Botany; Geotropism; Plants -- Irritability and movements
The peg is developed with extraordinary rapidity; for it could only
just be distinguished in two seedlings, having radicles .35 inch in
length, but after an interval of only 24 hours was well developed in
both. It is formed, according to Flahault, by the enlargement of the
layers of the cortical parenchyma at the base of the hypocotyl. If,
however, we judge by the effects of a solution of permanganate of
potassium, it is developed on the exact line of junction between the
hypocotyl and radicle; for the flat lower surface, as well as the
edges, were coloured brown like the radicle; whilst the upper slightly
inclined surface was left uncoloured like the hypocotyl, excepting
indeed in one out of 33 immersed seedlings in which a large part of the
upper surface was coloured brown. Secondary roots sometimes spring from
the lower surface of the peg, which thus seems in all respects to
partake of the nature of the radicle. The peg is always developed on
the side which becomes concave by the arching of the hypocotyl; and it
would be of no service if it were formed on any other side. It is also
always developed with the flat lower side, which, as just stated, forms
a part of the radicle, at right angles to it, and in a horizontal
plane. This fact was clearly shown by burying some of the thin flat
seeds in the same position as in Fig. 62, excepting that they were not
laid on their flat broad sides, but with one edge downwards. Nine seeds
were thus planted, and the peg was developed in the
same position, relatively to the radicle, as in the figure;
consequently it did not rest on the flat tip of the lower half of the
seed-coats, but was inserted like a wedge between the two tips. As the
arched hypocotyl grew upwards it tended to draw up the whole seed, and
the peg necessarily rubbed against both tips, but did not hold either
down. The result was, that the cotyledons of five out of the nine seeds
thus placed were raised above the ground still enclosed within their
seed-coats. Four seeds were buried with the end from which the radicle
protrudes pointing vertically downwards, and owing to the peg being
always developed in the same position, its apex alone came into contact
with, and rubbed against the tip on one side; the result was, that the
cotyledons of all four emerged still within their seed-coats. These
cases show us how the peg acts in co-ordination with the position which
the flat, thin, broad seeds would almost always occupy when naturally
sown. When the tip of the lower half of the seed-coats was cut off,
Flahault found (as we did likewise) that the peg could not act, since
it had nothing to press on, and the cotyledons were raised above the
ground with their seed-coats not cast off. Lastly, nature shows us the
use of the peg; for in the one Cucurbitaceous genus known to us, in
which the cotyledons are hypogean and do not cast their seed-coats,
namely, Megarrhiza, there is no vestige of a peg. This structure seems
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