Seed - good-quality seed, parts of a seed

 

Seed

A seed is an embryonic plant enclosed in a protective external covering. Seed formation is part of the process of reproduction in seed plants, spermatophytes, including gymnosperms and angiosperm plants.

Characteristics of the good-quality seed:

• Approved/recommended a variety
• Suit the prevailing ecological conditions
• Higher genetically purity
• Posses good shape, size, colour, etc., according to specifications of variety.
• Free from impurities such as foreign matter like stones, weed seeds, chaff, empty shells, etc.
• Higher physical soundness and weight.
• Free from seed-born diseases/insects
• Higher germination ( 90 to 35 % depending on the crop)
• Should be sound and uniform in size
• The varietal purity should be 100%
• Higher physiological vigour and stamina.
• Germination of the seed should be more than 90%

Parts of a seed:

A typical seed consists of three main parts;

1. Seed coat
2. Endosperm
3. Embryo

1. Seed coat

The seed coat is the protective outer structure of a seed that is usually hard, thick, and gray. The seed coat is formed from the external covering of the ovule called the integument. It contains two layers: (a) Testa - outer layer (b) Tegmen - the inner layer

Seed coat has the following four parts:

• Micropyle: A minute opening in the ovule of a seed plant through which the pollen tube generally enters
• Funiculus: The funiculus is the tissue that attaches the developing seed to the mother plant (fruit). It is the conduit for food and water to the growing seed.
• Hilum: The scar on a seed marking the point of attachment to its seed vessel.
• Raphe: The base of the funiculus is fused with the integument.

Functions of seed coat:

• It protects the embryo and the endosperm from drying up, mechanical injury, unfavorable temperature, attacks by pathogenic and decaying organisms i.e. bacteria, fungi, insects, etc.
• Preventing seeds from germination even under suitable growth conditions (seed dormancy).
• Preventing the excessive loss of water from the seeds.
• Helps to disperse seeds by acquiring special features such as wings (oroxylon), hairs (cotton), rail (litchi).

2. Endosperm

The part of the seed that serves as the food store for the embryo of the developing plant, which is usually starched with protein and other nutrients. The endosperm is formed by the triple fusion of the primary endosperm nucleus during double fertilization of angiosperms.

Functions of endosperm:

• Endosperm stores nutrients needed for embryonic development during seed germination.
• Acting as a mechanical barrier, protect the embryo, the next part of the seed.

They are the young plants that are growing inside the seed coat. An embryo contains the underdeveloped tissues of leaves, stem, and roots of a plant.

The embryo has the following parts:

• Epicotyl: The region of an embryo or seedling stem above the cotyledon.
• Hypocotyl: The part of the stem of an embryo plant beneath the stalks of the seed leaves or cotyledons and directly above the root.
• Radicle: The part of a plant embryo that develops into the primary root.
• Cotyledons: They are the leaves of the embryo that provide nourishment to the developing plant. There are two types of cotyledons present in flowering plants: i) monocotyledonous or monocots – embryo with one cotyledon ii) dicotyledonous or dicots – embryo with two cotyledons.

Functions of an embryo:

• Giving rise to a new complete new plant.
• Storing food and nourishing the baby plant.