426 LIFE MOVEMENTS IN PLANTS 



Thus light falling on one side of a shoot induces local 

 contraction, the rays being cut off from acting on the 

 further side by the opacity of the intervening tissue. Hut 

 there is no opaque screen to cut off the vertical linf-s of 

 gravity,* which enter the upper side of a horizontally laid 

 shoot and leave it by the lower side. Though lines of 

 force of gravity are transmitted without hindrance, yet a differ- 

 ential action is found to take place, for the upper side, 

 Avhere the lines of force enter, becomes concnve, while the 

 lower side where they emerge becomes convex. Why 

 should there be this difference ? 



For the removal of various obscurities connected with 

 geotropiam it is therefore necessary to elucidate the follow- 

 ing : 



1. The sign of excitation is, as we found, a contrac- 

 tion and concomitant galvanometric negativity. Does gravi- 

 tational stimulus, like stimulus in general, induce this 

 excitatory reaction ? 



2. What id the effective direction of geotropic sti- 

 mulus ? In the case of light, we are able to trace the 

 rays of light which is incident on the proximal side and 

 measure the angle of inclination. In the case of gravity, 

 the invisible lines of force enter by one side of the 

 organ and leave by the other side. Assuming that the 

 direction of stimulus is coincident with the vertical lines 

 of gravity, is it the upper or the lower side of the organ 

 that undergoes eft'ective stimulation ? 



3. What is the law relating to the ' directive angle ' 

 and the resulting geotropic curvature ? By the directive 

 angle (sometimes referred to as the angle of inclination) 



* I aliall in what follows take the direction of vertical lines of gravity as 

 that of movement of falling bodies, from above towards the centre of the 

 earth. 



