GEOTROPISM OF PARAMECIUM AXD SPIROSTOMUM. 13 



as in the preceding experiments. The majority of the animals 

 were found with anterior ends directed toward the open outer 

 end of the capillary tube. Owing to the animal's extreme 

 contractility and long form, the writer could not obtain such 

 invariable uniformity of results as he did in the case of Para- 

 mecium. Nevertheless, the proportion thus directing their 

 anterior end with reference to the centrifugal force was suffi- 

 ciently great to show that the position was not accidental. 



Experiment b. An attempt was made to repeat the iron- 

 ingestion experiments on spirostoma, but without success. The 

 writer mixed iron filings with about 2 c.c. of the "wild" culture 

 containing a dense suspension of spirostoma in the way that is 

 already described. Within five minutes, all of them broke into 

 pieces beginning at the anterior end and continuing to the 

 posterior, even though none of them ingested iron. Here we see 

 the great injury of iron to the animals; so that the iron ingested 

 by paramecia might not be "harmless" to these animals or at 

 least it might produce some chemical effect on them. 



III. THE PRESSURE THEORY AND ITS CRITICISM. 



Jensen, having been dissatisfied by Yerworn's mechanical 

 theory, proposed the "pressure theory" of geotropism (n, pp. 

 462-464). According to Jensen, the geotropic orientation of 

 paramecia is determined by the difference in hydrostatic pressure 

 between the upper and lower surfaces of the animal. In the 

 negative geotropism, therefore, the animal moves from the place 

 of high pressure to that of low; in positive geotropism, the 

 animal moves from the place of low pressure to that of high 

 (11, pp. 462-463). Davenport first criticized this theory of 

 Jensen's, and suggested a third theory (2, pp. 122-123). Jen- 

 nings (9, pp. 473-477) and Lyon (16, pp. xv-xvi) independently 

 disproved Jensen's theory from a physical point of view. 



Jennings calculated that "the difference in pressure between 

 the two sides of the organisms is only 1/1,000,000 of the pressure 

 acting everywhere on the surface. Furthermore, Jensen showed 

 that the reaction still occurs when the atmospheric pressure is 

 more than doubled; the effective difference in pressure would 

 be less than I 2,000,000 of the general pressure" (9, pp. 475- 



