They
were, however, cast off in the course of two or three days by the swelling
of the cotyledons. Until this occurs light is excluded, and the cotyledons
cannot decompose carbonic acid; but no one probably would have thought that
the advantage thus gained by a little earlier cast-
* 'Bull. Soc. Bot. de France,' tom. xxiv. 1877, p. 201.
[page 103]
ing off of the seed-coats would be sufficient to account for the
development of the peg. Yet according to M. Flahault, seedlings which have
been prevented from casting their seed-coats whilst beneath the ground, are
inferior to those which have emerged with their cotyledons naked and ready
to act.
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.
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