Seeds and Seedlings - Seed Testing - page 23

SEEd unIT STruCTurE
Although a seed is a mature ovule, a seed unit may include attached loral parts as well. Thus, “seeds” of
the grass family consist of a caryopsis in which the ovary wall (pericarp) is fused with the wall (testa) of the
ovule; therefore, the outer coating is actually pericarp rather than integumentary tissue. In addition, seed
units of some grass species include extra-loral bracts (lemma, palea, glumes).
It is important to recognize the differences between common and botanical terminology. The follow-
ing discussion about seed structure will refer to both common and botanical terms, but emphasis will be
given to the botanical aspects. In seed testing, the general term “seed unit” is used to describe the dispersal
unit of plants in order to avoid misuse of the term “seed” in the strict botanical sense. Seed units have three
basic parts: (1) embryo, (2) food storage tissue(s) and (3) protective coverings.
Embryo
The embryo is usually differentiated into an axis consisting of a root, shoot and cotyledon(s). The volume
of the seed occupied by the embryo varies from species to species. Also, the developmental maturity of the
embryo at dispersal is species dependent.
food Storage Tissues
Food storage in seeds may occur in one or more of the following tissues: (1) endosperm, (2) cotyledons,
(3) perisperm, or (4) in the case of gymnosperm seeds, megagametophyte. Figure 2.11 shows examples of
these kinds of storage tissues.
Endosperm tissue is formed from the union of two polar nuclei and one male gamete within the
embryo sac. It is therefore usually triploid tissue. Seeds such as cereal grains in which the predominant
storage tissue is endosperm are called albuminous seeds. Those lacking endosperm or in which the endo-
sperm tissue is greatly reduced are called exalbuminous seeds, whose major storage tissue is usually the
cotyledonary tissues of the embryo. In this case, the embryo develops at the expense of endosperm, which
is consumed while the embryo grows and develops. Cotyledon tissue, being embryonic, is diploid.
The single cotyledon of monocot grass seeds is called the scutellum . It is not a storage tissue except
perhaps in the early stages of germination, but serves to absorb nutrients for the embryo from the endo-
sperm during germination.
Perisperm is nutritive tissue derived from the nucellus, and is therefore diploid. Seeds of sugar beet
contain a large amount of perisperm tissue. In the development of these seeds, triple fusion occurs, result-
ing in early endosperm development. But the endosperm does not develop beyond the free-nuclear stage,
leaving the nucellus as the major storage tissue. The storage tissue of gymnosperm seeds develops from the
female gametophyte and is therefore haploid.
Seed Protective Coverings
The layers surrounding the seed serve to protect it in several ways, and may also aid in its dispersal. Seed
coats also help to regulate the entry of water and gases and thereby inluence germination. They protect the
seed from mechanical injury and invasion by fungi. They may also enable the seed to pass through animal
digestive tracts without being digested.
The seed coat may be of several types. Morphologically, it is a protective layer composed of one
or more integuments, collectively called the testa. Often the ovary wall (pericarp) forms the protective
covering around the seed and may contain features (e.g., shape, barbs, hooks) which aid in its dispersal.
Occasionally, extra loral parts may also surround the seed and fruit. Examples include grasses, beet, and
atriplex.