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I sperm nucleus unites with two polar nuclei present
in the embryo sac to form an endosperm nucleus, which later produces
the nutritive endosperm tissue surrounding the embryo in the seed.
In gymnosperms, the endosperm is formed from the tissue of the
embryo sac itself. The nucellus, or megasporangium, is the
tissue composing the main part of the ovule; it is partially
digested during the development of the embryo and endosperm tissue.
Surrounding the seed is a hard, tough seed coat, derived from the
integument of the ovule and known as the testa. In flowering
plants a second seed coat occurs within the testa; this second coat
is thin and membranous and is known as the tegmen. Some
seeds, in addition, have projects from the seed coat that serve to
aid in the absorption of water when the seed is about to germinate
or that merely form an additional protective coating about the seed.
In almost every seed, the micropyle through which the pollen tube
entered the ovule persists as a small opening in the seed coat.
Close to the micropyle in flowering plants, a stalk, or funiculus,
attaches the seed to the placenta on the inside of the fruit wall.
When the seed is removed, a small scar, known as the hilum,
marks the former attachment of the stalk.
In a few plants, such as the orchids,
the embryo is a small, undifferentiated mass of cells until after
the seed has parted from the parent plant; during the period between
separation from the parent plant and eventual germination, the
undifferentiated cells develop into an embryonic root, bud, stalk,
and leaf. In most other plants this development occurs prior to seed
dispersal: the embryonic root, or radicle, usually grows
toward the micropyle; the embryonic bud, called plumule, or
epicotyl, is at the end of the embryo opposite to the radicle;
the embryonic stem, or hypocotyl, connects the radicle with
the seed leaves, or cotyledons. In gymnosperms, several
cotyledons are usually present; among angiosperms two great groups
of plants exist, one group having but one cotyledon in the seed and
known as the monocotyledons, and the other with two
cotyledons and known as dicotyledons. The cotyledons serve as
centers of absorption and storage, drawing nutritive material from
the endosperm. The cotyledons of many plants, such as the sunflower,
function as primary photosynthetic organs after germination and
before the development of foliage leaves from the plumule. |
