756 GENERAL CONDITIONS OF PLANT- LIFE. 



one of ammoniacal copper oxide, the internodes of the first remain quite straight 

 and lengthen considerably as if they were in the dark, while those exposed to the 

 mixed blue light grow less and at the same time bend strongly towards the light. 

 It follows from this that only rays of high refrangibility, the blue, violet, and 

 ultra-violet, cause the curvature by retarding growth ^ 



In addition to the large number of the parts of plants which, when illumi- 

 nated unequally, bend so as to make the more strongly illuminated side concave, 

 there are a much smaller number which bend in the opposite direction, i. e. become 

 concave on the shaded side. In order to distinguish between them the former are 

 termed positively^ the latter negatively heliotropic *. 



Both positive and negative heliotropism occur not only in organs containing 

 chlorophyll, but also in those that are colourless; among the former in the green 

 tendrils of Vitis and Ampelopsis^ ; among the latter in the colourless root-hairs 

 of Marchantia *, the aerial roots of Aroidese, Orchideae, and Chlorophytum Gayanum, 

 and the rootlets of some Dicotyledons, as Brassica Napus and Sinapis alba^. 

 From the statement that positive heliotropism depends on a retardation of the 

 growth of the organ exposed to the stronger light, it might be inferred that negative 

 heliotropism is occasioned conversely by a more vigorous growth of the side 

 exposed to the stronger light. This conclusion would be confirmed by a superficial 

 examination of the phenomena; but if the attendant circumstances are observed 

 more closely, some considerations arise which I shall examine in detail in Chap. 

 IV. It need only be mentioned here that according to a theory started by Wolkoff, 

 two different explanations are possible : — Very transparent organs, like the apices 

 of the roots of Aroidese and of Chlorophytum, refract the light which falls upon 

 them in such a manner that the shaded side of the organ may actually be more 

 strongly illuminated than the other; and its negative heliotropism is then only 

 a special case of positive heliotropism. But in other cases, as in the Ivy and 

 Tropceolum majus^ the internodes are positively heliotropic when young, but 

 negatively when old before growth ceases ; and Wolkoff supposes that the curvature 

 which is in these cases convex on the illuminated side is caused by the more 

 vigorous assimilation and consequent longer duration of growth. It depends therefore 

 upon nutrition which only affects the mechanism of growth in a secondary degree. 



{/) Action of Light on the tensibn of the tissue of the contractile organs of leave's 



' See Sachs, Bot. Zeitg. 1865, On the action of coloured light on plants, where the literature is 

 also quoted. I consider experiments with absorbent fluids more decisive than those with the spec- 

 trum ; in this latter Guillemin states that not only do all the rays act heliotropically, but that there 

 is even a lateral curvature towards the blue end of the spectruni. When the light is sufficiently 

 strong the spectrum is certainly never free from diffused white light, which will cause heliotropism 

 even when its intensity is very small. [Wiesner has found (Heliotropische Erscheinungen, noticed 

 in detail in Chap. IV) that although the yellow rays do not give rise to heliotropic curvatures, 

 they exercise, nevertheless, a retarding influence on growth.] 



^ [Darwin (Movements of Plants) uses the terms ' heliotropic ' and • apheliotropic ' instead of 

 • positively ' and ' negatively heliotropic' He considers that the heliotropic movements are modified 

 forms of circumnutation (see infra).'] 



3 Knight, Phil. Trans. 181 2, Pt. I. p. 314. 



* Pfeffer, Arbeiten des bot. Inst, in Wiirzburg, 1871, Heft I. Div. 2. 



^ For the literature on this subject see Sachs, Exp.-Phys, p. 41. 



