Protective botanical layers

Usage examples of seedcoats.

But with most of the Cucurbitaceae there is a curious special contrivance for bursting the seedcoats whilst beneath the ground, namely, a peg at the base of the hypocotyl, projecting at right angles, which holds down the lower half of the seedcoats, whilst the growth of the arched part of the hypocotyl lifts up the upper half, and thus splits them in twain.

After a time the apex is drawn out of the empty seedcoats, and rises up, forming a right angle, or more commonly a still larger angle with the lower part, and occasionally the whole becomes nearly straight.

Long after the arch has risen above the surface the apex remains within the seedcoats, evidently absorbing the still abundant contents.

For instance, with the common bean, the epicotyl or plumule is bowed into an arch whilst breaking through the seedcoats, as shown in Fig.

It may, however, be suggested with some degree of probability that the arching was aboriginally caused by mechanical compulsion, owing to the confinement of the parts in question within the seedcoats, or to friction whilst they were being dragged upwards.

This movement facilitates the withdrawal of the tip of the epicotyl or of the cotyledons, as the case may be, from within the seedcoats and from the ground.

But the cotyledons often emerge from the ground still tightly enclosed within the seedcoats, which apparently serve to protect them.

The seedcoats are afterwards ruptured and cast off by the swelling of the closely conjoined cotyledons, and not by any movement or their separation from one another.

Cucurbitaceae, the seedcoats are ruptured by a curious contrivance, described by M.

Flahault, seedlings which have been prevented from casting their seedcoats whilst beneath the ground, are inferior to those which have emerged with their cotyledons naked and ready to act.

Cucurbitaceous genus known to us, in which the cotyledons are hypogean and do not cast their seedcoats, namely, Megarrhiza, there is no vestige of a peg.

It is a singular fact that after the ridge has done its work and has escaped from the seedcoats, it is developed into a frill all round the summit of the radicle.

But as both legs of the arch grow, their power of breaking through the ground will be much increased as long as the tip remains within the seedcoats and has a point of support.

As soon as any part of the arch protrudes from the seedcoats it is acted upon by apogeotropism, and both the legs bend upwards as quickly as the surrounding earth will permit, until the arch stands vertically.

The seedcoats are either left behind buried, or are retained for a time still enclosing the cotyledons.