* The latter, or heliotropic movements are
determined by the direction of the light, whilst periodic movements are
affected by changes in its intensity and not by its direction. The
periodicity of the circumnutating movement often continues for some time in
darkness, as we have seen in the last chapter; whilst heliotropic bending
ceases very quickly when the light fails. Nevertheless, plants which have
ceased through long-continued darkness to move periodically, if re-exposed
to the light are still, according to Sachs, heliotropic.
Apheliotropism, or, as usually designated, negative
* 'Physiologie Veg.' (French Translation), 1868, pp. 42, 517, etc.
[page 419]
heliotropism, implies that a plant, when unequally illuminated on the two
sides, bends from the light, instead of, as in the last sub-class of cases,
towards it; but apheliotropism is comparatively rare, at least in a
well-marked degree. There is a third and large sub-class of cases, namely,
those of "transversal-Heliotropismus" of Frank, which we will here call
diaheliotropism. Parts of plants, under this influence, place themselves
more or less transversely to the direction whence the light proceeds, and
are thus fully illuminated.
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