See, also, Stahl, 'Ueber den einfluss der Lichts auf die Bewegungs-erscheinungen der Schwärmsporen' Verh. d. phys.-med. Geselsshalft in Würzburg, B. xii. 1878. [page 489]
are apheliotropic. Some tendrils which consist of modified leaves--organs in all ordinary cases strongly diaheliotropic--have been rendered apheliotropic, and their tips crawl into any dark crevice.
Even in the case of ordinary heliotropic movements, it is hardly credible that they result directly from the action of the light, without any special adaptation. We may illustrate what we mean by the hygroscopic movements of plants: if the tissues on one side of an organ permit of rapid evaporation, they will dry quickly and contract, causing the part to bend to this side. Now the wonderfully complex movements of the pollinia of Orchis pyramidalis, by which they clasp the proboscis of a moth and afterwards change their position for the sake of depositing the pollen-masses on the double stigma--or again the twisting movements, by which certain seeds bury themselves in the ground*--follow from the manner of drying of the parts in question; yet no one will suppose that these results have been gained without special adaptation. Similarly, we are led to believe in adaptation when we see the hypocotyl of a seedling, which contains chlorophyll, bending to the light; for although it thus receives less light, being now shaded by its own cotyledons, it places them--the more important organs--in the best position to be fully illuminated. The hypocotyl may therefore be said to sacrifice itself for the good of the cotyledons, or rather of the whole plant. But if it be prevented from bending, as must sometimes occur with seedlings springing up in an entangled mass of vegetation, the cotyledons themselves bend so as to face the light; the one farthest off rising
* Francis Darwin, 'On the Hygroscopic Mechanism,' etc., 'Transactions Linn. Soc.,' series ii. vol. i. p. 149, 1876. [page 490]
up, and that nearest to the light sinking down, or both twisting laterally.* We may, also, suspect that the extreme sensitiveness to light of the upper part of the sheath-like cotyledons of the Gramineae, and their power of transmitting its effects to the lower part, are specialised arrangements for finding the shortest path to the light. With plants growing on a bank, or thrown prostrate by the wind, the manner in which the leaves move, even rotating on their own axes, so that their upper surfaces may be again directed to the light, is a striking phenomenon. Such facts are rendered more striking when we remember that too intense a light injures the chlorophyll, and that the leaflets of several Leguminosae when thus exposed bend upwards and present their edges to the sun, thus escaping injury. On the other hand, the leaflets of Averrhoa and Oxalis, when similarly exposed, bend downwards.
It was shown in the last chapter that heliotropism is a modified form of circumnutation; and as every growing part of every plant circumnutates more or less, we can understand how it is that the power of bending to the light has been acquired by such a multitude of plants throughout the vegetable kingdom. The manner in which a circumnutating movement--that is, one consisting of a succession of irregular ellipses or loops--is gradually converted into a rectilinear course towards the light, has been already explained. First, we have a succession of ellipses with their longer axes directed towards the light, each of which
* Wiesner has made remarks to nearly the same effect with respect to leaves: 'Die undulirende Nutation der Internodien,' p. 6, extracted from B. lxxvii. (1878). Sitb. der k. Akad. der Wissensch. Wien. [page 491]
is described nearer and nearer to its source; then the loops are drawn out into a strongly pronounced zigzag line, with here and there a small loop still formed. At the same time that the movement towards the light is increased in extent and accelerated, that in the opposite direction is lessened and retarded, and at last stopped.