]
These several cases show beyond doubt that the irritation of one side of
the apex, excites the upper part of the radicle to bend slowly towards the
opposite side. This fact was well exhibited in one lot of five seeds pinned
to the cork-lid of a jar; for when after 6 days the lid was turned upside
down and viewed from directly above, the little black marks made by the
caustic were now all distinctly visible on the upper sides of the tips of
the laterally bowed radicles. A thin slice was shaved off with a razor from
one side of the tips of 22 radicles, in the manner described under the
common bean; but this kind of irritation did not prove very effective. Only
7 out of the 22 radicles became moderately deflected in from 3 to 5 days
from the sliced surface, and several of the others grew irregularly. The
evidence, therefore, is far from conclusive.
Quercus robur: Sensitiveness of the apex of the Radicle.--The tips of the
radicles of the common oak are fully as sensitive to slight contact as are
those of any plant examined by us. They remained healthy in damp air for 10
days, but grew slowly.
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