What is Seed? – Definition
Fruit is a matured or ripened ovary which contains one or more ovules that develop into seed.
Botanically seed is defined as matured (after fertilization) and ripened ovule which contains an embryo with food reserve and protective coat.
As per seed technology or agriculture seed is any plant part which is used for raising or propagation or multiplication of commercial crops e.g. true seed, tubers, suckers, bulbs, cuttings, setts and grafts.
Classification of Seed
| Monocotyledonous Seed | Dicotyledonous Seed | Polycotyledonous Seed | ||
|---|---|---|---|---|
| Albuminous or endospermic seed | Exalbuminous or nonendospermic seed | Albuminous or endospermic seed | Exalbuminous or nonendospermic seed | – |
| wheat, rice, jowar, maize, other cereals | orchids | castor, papaya, custard apple | gram, pea, sunflower, safflower, cotton, mango, gourds | gymnosperms |
Structure of typical Seed
Basic terms related to seed structure are as follows
- Seed Coat
- Endosperm
- Embryo
- Cotyledon
- Epicotyl and Hypocotyl
- Radicle
- Plumule
- Coleoptile
Seeds of all the species do not have the same structure. Some seeds have, the endosperm retained as the storage tissue, while in other, the endosperm is more or less used up to put store chemicals into the embryo itself (usually in the cotyledons).
Seed Coat
It is a three-layered outer covering of seed. The outer thick layer is called as testa and inner thin one as tegmen. The seed develops from the tissue, the integument, originally surrounding the ovule. It protects the embryo sac, from external environment and regulates movements of water and gases into and out of seed during germination of seed. They can be of varying thicknesses, depending on the plant seed, can be a paper-thin layer (e.g. peanut) or thick and hard (e.g. coconut).
Endosperm
It is a triploid (albumin) tissue found in the seeds of most of the flowering plants. It provides nutrition to the developing embryo, in the form of starch, though it can also contain oils and protein. Endosperm is formed when the two male gametes inside a pollen grain reach the interior of an ovary. One gamete fertilizes the ovule, forming a fertilized egg. The other gamete fertilizes two polar nuclei (fused to form secondary nucleus) within the center of the ovary, creating endosperm.
Some plant seeds (e.g. peas) absorb the endosperm into their cotyledons, which then becomes the major source of nutrition during development. Most, however, keep it.
There are two different types of endosperm formation, the nuclear type and cellular type.
In nuclear type formation of cell wall is delayed for a number of cell divisions, it is common in angiosperms. While in cellular type cell wall formation is initiated instantly.
Cotyledon
cotyledon is the embryonic first leaf of a germinating seed, often stores food materials. Cotyledons may be either epigeal or hypogeal. The number of cotyledons present is one of the characteristic used to classify flowering plants (angiosperms). Species with one cotyledon are called as monocotyledonous – monocots and those with two cotyledons are dicotyledonous – dicots.
Radicle
It is the first part of a seedling (a growing plant embryo) to emerge from the seed when it germinates. The radicle is the embryonic root of the plant. It grows downward in the soil. Above the radicle hypocotyl is present, which supports cotyledons.
Hypocotyl
Hypocotyl is the part of the stem of a seedling below the cotyledons. It develops into the roots of the plant.
Plumule
plumule is a part of seed embryo that develops into the shoot. It bears the first true leaves of a plant. It is enclosed by the coleoptile.
Epicotyl
Epicotyl is the stem of a seedling or embryo between the cotyledons and the first true leaf. In most plants the epicotyl will eventually develop into the stem and the leaves of the plant.
Coleoptile
Protective sheath covering the shoot apex of the embryo in monocotyledonous plants. It protects the plumule as it emerges through the soil.