In angiosperms, the seed is the final product of sexual reproduction. It is often described as a fertilised ovule. Seeds are formed inside fruits. A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis. The cotyledons of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes). Mature seeds may be non-albuminous or albuminous.
Non-Albuminous Seeds: Seeds with no residual endosperm are called non-albuminous seeds. Endosperm is completely consumed during embryo development in these seeds, e.g. pea, groundnut, etc.
Albuminous Seeds: Seed which retain a part of endosperm are called albuminous seeds, e.g. wheat, maize, barley, castor, sunflower, etc.
Perisperm: In some seeds, remnants of nucellus are also persistent, e.g. black pepper and beet. This residual, persistent nucellus is the perisperm.
Seed Coat: Integuments of ovules harden as tough protective seed coats. The micropyle remains as a small pore in the seed coat. This facilitates entry of oxygen and water into the seed during germination.
As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass). The general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity called dormancy, or if favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate.
Development of Fruit
The transformation of ovules into seeds and ovary into fruit proceeds simultaneously. The wall of the ovary develops into the wall of fruit called pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc. Many fruits have evolved mechanisms for dispersal of seeds.
False Fruit: In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off. But in a few species, the thalamus also contributes to fruit formation. Such fruits are called false fruits. Example: apple, strawberry, cashew, etc.
True Fruit: Most fruits develop only from the ovary. Such fruits are called true fruits.
Parthenogenesis: In some species, fruits develop without fertilization. Such fruits are called parthenocarpic fruit. This phenomenon is called parthenocarpy. Example: Banana.
Advantages of Seeds
- Seed production is more dependable because reproductive processes in flowering plants are independent of water.
- Seeds have better adaptive strategies for dispersal. This helps the flowering plants to spread into new habitats and colonise different areas.
- Seeds have enough food reserves. This helps young seedlings to get nourishment until they become capable of photosynthesis on their own.
- Seed coat protects the young embryo.
- Dehydration and dormancy of mature seeds are crucial for storage of seeds. Thus, seeds can be used as food throughout the year. Due to this, seeds can also be used to raise crop in the next season. Thus, seeds have become the basis of our agriculture.
Apomixis and Polyembryony
Some plants produce seeds without fertilization. This phenomenon is called apomixis. Example: some species of Asteraceae and grasses.
There are several ways of development of apomictic seeds. In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation. More often, as in many Citrus and Mango varieties some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. In such species each ovule contains many embryos. Occurrence of more than one embryo in a seed is referred as polyembryony.