450 



LIFE MOVEMENTS IN PLANTS 



Experiment 172. — I shall, as a typical example, give a 

 detailed account of experiments with the petiole of 

 Tropmolum which was found so highly excitable to 

 geotropic stimulus (p. 434). The specimen was held horizontal 

 with two symmetrical contacts at the two sides, the electrodes 

 being connected in^ the usual manner with the indicating 

 galvanometer. When the plant is rotated through +90° 

 there is an immediate current of response, the upper 

 side becoming galvanometrically negative. This excitatory 

 reaction on the upper side finds, as we have seen, mecha- 

 nical expression by contraction and concavity, with posi- 

 tive or up-curvature. 



The differential stimulation of A and B disappears on 

 rotation of the axis back to zero position, and the induced 

 electro-motive response also disappears at the same time. 

 If now the axis be rotated through - 90°, A will become 

 the lower, and B the upper and the excited side. The 

 electro-motive change is now found to have undergone a 

 reversal, B becoming galvanometrically negative. This 

 induced electro-motive variation under geotropic stimulus is 



Fig. IriT. — ^Uiagranimatic lepreseatation of the geo-electric response of the 

 shoot (see text). 



of considerable intensity often exceeding 15 millivolts. The 

 characteristic electric change is shown diagrammatically in 

 figure 167 in which the middle figure shows the symme- 



