§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- 

 mulina 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. 



Jensen* ('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 Jensen worked 

 with were these Ciliata : Paramecium, 

 Urostyla, Spirostomum, Colpoda, Col- 

 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. On 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, 



J), — and they shun the uppermost layer, apparently because, 

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

 24, c). Whether light inhibits the geotactic response was 

 one of the questions asked and answered by Jensen. When 



Fig. 24. — Glass tubes, about 0.5 

 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 ; &, 

 aggregation of Paramecium 

 around bacteria suspended 

 in the water — chemotaxis 

 veiling geotaxis; e shows 

 that occasionally the Para- 

 mecia avoid the uppermost 

 layer of the water. (From 

 Jensen, '93.) 



* 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. 



