When the apex
came into contact with the polished level surface it turned at right angles
and glided over it without leaving any impression; yet the tin-foil was so
flexible, that a little stick of soft wood, pointed to the same degree as
the end of the radicle and gently loaded with a weight of only a quarter of
an ounce (120 grains) plainly indented the tin-foil.
Radicles are able to penetrate the ground by the force due to their
longitudinal and transverse growth; the seeds themselves being held down by
the weight of the superincumbent soil. In the case of the bean the apex,
protected by the root-cap, is sharp, and the growing part, from 8 to 10 mm.
in length, is much more rigid, as Sachs has proved, than the part
immediately above, which has ceased to increase in length. We endeavoured
to ascertain the downward pressure of the growing part, by placing
germinating beans between two small metal plates, the upper one of which
was loaded with a known weight; and the
[page 74]
radicle was then allowed to grow into a narrow hole in wood, 2 or 3 tenths
of an inch in depth, and closed at the bottom.
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