PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 623 



planation, applicable to the phenomena both of photonasty 

 and of para-heliotropism. It will be noticed that in the first 

 case we have to deal with the effect of light on all growing 

 dorsi-ventral organs, and in the second with its effect on 

 mature dorsi-ventral pulvini ; and in dealing with both 

 these classes I shall pass step by step from the consideration 

 of simple to that of more complex types. 



The response of Tropaeolum majus. — Sachs found that 

 when the stem of Tropceolum majus is exposed to intense 

 and long-continued unilateral illumination, a negative helio- 

 tropic curvature is induced ; but when the plant is exposed 

 to moderate unilateral illumination it exhibits positive helio- 

 tropic movement. The explanation of this difference is 

 made quite clear from the experiments which I have already 

 described, with reference to seedlings of Sinapis nigra and to 

 the tendril of Vitis ; for it has been shown that in these 

 cases moderate unilateral stimulus of light caused a positive 

 heliotropic effect ; whereas, under the action of intense stimu- 

 lation, a transient anisotropy was induced, on account of 

 which the excitability of the over-stimulated proximal side 

 was diminished. Hence the distal side was rendered the 

 relatively more excitable, and the intense stimulus becoming 

 internally diffused caused a concavity of the distal side, and 

 gave rise to a negative heliotropic effect (p. 609). 



The negative effect is thus due to anisotropy, and internal 

 diffusion of excitation from the proximal to the distal sides, 

 this latter factor evidently tending to be facilitated by any 

 agency that increases the conductivity of the tissue (p. 603). 

 We have also seen elsewhere that this power of conductivity 

 is very much augmented in summer (p. 475). In connection 

 with this, it is interesting to note that the negative heliotropic 

 curvature of Tropceolum majus, clue to transmitted excita- 

 tion, has only been observed by Sachs during summer, while 

 in autumn the effect observed by him was always positive, 

 owing to stimulus remaining localised. Another instance of 

 the same kind is furnished by the hypocotyl of Ivy {Hedera 

 helix), which from the normal positive heliotropic curvature, in 



