3] 



GEOTAXIS 



117 



upper part of the tube and a lot at the lower part ; and the 

 individuals of both the upper and lower lot were active. The 

 sense of response depends upon temperature also. Thus Chro- 

 mtilina woroniniana is negatively geotactic at 15 to 20 C., 

 and positively geotactic at 5 to 7 C. The other species men- 

 tioned above are negatively geotactic i.e. move in the direc- 

 tion opposite to that in which the 

 force tends to carry them. 



JEXSEX* ('93) finally has greatly 

 extended our knowledge of the spe- 

 cies responsive to gravity, has shown 

 the necessity for regarding carefully 

 the other agents acting during the 

 experiment, and has entered more 

 carefully into the cause of the phe- 

 nomenon than previous authors. \The 

 new forms which JEXSEX worked 

 with were these Ciliata : Paramecium, 

 Urostyla, Spirostomum, Colpoda, Col- ^ ^.-GlaJtubes, about 0.5 

 pidium, Ophryoglena, and Coleps ; 

 also the more commonly used species, 

 Euglena and Chlamydomonas. The 

 other agents whose action may mod- 

 ify that of gravity are chemical stuffs, 

 density, warmth, light, etc. Light 

 may be easily excluded. yOn warm 

 days the typical geotactic phenomena 

 are often absent, the Paramecia sink- 

 ing to the deeper, cooler layers. The 

 Infusoria aggregate around bacteria 

 in the water, chemotaxis (Fig. 24, 



&), and they shun the uppermost layer, apparently because, 

 owing to evaporation, this layer is denser tonotaxis (Fig. 

 24, <?). Whether light inhibits the geotactic response was 

 one of the questions asked and answered by JENSEN. When 



* JENSEN used glass tubes of 0.5 to 1 cm. diameter, and 5 to 100 cm. length, 

 fused at one end. To prevent the free end becoming richer in oxygen, a layer 

 of oil 2 to 3 cm. high was poured over that end, or, air being carefully excluded, 

 it was sealed by an impermeable plug of wax. 



cm. in diameter and 20 cm. 

 long, fused at one end, and 

 filled with water containing 

 Paramecium ( represented 

 by points) . a shows aggre- 

 gation of the Paramecium 

 at upper end of tube: 6, 

 aggregation of Paramecium 

 around bacteria suspended 

 in the water chemotaxis 

 veiling geotaxis ; c shows 

 that occasionally the Para- 

 mecia avoid the uppermost 

 layer of the water. (From 

 JENSEN, '93.) 



