ROOT OF SINAPIS I45 



the root, especially when it is thin, a condition which 

 facilitates transverse conduction of excitation to the distal 

 side. 



Should the power of transverse conduction in the shoot 

 be less than that in the root, then under the action of the 

 same unilateral stimulation b}^ light the stem would exhibit 

 a positive, while the root would show a negative, curvature. 

 This is seen in the illustration (fig. 82), reproduced from 

 a photograph, of the responsive curvature of stem and 

 root of Sanchezia nobilis. The stem, being thick, failed 

 to transmit excitation to the distal side ; the phototropic 

 curvature was therefore positive. But transverse conduc- 

 tion occurring in the root induced the negative phototropic 

 curvature. 



Phototropic Curvature of Root of Sixapis 



I next investigated the effect of unilateral light on the 

 thin root of Sinapis by taking a continuous record of its 

 movements. The root was too thin to give a direct record 

 by exerting a pull on the recording lever. The experiment 

 was therefore modified as follows : 



Experiment 89.^ — The plant was mounted with its root 

 irhmersed in a cubical glass vessel. A microscope with 

 micrometer eye-piece was focused on the tip of the root. 

 Light from a lOO-candle-power Pointolite was applied 

 laterally on one side, say the left. The incidence of the 

 lateral light at first induced a movement towards the light 

 (positive curvature), which went on increasing for 15 minutes, 

 after which there was a turning away from light. There 

 was neutralisation in the course of about 28 minutes, after 

 which an increasing negative . curvature was produced. 

 The following table gives readings of the scale taken every 

 5 minutes, the positive readings indicating the movement of 

 the root towards, and the negative readings that away from, 

 the light. 



