, as the case may be. In the next chapter
some observations on the sensitiveness of plants to light, their
[page 420]
rate of bending towards it, and the accuracy with which they point towards
its source, etc., will be given. Afterwards it will be shown--and this
seems to us a point of much interest--that sensitiveness to light is
sometimes confined to a small part of the plant; and that this part when
stimulated by light, transmits an influence to distant parts, exciting them
to bend.
Heliotropism.--When a plant which is strongly heliotropic (and species
differ much in this respect) is exposed to a bright lateral light, it bends
quickly towards it, and the course pursued by the stem is quite or nearly
straight. But if the light is much dimmed, or occasionally interrupted, or
admitted in only a slightly oblique direction, the course pursued is more
or less zigzag; and as we have seen and shall again see, such zigzag
movement results from the elongation or drawing out of the ellipses, loops,
etc., which the plant would have described, if it had been illuminated from
above.
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