A seed develops from an Ovule after fertilization. The seeds are the embryo of flowering plants in dormant state. Seeds provide protection and nourishment to the embryo.
Structure of Dicot Seed:
Seed consists of following parts:
1) Seed Coats:
Seed-coats is made up of two layers the outer thicker layers is called testa and inner thin layer is called legmen. Its function is to protected the embryo.
The hilum is a scar which shows the place of attachment of seed-stalk with the testa.
The micropyle is the opening in the seed-coat. The water enter into the seed through this opening before germination.
The embryo consists of very short axis to which one or two seed leaves called cotyledons are attached. In many of the seeds endosperm is not present then food is stored in cotyledons.
5) Plumule and Radicle:
The end of the axis lying between two cotyledons is called plumule and the other end is called radical. Plumule develops into shoot and radical into root.
The food far the embryo is stored in endosperm or albumen.
Structure of Monocot Seeds:
In many of the seeds, ripened ovary wall called pericarp get fused with seed coat e.g Maize (Corn) grains. These grains have no hilum and no micropyle. Then can absorb water from their surface.
Internal Structure of the Maize-grain seeds:
Internally it is divided into unequal parts by a thin layer called epithelium. The bigger portion is the endosperm and the smaller is the embryo. The large shield-shaped cotyledons is called scutellum. The plumule and radical are enclosed in protective sheath called coeoptile (plumule) and coleorhizae (Radicle).
The process in which a dormant embryo (sleeping embryo) awakes up and beings to develop into a seedling is called germination.
Kinds of germination:
There are three types of germination.
i) Hypogeal germination (hypo = below, geo = earth):
In this type of germination the cotyledons remains under the soil due to rapid growth of epicotyle e.g Gram, pea etc.
ii) Epigeal germination (Eip = above, geo = earth):
In the type of germination the cotyledons come out of the soil due rapid growth of hypocotyle e.g castor oil seeds tomato etc.
iii) Vivipary germination:
The seeds of coastal or marshy plants show special type of germination called viviparous. In this type of germination, seed starts germination in the fruit it is still attached to parent plants.
Radicle came out of the fruit which becomes swollen and heavy due to its increasing weight; the seedling gets detached and falls vertically into the soft mud. It is changed into a root and finally produced new plant. Rhizophorae, Coconut, Date palm.
Conditions necessary for seed germination:
The seed germinates during favourable condition such as moisture, oxygen and temperature.
1) Role of Water:
The seed absorbs water through micropyle and swells up, causing the seed coat burst. In this way embryo comes out to grow. Enzyme become activated by water and solid reserve food changes into solution.
2) Role of Oxygen:
Oxygen is very important for germination. During respiration energy is liberated from food molecules. It is required for their growth.
3) Role of Temperature:
Temperature is necessary for the proper function of enzymes. Most require a suitable temperature ranging from 25oC to 37oC. Seed don’t germinated at temperature below 0oC or above 45oC.