This difference in their behaviour seems at first inexplicable, but can, we
believe, be simply explained by the difference between the initial power of
the two forces under the above circumstances, combined with the well-known
principle of the after-effects of a stimulus. When a young and sensitive
radicle is extended horizontally, with a square attached to the lower side
of the tip, geotropism acts on it at right angles, and, as we have seen, is
then evidently more efficient than the irritation from the square; and the
power of geotropism will be strengthened at each successive period by its
previous action--that is, by its after-effects. On the other hand, when a
square is affixed to a vertically dependent radicle, and the apex begins to
[page 154]
curve upwards, this movement will be opposed by geotropism acting only at a
very oblique angle, and the irritation from the card will be strengthened
by its previous action. We may therefore conclude that the initial power of
an irritant on the apex of the radicle of the bean, is less than that of
geotropism when acting at right angles, but greater than that of geotropism
when acting obliquely on it.
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