Such abnormalities would probably never, or
very rarely, occur with forms developed in a state of nature and subjected
to natural selection. One of the four radicles just mentioned in doubling
backwards came into contact with the pin by which the pea was fixed to the
cork-lid; and now it bent at right angles round the pin, in a direction
quite different from that of the first curvature due to contact with the
ruptured skin; and it thus afforded a good illustration of the tendril-like
sensitiveness of the radicle a little above the apex.
Little squares of the card-like paper were next affixed to radicles of the
pea at 4 mm. above the apex, in the same manner as with the bean.
Twenty-eight radicles suspended vertically over water were thus treated on
different occasions, and 13 of them became curved towards the cards. The
greatest degree of curvature amounted to 62o from the perpendicular; but so
large an angle was only once formed. On one occasion a slight curvature was
perceptible after 5 h. 45 m., and it was generally well-marked after 14 h.
There can therefore be no doubt that with the pea, irritation from a bit of
card attached to one side of the radicle above the apex suffices to induce
curvature.
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