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
Schw?rmsporen' Verh. d. phys.-med. Geselsshalft in W?rzburg, 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.
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