GEOTROPISM OF PARAMECIUM AND SPIROSTOMUM. 15 



If the weight of organisms or their resistance to the pull of 

 gravity is the only cause of geotropism, negatively geotropic 

 animals should become positively geotropic, and positively 

 geotropic animals should become negatively geotropic in solutions 

 of greater specific gravity than their own, as Platt points out 

 (20, p. 31). Platt at the suggestion of Davenport attempted 

 in vain to solve this problem, because she could not find "para- 

 mecia which showed decided geotropic reaction" (20, p. 32). 

 The first approach to this problem was to determine the specific 

 gravity of the Paramecium. Jensen first reported it as 1.25 (12, 

 p. 544). He killed Paramecium aurelia in solutions of potas- 

 sium carbonate (12, p. 543) and determined their specific gravity 

 when they were suspended in a solution of known specific gravity. 

 But as Platt has shown (20, p. 34), the specific gravity of the 

 animal when so treated was certainly increased in the solution 

 to some extent. The value of the specific gravity of paramecia 

 obtained by Jensen, therefore, is hardly worth serious con- 

 sideration. Platt then tried to determine the specific gravity 

 by killing the animals in water to which was "added a few 

 drops of 0.5 per cent, acetic acid." She "also killed paramecia 

 by the fumes of osmic acid." Then she placed the killed para- 

 mecia "in a gum-arabic solution of 1.018 specific gravity," 

 whereupon they "remained long suspended in the solution' 

 (20, pp. 35-36). So she thinks that the specific gravity of 

 paramecia so treated "is about 1.017" (20, p. 38). But Platt's 

 method was no better than Jensen's. As she observed that 

 the "reagents caused the paramecia to change their shape 

 somewhat in dying, by becoming wide and shorter," we are not 

 justified in assuming that the specific gravity of the dead animals 

 remained the same. "On the death of the cell," according to 

 Lillie, "there follows a marked and permanent increase in the 

 general permeability, and this change is always associated with a 

 permanent fall in the potential difference between surface and 

 interior" (14, p. 196). Furthermore, as Lyon rightly points out, 

 a viscous gum-arabic solution is likely to hold suspended for a 

 long time even bodies of considerably different density "unless a 

 greater force than gravity be used to separate them from the 

 solution" (17, p. 427). We can therefore place little value on 

 either Jensen's or Platt's results. 



