As the stems of most other
plants are heliotropic, we may feel almost sure that twining plants, which
are distributed throughout the whole vascular series, have lost a power
that their non-climbing progenitors possessed. Moreover, with Ipomoea, and
probably all other twiners, the stem of the young plant, before it begins
to twine, is highly heliotropic, evidently in order to expose the
cotyledons or the first true leaves fully to the light. With the Ivy the
stems of seedlings are moderately heliotropic, whilst those of the same
plants when grown a little older
* Strasburger has shown in his interesting work ('Wirkung des Lichtes...auf
Schw?rmsporen,' 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.
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