Nevertheless, this explanation did not seem to
us satisfactory. The radicles did not present the appearance of having been
subjected to a sufficient pressure to account for
[page 131]
their curvature; and Sachs has shown* that the growing part is more rigid
than the part immediately above which has ceased to grow, so that the
latter might have been expected to yield and become curved as soon as the
apex encountered an unyielding object; whereas it was the stiff growing
part which became curved. Moreover, an object which yields with the
greatest ease will deflect a radicle: thus, as we have seen, when the apex
of the radicle of the bean encountered the polished surface of extremely
thin tin-foil laid on soft sand, no impression was left on it, yet the
radicle became deflected at right angles. A second explanation occurred to
us, namely, that even the gentlest pressure might check the growth of the
apex, and in this case growth could continue only on one side, and thus the
radicle would assume a rectangular form; but this view leaves wholly
unexplained the curvature of the upper part, extending for a length of 8-10
mm.
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