75^ 
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 Aroideae, Orchidese, 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 grow^th 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 Aroideae 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 
TropcBolum 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. 
(y) Action of Light on the tension of the tissue of the contractile organs of leaves 
1 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 spectrum. 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) 7\ 
2 Knight, Phil. Trans. 181 2, Pt. I. p. 314. 
* Pfeffer, Arbeiten des bot. Inst, in Würzburg, 1871, Heft I. Div. 2, 
For the literature on this subject see Sachs, Exp.-Phys. p, 41. 
