474 LIFE MOVEMENTS IN PLANTS 



growing region and induces there an expansion and ti 

 convex curvature. 



GEO-PERCEPTION AT THE ROOT TIP. 



The results given above fully confirm Charles Darwin's 

 discovery that it is the root tip that perceives the stim- 

 ulus of gravity *; he found that removal of the tip 

 abolished the geotropic response of the root. Objection 

 has been raised about the shock-eflLect of operation itself 

 being the cause of abolition of response. But subsequent 

 observations have shown that Darwin's conclusions are in 

 the main correct. 



The experiments which I have described on tiie geo- 

 electric response of the root tip and of the growing 

 region offer convincing proof of the perception of the 

 stimulus at the tip, and the transmission of the effect of 

 indirect stimulus to the growing region. These experi- 

 ments exhibit in an identical uninjured organ : the excita- 

 tory reaction at the upper side of the tip, the cessation 

 of excitation, and the excitation of the opposite side of 

 the tip, following the rotation of the organ through 

 -H 90°, 0° and -90°.- The effect at the growing zone is 

 precisely the opposite to that at the tip, i.e., an expansive 



'^ " This view has been the subject of a considerable amount of controversy. 

 Wiesner denies the localisation of geotropic sensitiveness. Czapek, on the 

 other hand, supports Darwin's theory. Recently Picard has attacked the 

 problem in a new way (and) concludes that not only the root tip but also the 

 entire growing zone is capable of pevceiving gravitational stimuli. ... As 

 both Picard's experimental method and his interpretation are open to criticism, 

 the author has repeated his experiments with a more satisfactory apparatus. 

 He finds that in Vicia Faha, Phaseolus multejlorus and Lupinns alhus, both 

 apex and growing zone are geotropically sensitive, the former being by far the 

 more sensitive of the two, and the curvature of the growing zone being 

 without a doubt largely induced by secondary stimuli transmitted from the 

 apical region. ; harles Darwin's views were therefore in the main correct." — 

 Haberlandt — Ibid, p. 748. 



