GEOTROPISM OF PARAMECIUM AND SPIROSTOMUM. 1 9 



The distribution shown by the animals was then compared with 

 the distribution shown in the original determinations of their 

 specific gravity. If the animals in the solution of 1.037 increased 

 their density about 0.003 after 13 hours, they should be dis- 

 tributed in the tube when centrifuged at this time as they had 

 formerly been in the solution of 1.034. The results of Lyon and 

 those of the writer agree fairly with one another in this respect 

 (17, p. 428). 



From the results of all the above experiments, we have found 

 that the weight or resistance theory of geotropism is untenable. 

 We have found that no animals which are negatively geotropic 

 become positively geotropic in gum-arabic solutions of greater 

 specific gravity than their own. Thus Lyon's results in all 

 respects are fully confirmed except as regards the exact value 

 of the specific gravity of the animals. 



2. Experiments on Spirostomum teres. 



The same experiments as the above were made with Spirostoma 

 in the same ways. The density of the animals was found: 



f Tube i. Density of gum-ar. sol. 1.022, 1,000 revolutions; some go to 

 I bottom. 



'] Tube 2. Density of gum-ar. sol. 1.023, 1,000 revolutions; a few go to 

 t bottom. 



("Tube i. Density of gum-ar. sol. 1.024, 1,000 revolutions; 4 go to bottom. 

 11.4 Tube 2. Density of gum-ar. sol. 1.025, 1,000 revolutions; none go to 



bottom. 



{Tube i. Density of gum-ar. sol. 1.026, 1,000 revolutions; a few go to 

 lower part. 

 Tube 2. Density of gum-ar. sol. 1.027, 1,000 revolutions; a few go to 

 middle. 



C Tube i. Density of gum-ar. sol. 1.019, 1,000 revolutions; most go to 

 TV J bottom. 



I Tube 2. Density of gum-ar. sol. 1.030, 1,000 revolutions; most stay near 



top. 



These experiments show that the specific gravity of Spirosto- 

 mum teres lies between 1.024 and 1.025, or very near to 1.025. 

 If we add 0.003 to the 1.025, as we did before, it becomes about 

 1.028. 



Platt obtained a specific gravity of spirostoma of about 1.017 

 (20, p. 34), which varies by about 0.008 from 1.025. As we have 



