186 E. 11. HARPER 



2. Ingestion of iron for a moderate period, two minutes. 



In such a culture as this it was found that a large number of 

 the animals in a control orient upward and aggregate toward 

 the top of the water. But a considerable number showed evi- 

 dence of overloading and aggregated toward the lower part of 

 the jar. Under the influence of the magnetic field, a culture of 

 this sort showed the streaming movements distinctly. There 

 was this difference, that an aggregation was formed overhead 

 of the field of the magnet at the water surface. The animals 

 which are oriented upward continue in many cases to the top and 

 there may remain, often forming a dense ring at the water's 

 edge. These are therefore an aggregation of active animals 

 able to maintain themselves at the top, and contrasting with 

 the aggregation in the lower outside corner, consisting of slug- 

 gish individuals. The latter sort, if near enough to the field of 

 the magnet are sometimes noticed to move as if passively pulled 

 by its attraction. 



3. Ingestion of iron for a brief period, 20-30 seconds. 



In this length of time only a light load is taken in. With the 

 cultures used this produced a marked upward tendency in the 

 control, so as to leave the lower region approximately free from 

 Paramoecia. When this culture was brought under the influence 

 of the magnet it appeared that the upward streaming was for 

 a short time visibly greater on the side of the magnet, but there 

 was no noticeable compensating movement downward, and in 

 consequence the upward streaming was of short duration. The 

 aggregation above the magnetic pole was distinctly greater than 

 elsewhere, so that this case gives only a single region of aggre- 

 gation. 



THEORIES OF GEOTROPISM OF PARAMOECIUM 



Various explanations have been offered for the geotropic 

 response, such as sensitiveness to the difference in pressure 

 between the upper and lower portions of the organism; sensi- 

 tiveness to the greater resistance encountered in swimming 

 upward than downward ; response to internal localized stimuli 

 by heavier particles in the endoplasm — the statocyst theory. 

 All have taken for granted that orientation to gravity was 

 brought about by a response to some sort of stimulus. 



