photOnastic curvatures 381 



from above, exhibit concavity of the shaded and more 

 excitabU' s^ide. 



PARA-HELIOTROPISM. 



Under strong sunlight, the leaflets oO various plants move 

 sometimes upwards, at other times downwards, so as to 

 pUice the blades of leaflets parallel to incident light. This 

 'mi(l(biy sleep' has been termed para-?ieliotropism by 

 Darwin. It has I)een thought that para-heliotropic action 

 has nothing to do with the directive action of light 

 since many leaflets either fold upwards or downwards, 

 irrespective of the direction of incident light. I shall 

 for convenience distinguish the leaflets which fold upwards 

 under light as iiositiveln para-heliotropic, and those which 

 fold downwards as Jiegatively para-heliotropic. This is 

 merely for convenience of description. There is no specific 

 irritabality which distinguishes one from the other. 



POSITIVE PARA-HELIOTROPISM. 



Para-heliotrupic response of Erythrina indict and of 

 Olitoria ternatea : Experiment 142. — For the purpose of sim- 

 plicity I have described the type of movement of these 

 leaflets as upwards; but the actual direction in which the 

 leaflets point their apices is towards the sun. Both the 

 plants mentioned here are so remarkably sensitive that 

 the leaflets follow the course of the sun, in such a way 

 that the axis of the cup, formed by the folding leaflets 

 at the end and the sides of the petiole, is coincident with 

 the rays of light. The pulvinus makes a sharp curva- 

 ture which is concave to light, the blade of the leaflet 

 being parallel to light. I have taken record of continuous 

 action of strong light acting on the responding pulvinus 

 of the leaflets from above. The result is an increasing 

 positive curvature which reached a limit (Fig. 141). There 



