SEED STRUCTURE AND FUNCTION OF SEED

SEED STRUCTURE AND FUNCTION OF SEED

SEED:

      A mature fertilized ovule is called a seed.

Structure of seed:

       A seed Consists of the following different parts;

1)Seed coat:

       Seed is covered by seed coat or testa. It may be a thin layer as in peanut or a thick hard coat as in coconut. It is of different colour and covers the embryo. A thin coat beneath the testa is called tegmen which is fused with the testa.

ii) Hilium:

          There is a small scar on one end of the seed, called hilum. It shows the place of attachment of the seed in the fruit.

iii) Micropyle:

         It is a minute pore near to hilum. The seed absorbs water through this pore at the time of germination.

iv) Embryo:

         Embryo is an immature plant, Embryo is present beneath the testa and tegmen. The embryo consists of a short axis and one or two leaves or cotyledons. The axis has three parts, plumule, hypocotyl and radicle.

Main axis:

       The main axis has three parts. Which are defined below;

a) Plumule:

        Plumule is a minute bud. It develops into a new shoot. it arises from the upper end of the hypocotyl, It lies between the cotyledons.

b) Radicle:

        The lower part of the hypocotyl is called radicle which develops into a new root, The radicle lies towards the pointed end of the seed.

c) Cotyledons:

           The cotyledons are two in dicot plants or one in case of monocot plants. They may be large due to stored food in them. They attached to the side of hypocotyl by mean of short delicate stalks.

 d)Endosperm:

            Endosperm stores the food material in endospermic seeds. This food material is used by the seed during the process of generation.

SEED GERMINATION

Seed germination:

         The process by which inactive embryo become active and develop into seedling is called germination. A seed contains an embryo plant. The process in which inactive embryo in the seed becomes active and develops into seedling is called germination.

        Seed germinates only under suitable conditions such as availability of presence of water, oxygen and proper temperature.

Types of germination:

      Germination of seeds may be epigeal or hypogeal.

1.Epigeal germination:

Epi mean above and geo mean earth. So it is define as;

       'The type of germination which bring cotyledon out of soil is called epigeal germination. The epicotyls elongate and form a hook and pull the cotyledons above the soil. The cotyledons become photosynthetic. Seed coat also comes out from the soil.

Examples:

        Seeds of beans, cotton, papaya, cucumber and melons etc.

2. Hypogeal germination:

         Hypo mean beneath and go mean earth. So it is defined as;

       'The type of germination in which cotyledon remain inside the soil and only the plumule grows upward into the air. Epicotyl elongates bringing only plumule above the soil. Cotyledons do not become photosynthetic. Seed coat also remains in the soil.

Examples:

       Seeds of pea, coconut, gram and maize etc.

CONDITION NECESSARY FOR GERMINATION

Conditions for seed germination:

          Seed germination requires both internal and external conditions. Seed must have a living embryo and sufficient stored food. The external conditions are:

1.Water

2.Oxygen

3.Suitable temperature

1.Water:

        Water is very necessary for the germination of seed. The seeds absorb water through micropyle. Seed swells, causing the seed coat to burst. This helps the root to come out of seed. Water also helps to activate the enzymes and to digest the food present in the seed. This food is required for the growth of embryo.

2. Oxygen:

        Air contains about 21% oxygen. Oxygen is used during respiration in the cells of embryo.As a result of respiration energy is produce which is utilized by embryo for germination.

3. Suitable temperature:

          Seed germination needs temperature ranging between 5°C to 30°C. This suitable temperature is necessary for the enzymes to function properly. A plant hormone called gibberellins. It is also activated by water during germination of seeds.

Theophrastus (371-287 BC):

        Theophrastus is also called father of botany.

Books:

       His famous books include "Historia de plantis (enquiry in plants) and 'De Causis plantarum (oof plant, explanation).

Contribution:

● He mentioned 300 species of plant particularly Cultivated and mostly horticultural.

● He distinguishes between sexes in plants.

● He distinguishes between sexual Vegetative reproduction, monocot and dicot plant, angiosperm and gymnosperm plant.

● He also gives technical description of grafting and cutting.

● He also discusses the role of pollination.

QUESTIONS.(1)

       Why it is said that seed is dormant embryo, why plants produce seeds?

Answer.

       A seed contains an embryo that remains inactive or dormant in unfavorable conditions. In a dry seed, the embryo is alive but inactive. Seed germinates only under suitable conditions. Plants produce seeds for the survival of their generations. Each seed germinates and grows into a new plant.

Question 2.

        How you can produce seedless fruits?

Answer.

         If anthers are removed from a flower, then there will be no fertilization. The fruit produced will be seedless.