POSITIVE HELIOTROPISM 



593 



In this experiment, on a seedling of Sinapis, light was 

 allowed to strike the growing organ horizontally. The 

 specimen was very sensitive, and the source of light employed 

 at first was a candle placed at a distance of 20 cm. acting on 

 the plant for three minutes. The responsive movement began 

 within five seconds, and though the light was cut off, there 

 were produced three multiple responses, after which the plant 

 underwent a complete recovery and resumed its former 

 position. Sunlight was next applied, and a continuous move- 

 ment towards the light was induced (fig. 242). 



Fig. 242. Heliotropic Response of 

 Sinapis 



x Application of candle-light for three 

 minutes gave three multiple re- 

 sponses ; f application of sunlight 

 gave rise to continuous response. 



Fig. 243. Heliotropic Response of 

 Sinapis to Sunlight 



Dotted line shows after-effect and 

 recovery on cessation of light. 



In another case sunlight was applied for twelve minutes. 

 The response commenced almost immediately on application, 

 and the average rate of movement was 1 mm. per minute. 

 In this particular case, the positive after-effect persisted for 

 five minutes, even on the stoppage of light, after which 

 there was a gradual recovery (fig. 243). 



Recovery and theory of recti-petality. — On the cessation 

 of stimulus, there is a more or less perfect recovery of the 

 radial organ from its induced curvature. I have already 

 demonstrated the fact that the growing organ acts as a 

 diffused pulvinoid. We have just seen that the primary 

 action of light is to induce similar motile effects in both 



QQ 



