XXIX.— ON PHOTOTROPISM 



By 



Sir J. G. Bosb. 



In different organs of plants the stimulus of light in- 

 duces movements of an extremely varied character. Radial 

 organs exhibit tropic movements in which the position of 

 equilibrium is definitely related to the direction of incident 

 stimulus. Nastic movements nnder the action of light are, 

 on the other hand, regarded as curvatures of the organ 

 which show "no relation to the stimulus but is determined 

 by the activity of the plant itself".* There are thus 

 two classes of response to light which seem to be unrelat- 

 ed to each other. Returning to the directive action of 

 light, radial stems often bend towards the light, while 

 certain roots bend away from it. It may be thought that 

 this difference is due to specific difference of irritability 

 between shoot and root, the irritability of the former being 

 of a positive, and of the latter, of a negative character. 

 But there are numerous exceptions to this generalisation. 

 Certain roots bend towards the light, while a stem, under 

 different circumstances, moves towards light or away from 

 it. Again an identical organ may exhibit a positive or a 

 negative curvature. Thus the leaflets of Mimosa j^udica 

 acted on by light from above fold upwards, the photo- 

 tropic effect being positive. But the same leaflets acted on 

 by light from below exhibit a folding upwards, the photo- 

 tropic effect being now negative. Effects precisely the 

 opposite are found with the leaflets of BiopJiytum and 

 Averrhoa. They fold downwards whether light acts from 



* Jost— Ibid, p. 428. 



25 A 



