Growth Movements 



499 



while curvature occurs in the 

 region of greatest growth. 

 The time required for per- 

 ception (presentation time) 

 varies from a few minutes to 

 several hours. Reaction time 

 is often several hours, and re- 

 sponse is evident even if 

 meanwhile the position of the 

 organ is again shifted. The 

 mechanism of geotropic per- 

 ception is not clearly under- 

 stood. An early mechanical 

 theory of Knight has found 

 new life in the statolith theory 

 of Nemec, Haberlandt, and 

 others. By this hypothesis it 

 is assumed that in shifted 

 structures the sinking of cer- 

 tain cell products, especially 

 starch, to the bottom of the 

 cell produces a change of 

 pressure, and it is this change 

 which furnishes the excitation. 

 303. Thigmotropism. - The 

 capacity for thigmotropic re- 

 sponse, or growth curvature 

 induced by contact, is most 

 highly developed in tendrils. 



FIG. 143. Diagrammatic view of geo- 

 tropic curvature in an etiolated hypo- 

 cotyl, horizontally placed. [After 

 Noll.] 



