480 



LIFE MOVEMENTS IN PLANTS 



I was led to discover it. The experiments described in 

 Chapter XL showed that the upper surface of a horizon- 

 tally laid shoot exhibits sign of excitation by induced 

 galvanometric negativity ; that this was due to the stimulus 

 of gravity was made clear by restoration of the plant-organ 

 to the vertical position, when all signs of electric excitation 

 disappeared. Now the skin of the organ on which the 

 electrode was applied could not be the perceptive organ, for 

 the removal of the epidermis did not abolish the geotropic 

 action ; the perceptive layer must therefore lie somewhere 

 in the interior. As every side of a radial organ undergoes 

 geotropic excitation, the geo-perceptive cells must therefore 

 be disposed in a cylindrical layer, at some unknown depth 



Fig. 174. — Diagrammatic representation of the geo-perceptive layer in 

 nnexcited vertical, and in excited horizontal poKition. (See text.). 



from the surface. In a longitudinal section of the shoot, 

 they would appear as two straight lines G and G' (Fig. 174). 

 In a vertical position the geo-perceptive layer will remain 

 quiescent but rotation through -|- 90° would initiate the 

 excitatory reaction. Let us first centre our attention to the 

 geo-percc^ptive layer G, which occupies the upper position. 

 This sensitive layer perceives the stimulus and is therefore 



