412 



Harry Beal Torrey. 



cells, induced by a suspension of the stem from its distal end, in 

 which case the stem would come to a vertical position of its own 

 weight. This statement may be nearer the facts and yet not lead 

 us appreciably nearer an explanation. Why should the same cell 

 react in one way when the stem is attached proximally, and another 

 way or not at all when the stem is attached distally ? 



In seeking an answer for this question, it should be observed 

 that gravity may conceivably stimulate the stem in several ways: 

 (i) through the difference in the mechanical stresses on the two 



Fig. 3. 



Diagram to illustrate geotropism. Cc, compression, greater and less; Tt, tension, 

 greater and less. 



sides of the stem, (2) through the difference in the resistance en- 

 countered by the organism according as it goes upward (frictions- 

 weight) or downward (friction— weight) — Davenport's theory as 

 applied to free organisms, (3) by redistributing the contents of the 

 axial cells so that in any but a vertical position of the stem the cells 

 would be in a state of unstable equilibrium with respect to the 

 geotropic function. 



With reference to the first hypothesis, it may seem that the 

 difference in the response of the same cell may depend upon cer- 



