GEOTROPISM IN PARAMOECIUM 187 



Verworn is credited by Lyon with having earlier held that 

 the geotropic response of Paramoecium was explicable on the 

 basis of the form and structure of the organism, a view which 

 he is said to have later abandoned. The original reference is not 

 at hand. 



These theories are discussed by Davenport, with the exception 

 of the later one of Lyon, and he credits the mechanical factor 

 as having some influence at least in producing the geotropic 

 orientation. 



DISCUSSION OF RESULTS 



In presenting what appeared to the writer to be valid reasons 

 for accepting the view that in the case of experiments with 

 loaded animals the geotropic tendency is a result of passive 

 orientation rather than a response to any form of stimulus by 

 gravity, I would not have it supposed that I am opposing the 

 possibility of the action of gravity as a stimulus under any 

 conditions. Inasmuch as evidence has been offered in this paper 

 for the action of gravity as a stimulus upon heavily loaded animals, 

 tending to check their upward movement, it would be unwise to 

 harbor a predilection against the possibility of its acting as a 

 stimulus to account for movement in either direction. 



Jennings instances the case of the geotropic collection of 

 Paramoecia in distilled water which give the avoiding reaction 

 on starting to move downward and so are oriented upward 

 again. This furnishes an opportunity to compare my results 

 with those of Jennings. In no case have I a record of a complete 

 geotropic collection at the top of the tube in my experiments 

 with iron filings and ordinary w T ater. There have been aggre- 

 gations toward the top with no sharp dividing line and the 

 animals all show active swimming movements in all directions, 

 except those which collect on the walls of the vessel and in the 

 surface film. Hence there is no possibility of recognizing the 

 geotropic response per se as there is in the pure form of the reac- 

 tion described by Jennings. There is however abundant oppor- 

 tunity to observe the gradual swinging from a horizontal to a 

 vertical path among such animals and the recognition of it as a 

 prevailing type of movement. I have given the chief reason 

 for using ordinary water in these experiments instead of distilled 

 in which the pure response may be elicited. It is, to repeat, 

 because the greater negative tendency of the properly loaded 



