GEOTROPISM OF PARAMECIUM AND SPIROSTOMTM. 9 



The writer calculates the centrifugal force which he used for 

 the above experiments as follows: The revolutions with the 

 radius about 4 cm. long being about 15 per second, the cen- 

 trifugal force is given by the formula 



where M = mass in grams, r : radius of circle, n - -- number of 

 revolutions per second. This gives for r - 4 cm. and n - 15, 

 f M 35,494 dynes or M 36.2 grams. Moreover, the 

 revolutions were increased to about 20 per second, more or less, 

 as already stated, and then the centrifugal force was about 

 M 59,101 dynes or M 60.32 grams. Under these conditions, 

 is it thinkable that such a small animal as Paramecium "is able 

 to react at the outset of centrifugalization" against the cen- 

 trifugal force of the above calculation so as to direct itself with 

 its "heavier" posterior end toward the axis of the centrifuge as 

 Harper claims? 



Moreover, Harper states that "an excessive amount of iron 

 may overload 1 the animals apparently and caused them to aggre- 

 gate at the bottom" (3, p. 995). Even though an "overload of 

 iron" might be quite excessive for the animal, it could not possibly 

 be comparable with the centrifugal force which the writer used. 

 Nevertheless, Harper thinks that in the case of the strong 

 centrifugalization "the animal is able to react" with the sup- 

 posedly lighter anterior end directed away from the axis of the 

 centrifuge. Such mechanics we can hardly understand. 



Experiment b. With the same point in mind, the writer made 

 another series of experiments with iron-ingested paramecia, as 

 suggested by Harper's experiments. He mixed about a half- 

 gram of finely divided iron prepared by the hydrogen method 

 with about 2 c.c. of a dense culture of paramecia free from 

 sediment. The iron was kept in suspension by drawing the 

 water in and out with a pipette for about 5 minutes. All the 

 paramecia so treated ingested iron in the posterior end of the 

 body. Two or three drops of the culture containing paramecia 



1 Italic not in the original. 



