Agriculture Reference
In-Depth Information
2
Seeds
and
Seedlings
A seed has been described as a miniature plant packaged for storage and shipment. This is a very good dei-
nition, for a seed does contain a miniature plant in the form of the embryo, along with reserve food storage
(endosperm, cotyledons, or other nutritive tissue) and a protective wrapping (seed coat).
Seeds are well adapted for distribution and storage. Their structure enables them to be transported and
disseminated by natural forces, such as wind, water, and animals, as well as agricultural practices. Many
seeds possess dormancy factors which allow them to remain viable for long periods until conditions are
suitable to insure their survival upon germination.
Botanically, a seed is a fertilized, mature ovule consisting of an outer protective seed coat enclosing an
embryo and a reserve food supply. The seed coat, also called the testa , is composed of one or more integu-
ments . The testa should not be confused with the pericarp or ripened ovary wall, which forms the outer
covering of such one-seeded fruits as caryopses (corn, wheat), achenes (sunlower, sugar beet), and other
true fruit structures that are commonly called seeds.
The embryo consists of a root-shoot axis and attached cotyledon(s). The root-shoot axis with the nec-
essary meristematic tissue allows it to grow and develop into the new plant upon germination. The number,
size, shape, and function of the cotyledons varies depending upon the species. In general, monocots have
one cotyledon, dicots have two cotyledons, and gymnosperms can have from one to several cotyledons. In
monocotyledonous grass species, the cotyledon is called the scutellum .
The food supply can take one of several forms. Most dicotyledonous plants store their food
within the cotyledons. Therefore, their storage tissue is actually part of the embryo (e.g., bean or pea).
Monocotyledonous plants, such as corn and wheat, store their food in endosperm tissue which is not part
of the embryo. Other storage tissues include perisperm which originates from nucellar tissue, such as that
found in sugar beet, and megagametophyte tissue of gymnosperm, e.g., conifer seed.
Seed development
Since seeds are mature, ripened ovules, a study of their development must begin with lower development.
The discussion below will address the development of both angiosperm and gymnosperm seeds. 1
1 Although the terms angiosperms and gymnosperms are commonly used, they are based on a previous classiication that had two
classes: Angiospermeae and Gymnospermeae within the division Spermatophyta (seed plants). At present, the superdivision Sper-
matophyta is subdivided into ive taxonomical divisions, one of which is Magnoliophyta (lowering plants - angiosperms), while
species collectively referred to as gymnosperms are now distributed among the remaining four divisions, i.e., Coniferiophyta,
Cycadophyta, Ginkyophyta, and Gnetophyta
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