the edges of the film of water under the cover-glass and, 

 if undisturbed, form a cluster in the middle of the film, or, if the 

 cover-glass be supported at one end, a little away from the center, 

 towards the supported end. 2 



Jensen attributed this to the increase in concentration at the 

 edges of the film due to evaporation. Since infusoria appear to 

 be very generally attracted by solutions of lower concentration 

 they tend to congregate in the more dilute part of the film, that 

 is, near the center. In considering the apparent contradiction to 

 this rule displayed by the infusoria when immersed in the above- 

 mentioned media it struck me that in all these cases we had the 

 converse of Jensen's experiment for these media tend to 

 become more dilute at the edge of the film. Thus methyl 

 alcohol evaporates more rapidly than water, so that a solution of 

 methyl alcohol becomes more dilute as it stands exposed to the 

 atmosphere while solutions of highly hygroscopic substances 

 CaCl 2 , NaOH and KOH absorb water-vapor from the atmos- 

 phere and so also tend to become more dilute at the surface. In 

 order to see whether this was the case I tried the effect of adding 



^Journal of Biological Chemistry, January, 1906. 

 2 Jensen, Pftiiger 1 s Archiv. /urges, physiol., Vol. 53 (1893), p. 428. 



