3] GEOTAXIS 115 



Euglena and desmids ; MASSART ('91), who worked upon 

 bacteria, and ciliate and flagellate Infusoria ; and JENSEN ('93), 

 who experimented with Euglena, Chlamydomonas, and eight 

 species of Ciliata. 



The observation that led SCHWARZ to his study was that 

 Euglena and Chlamidomonas, shaken up with sand and covered 

 by it, constantly, even in the dark, rose to the surface. The 

 experiments now made by SCHWAKZ to determine the true 

 cause of the pheriornenon were a model of experimental investi- 

 gation. In the first place only fresh and actively moving 

 individuals were used, and light was carefully excluded, either 

 by enveloping the culture vessel in black paper, or by working 

 in a dark chamber. I shall now give in detail the experiments 

 and their results. 



When the Flagellata were placed in water they responded 

 like those 1 in sand they soon came to the upper surface. But 

 may not this upward movement be purely passive due to the 

 small specific gravity of the algse or to currents in the water ? 

 To get an answer to this question SCHWARZ heated the sand 

 to 70 C a fatal temperature and no aggregation occurred. 

 Again, the algae were subjected to vapor of chloroform ; no 

 aggregation. Again, to a low temperature (5 to 6) ; no 

 aggregation. An aggregation occurred, however, when the 

 temperature of the same culture was raised to 22. Finally, 

 Lycopodium spores and Euglena in the resting stage do not 

 move upwards ; hence no currents are passing in this direction. 

 On the contrary, these experiments show that the upward 

 movements of the algae are the results of its own active loco- 

 motion. 



Nor can it be that anything else than gravity determines the 

 direction of the locomotion. That the greater amount of oxy- 

 gen at the upper level is not the controlling agent was shown 

 by smearing the sides of a glass cylinder with a thin layer of 

 sand containing the algae. In this thin layer, permeated by 

 oxygen, they still accumulated at the upper margin. That the 

 locomotion was not directed by currents in the water (Rheo- 

 taxis, p. 108) was indicated by the fact that whether the free 

 end of the tube, at which evaporation is occurring, be up or 

 down, migration is always upwards. Thus, since the stimulus 



