Agriculture Reference
In-Depth Information
one to three layers of cell wall may form around the periphery, with free nuclei inside; (2) a cell wall may
form in the micropylar area, with the rest remaining in a free-cell state; or (3) the entire endosperm may be
illed with walled cells. The helobial endosperm is intermediate between the nuclear and the cellular types.
Free-nuclear divisions occur, but cell wall formation accompanies nuclear division in some parts of the
endosperm as well. In some cases (sugar beet, pigweed) the endosperm does not proceed beyond the free-
nuclear state and the nucellus becomes the dominant nutritive tissue. This tissue is called
perisperm
rather
than endosperm. The female gametophyte in gymnosperms is analogous to endosperm in the angiosperms;
they serve the same function, but their origin is different.
In seeds of many species, especially dicotyledons, the endosperm develops only a few cells; while
in others it may be highly modiied and hardly recognizable. In Orchidaceae, it is completely suppressed.
Triple fusion occurs in Orchidaceae, but the products soon degenerate after one or two cell divisions. In
most dicotyledons, the endosperm is formed but is almost completely consumed during seed development
so that the mature seed is composed almost entirely of embryo. Considerable speculation exists about the
status of the endosperm. It has been called an anomalous embryo, since the egg and the two polar nuclei
are genetically identical. Regardless of their genetic similarity, the fusion of the polar nuclei and the male
gamete yield the endosperm, while the fusion of an egg with the other male gamete yields a zygote.
One of the principal endosperm functions is to provide nutrition for the developing embryo; therefore,
its composition is compatible with the embryo's needs. But the endosperm must also draw its nutritive
support from the embryo sac and surrounding tissues. The net effect is to surround the embryo with a rich
nutritive tissue from which it can draw for development and growth. This creates competition for nutrients,
both within and outside the embryo sac.
GyMnoSPErM SEEd
Gymnosperms comprise only about 520 species and are those plants producing seeds not enclosed within
an ovary (i.e., naked seeds). They are commonly represented by cone-bearing coniferous trees. Unlike
angiosperms, most gymnosperm seeds are the product of single fertilization; however, double fertilization
does occur in
Gnetum
and
Ephedra
, but no endosperm is produced.
The study of gymnosperm seed development should begin with an understanding of the female cone
and its parts. The seed (or mature ovule) of gymnosperms is borne on an
ovuliferous
scale,
which is attached
to a central cone axis. Photosynthetic bracts are present in some species. The grouping of the scales in the
form of a cone protects the seed from environmental factors and serves to nourish developing seeds.
Gymnosperm seed development may be divided into four distinct stages, including (1) ovule develop-
ment, (2) development of the female gametophyte with egg-illed archegonia, (3) pollination and fertiliza-
tion, and (4) embryo and seed development.
The following discussion will cover seed development of Douglas ir (
Pseudotsuga
sp.). Although
slight variations occur among species, the processes discussed are typical for most gymnosperm seeds.
ovule development
The ovuliferous scales on which the seeds are borne are fused to the base of the photosynthetic bract and
gradually grow larger than the bract. A slight swelling develops on either side of the ovule and gradually
takes the shape of two large integuments. One grows faster than the other and is covered with pubescence
to entrap pollen. Two ovules develop on each ovuliferous scale frame, forming the integuments or seed
coat surrounding the ovule as in angiosperm seeds (Fig. 2.6). The most important difference between ovule
development in gymnosperm seeds from that in angiosperms is the absence of an ovary which serves as a
protective and nutritive organ. The cone made up of ovuliferous scales provides the same functions in gym-
nosperm seed development. The absence of the ovary in gymnosperm seeds leaves them “naked” (gymnos)
to the environment.
