Seed:

A seed is defined as "a fertilized and metamorphosed ovule containing an embryo enclosed in resistant protective coats."

 

Seed is also described as the initial, dormant stage of the diploid sporophytic generation in the life cycle of Spermatophytes or Phanerogams.

 

Structure of a Dicotyledonous Seed:

A typical dicotyledonous seed consists of seed coat made up of two layers-

- outer thick and resistant layer is called testa

- the inner thin and membranous layer called tegmen

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A seed is attached to the inner wall of fruit by a small stalk called funicle at a point called hilum, which is visible as a scar on the surface of testa.

A small-pore is present close to the hilum in the testa, called the micropyle. Through this pore, the seed absorbs water and radicle emerges out during seed germination.

A ridge may be seen in continuation with the hilum on testa, which is called raphe. Enclosed within the seed coat, there is an embryo consisting of embryo

- axis (tigellum)

- two cotyledons.

 

The contents enclosed within the seed coat are together called kernel.

The embryo axis consists of two ends

- the radicle: gives rise to root system

- the plumule: produces the shoot system.

 

The cotyledons are attached to the embryo axis at the region, which represents first node of embryo.

The part of embryo axis between radicle and first node is called hypocotyl.

The part between plumule and cotyledons is called epicotyl.

 

Most of the dicotyledonous seeds are called exalbuminous or non-endospermic because they lack the endosperm at maturity.

In such cases, entire endosperm is consumed during its development.

In ex-albuminous seeds, the cotyledons are thick and fleshy as they store necessary reserve food material.

 

A few dicotyledonous seeds like castor, possess endosperm and are called albuminous or endospermic seeds.

In albuminous seeds, cotyledons are thin and papery.

 

 

Structure of a monocotyledonous seed:

A maize grain represents a single-seeded fruit in which the seed coat and the fruit wall are inseparable.

On one side of the grain a small, opaque, whitish, area is seen. Embryo lies embedded in this area.

A thin layer lying around the grain is formed by fusion of seed-coat and the fruit wall which is called hull.

 

The grain is divided into two unequal portions by a definite layer known as the epithelium.

The bigger portion is the endosperm, and the smaller portion, the embryo.

The endosperm is food storage tissue.

The embryo consists of a single shieldshaped cotyledon (known as the scutellum) and the axis.

The upper portion of the axis is the plumule and the lower portion is the radicle.

The plumule is surrounded by a protective sheath called coleoptile.

The radicle is surrounded by another sheath called coleorhiza.