A somewhat analogous structure occurs in Mimosa
pudica and some other plants. Before the cotyledons are fully expanded and
have diverged, the hypocotyl generally straightens itself by increased
growth along the concave side, thus reversing the process which caused the
arching. Ultimately not a trace of the former curvature is left, except in
the case of the leaf-like cotyledons of the onion.
The cotyledons can now assume the function of leaves, and decompose
carbonic acid; they also yield up to other parts of the plant the nutriment
which they often contain. When they contain a large stock of nutriment they
generally remain buried beneath the ground, owing to the small development
of the hypocotyl; and thus they have a better chance of escaping
destruction by animals. From unknown causes, nutriment is sometimes stored
in the hypocotyl or in the radicle, and then one of the cotyledons or both
become rudimentary, of which several instances have been given. It is
probable that the extraordinary manner of germination of Megarrhiza
Californica,
[page 557]
Ipomoea leptophylla and pandurata, and of Quercus virens, is connected with
the burying of the tuber-like roots, which at an early age are stocked with
nutriment; for in these plants it is the petioles of the cotyledons which
first protrude from the seeds, and they are then merely tipped with a
minute radicle and hypocotyl.
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