CHAPTER XLVI 



ON DIA-HELIOTROPISM AND DIA-GEOTROPISM 



Difficulty of distinguishing between effect of light and other reactions— Theories 

 of Frank and De Vries— Subsidiary factors : (i) Epinasty and hyponasty ; 

 (2) Effect of gravity ; (3) Effect of suctional activity and of turgescence ; 

 (4) Modification of effect by characteristic limits of flexibility — Discrimination 

 of the part played by heliotropism in the movement of the leaf — Proof of 

 absence of any specific dia-heliotropic tendency in leaves— The lamina not 

 the perceptive organ— Principal types of the response of leaves to stimulus of 

 light— Positive type of response : mango leaf— Negative type of response : 

 leaf of Artocarpus. 



We shall now take up the question of the effects of 

 illumination on ordinary leaves, which are generally supposed, 

 on account of some special dia-heliotropic sensibility, to have 

 the habit of placing themselves at right angles to incident 

 light. We have seen in the last chapter that the movement 

 of anisotropic organs under heliotropic stimulation is not the 

 result of any specific sensitiveness, but constitutes a simple 

 instance of the response of plant-organs to all forms of 

 stimulus, the response being appropriately modified in this 

 particular case by the anatomical and physiological pecu- 

 liarities of the responding organ. We have been able to 

 demonstrate the continuity of this responsive phenomenon 

 by analysing it in two extreme cases of the anisotropic 

 differentiation, those namely of the plagiotropic stem, in 

 which we see one of its earlier phases, and of dorsi-ventral 

 pulvinated organs, in which it attains its highest development. 

 In the movements of ordinary leaves we have what is merely 

 an intermediate stage between these, hence any explanation 

 which would elucidate their action must be one which is 

 applicable to both the extremes. 



