* Strasburger has shown in his interesting work (’Wirkung des Lichtes...auf Schwärmsporen,’ 1878), that the movement of the swarm-spores of various lowly organised plants to a lateral light is influenced by their stage of development, by the temperature to which they are subjected, by the degree of illumination under which they have been raised, and by other unknown causes; so that the swarm-spores of the same species may move across the field of the microscope either to or from the light.  Some individuals, moreover, appear to be indifferent to the light; and those of different species behave very differently.  The brighter the light, the straighter is their course.  They exhibit also for a short time the after-effects of light.  In all these respects they resemble the higher plants.  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]