CHAPTER XLII 



POSITIVE HELIOTROPISM 



Introduction — Theory of de Candolle — Inadequacy of de Candolle's theory — 

 Definition of terms positive and negative — Darwin's theory of modified circum- 

 nutation — Response of terminal leaflet of Desmodium — Extreme sensitiveness 

 of some plant-organs to light — Merging of multiple in continuous response- 

 Orientation induced by light — The perceptive region in the terminal leaflet of 

 Desmodium — Heliotropic response in radial organ— Magnetically controlled 

 recorder — Heliotropic response of hypocotyl of Sinapis — Recovery and theory 

 of recti-petality. 



Having demonstrated the fact that the stimulus of light 

 induces contraction in mature organs, and retardation of 

 growth in growing organs, in precisely the same manner as 

 any other form of stimulus, we shall now proceed to study 

 in detail the various effects produced by the unilateral 

 application of light. Such application, long continued, is 

 seen to induce movement of the organ, in some cases towards 

 the light, in others away from it, and in still other organs to 

 induce a position at right angles to it. While it is doubtless 

 convenient to distinguish such external effects as positive, 

 negative, and dia-heliotropic, it is nevertheless unfortunate 

 that these terms carry with them an assumption that the 

 movements in question are due to as many distinct sensi- 

 bilities on the part of the plant-organs. I shall, however, 

 endeavour to show that there is in the plant but one funda- 

 mental sensibility to light, as to other forms of stimula- 

 tion, which finds expression in contraction under its direct 

 action ; and that the resulting movements of the organs are 

 dependent on the question as to whether stimulus acts 

 directly or indirectly, unilaterally or diffusely, and also, 

 on anatomical or physiological peculiarities of structure 

 The complexity of the subject being very great, it has often 



