116 GRAVITY AND PROTOPLASM [Cn. V 



of chemical agents and currents was eliminated, gravity seemed 

 to remain as the only directing force. 



It only remained to show that the attractive force of the 

 earth can be replaced by centrifugal force, and this SCHWARZ 

 was able to do by means of the klinostat. By varying the 

 rate of rotation of this machine he varied the centrifugal force 



O 



and was able to determine the limits within which the Infusoria 

 move against an opposing force. The acceleration of the rota- 

 tion-force may be expressed in terms of the attraction of gravity 



f 



as a unit by the formula c = -, when c equals the acceleration 



9 



of centrifugal force in the required units ; /, the centrifugal 

 force found, as on p. 113 ; and g the acceleration due to gravity. 

 It appeared from the experiments that, in both living Euglena 

 and Chlamidomonas migration took place towards the central 

 end, thus against the centrifugal force, when the latter was 

 over 0.5 g., and under 8.5 g. Under the lower limit no migra- 

 tion occurred ; near the upper limit aggregation occurred at 

 both ends ; above the upper limit aggregation took place at the 

 peripheral end that is to say, with the centrifugal force. 

 Clearly, then, geotaxis is in these cases a movement against an 

 opposing force, provided that force is considerable (over 0.5 g.) 

 but not too great (over 8.5 g.). 



The workers in this field who followed SCHWARZ advanced 

 our knowledge of geotaxis in two principal ways : first, by 

 increasing the number of organisms known to be geotactic, and, 

 secondly, by revealing the fact that closely allied species may 

 have geotaxis of opposite sense. 



MASSART ('91, pp. 161-167) employed a simple but satisfac- 

 tory method. He placed Protista in a capillary tube which was 

 open, hence equally oxygenated at the two ends. By invert- 

 ing the tube the ends were brought into different relative posi- 

 tions with respect to the earth, causing the geotactic organisms 

 to migrate throughout its length. As a result of his experi- 

 ments it appeared that Spirillum ; the flagellata, Polytoma, 

 Chlamydomonas, and Chromulina ; and the ciliata, Anophrys 

 and Euplotes, are geotactic. The sense of geotaxis may be 

 different between individuals of the same genus ; thus, under 

 similar conditions Spirillum separated into a lot lying at the 



