CHAPTER XV 



THE PHOTONASTIC PHENOMENA 



Phototropic response, positive or negative, has been shown 

 to be determined by the directive action of hght. There is, 

 however, a different class of phenomena already referred 

 to in previous chapters which is supposed to be independent 

 of the directive action of incident stimulus. This is the 

 so-called photonastic action of light. Strong sunlight, for 

 example, brings about a para-phototropic movement, by 

 which the apices of leaves or leaflets turn towards or away 

 from the strong source of light. The upward movement of 

 the leaflet of Erythrina has already been described. Far 

 more anomalous are the movements of the leaflets of 

 Averrhoa Cammhola and of Mimosa pudica. In Averrhoa 

 the effect of strong light from above is a movement down- 

 wards, away from light. In Mimosa the movement under 

 similar circumstances is precisely the opposite. 



The above description relates to the action of light from 

 above. What happens when the hght acts from below ? 

 The results are apparently even more inexplicable, as would 

 appear from the detailed accounts given below. 



Response of the leaflet of Averrhoa. —Strong light acting 

 in a downward direction from above induces, as already 

 stated, a movement downwards, away from light. Speak- 

 ing from the phototropic point of view, this may be 

 described as negative phototropism. But when strong light 

 is incident on the leaflet in an upward direction from below 

 the response is also a down-movement towards the light, and 

 has therefore to be described as positive phototropism. This 

 leads to the paradoxical conclusion that an identical organ 

 possesses two different irritabilities, negative and positive. 



