The first change is the protrusion of the
radicle, which begins at once to circumnutate. This movement is immediately
modified by the attraction of gravity and rendered geotropic. The radicle,
therefore, supposing the seed to be lying on the surface, quickly bends
downwards, following a more or less spiral course, as was seen on the
smoked glass-plates. Sensitiveness to gravitation resides in the tip; and
it is the tip which transmits some influence to the adjoining parts,
causing them to bend. As soon as the tip, protected by the root-cap,
reaches the ground, it penetrates the surface, if this be soft or friable;
and the act of penetration is apparently aided by the rocking or
circumnutating movement of the whole end of the radicle. If the surface is
compact, and cannot easily be penetrated, then
[page 549]
the seed itself, unless it be a heavy one, is displaced or lifted up by the
continued growth and elongation of the radicle. But in a state of nature
seeds often get covered with earth or other matter, or fall into crevices,
etc., and thus a point of resistance is afforded, and the tip can more
easily penetrate the ground.
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