Seed may be defined as the ripened ovule. The gymnosperms and angiosperms are characterized by the presence of seeds. In gymnosperms, the megasporophylls do not form ovary, hence, the ovules are exposed and so also the seeds. The gymnosperms are, therefore, described as naked seeded plants.In angiosperms, on the other hand, the seeds are enclosed within the fruit, hence, they are known closed seeded plants.
A mature seed consists of two essential parts – the seed coat and the embryo.
Seed coat: The outer covering of the seed is known as seed coat. The seed coat develops from the integument of the ovules. The seed coat is made of two layers – the outer layer is called the testa and the inner layer is called tegmen.
The testa is usually thick and leathery while the tegmen is thin, papery and fused with the testa. The seeds developing from the untegmic ovules (ovule with one integument) have single-layered seed coat. The function of the seed coat is to protect the delicate embryo.
The seed remains attached to the pericarp by a short stalk called seed stalk or funiculus. In a mature seed, the position of the seed stalk is represented by a small oval depression, called hilum.
Just below the hilum is a small pore, called the micropyle. In some seeds, the stalk is continuous with the seed coat and the fused part appears as a small ridge just above the hilum. This ridge is known as raphe.
Embryo: Embryo may be defined as a young plant enclosed within the seed coat. The embryo of a mature seed consists of four distinct parts – cotyledon, plumule, hypocotyl and radicle. The radicle, plumule, and hypocotyl together form the embroynic axis or tigellum.
The cotyledons are attached to the embryonic axis. Dicotyledons have two cotyledons which are situated opposite to one another whereas the monocotyledons have only one cotyledon. In most of the plants, the cotyledons serve as a food storage organ and in others, they serve as photosynthetic organs after germination.
The part of the embryonic axis lying immediately above the point of attachment of the cotyledons is known as epicotyl and at the tip of the epicotyl lies the plumule.
It gives a small feather-like appearance due to the presence of one or more leaf primordia at its apex. The part of the embryonic axis below the point of attachment of the cotyledons is known as hypocotyl. The radicle is the basal tip of the hypocotyl. When seed germinates, the radicle becomes the primary root of the seedling.
Storage of food in the seed:
In seed, food is stored either in the cotyledons or in a special food storage tissue, called the endosperm. In legumes, for example, the food is stored chiefly in the cotyledon and there is no endosperm.
Absence of endosperm in the seeds indicates that the endosperm has been completely utilized by the developing embryo. Such seeds are known as non-endospermic or exalbuminous.
In some dicotyledons (e.g., castor) and monocotyledons (e.g., cereals and grasses) the food is stored mainly in the endosperm. Such seeds are known as endospermic or albuminous.
The food materials stored in the seed include proteins, fats and carbohydrates. Different types of seeds accumulate different foot materials, for example, seeds of wheat, rice, maize are rich in carbohydrates. Seeds of sunflower, soya bean, peanuts, castor etc are rich in fats and seeds of legumes are rich in proteins.
Structures of some of the seeds:
Structure of castor seeds (Ricinus communis):
The castor seed is relatively large and oblong. The testa or the outer seed coat is hard, brittle and mottled. The tegmen or inner seed coat is thin and is fused with the testa.
One of the characteristics features of the castor seed is the presence of a soft spongy outgrowth known as the caruncle at the micropylar region. It absorbs water and helps in the germination of the seeds. The hilum is almost hidden in the caruncle. A distinct raphe extends from the hilum.
Inner to the seed coat is the white endosperm or albumen, forming most of the interior part of the seed. It is the main food storage tissue of the seed. The endosperm is rich in oil and the castor oil is extracted from these seeds.
The embryo is enclosed within the endosperm. The plumule is blunt and undifferentiated. The hypocotyl and radicle are short and hidden in between the two cotyledons.
Structure of gram seeds (Cicer arietinum):
The gram seed is broader at one end and pointed at the other end. It is covered with a brownish seed coat, made of two distinct layers – the testa and the tegmen. The testa is thick and brownish whereas the tegmen is thin, whitish and fused with the testa.
The micropyle lies at the pointed end of the seed. Above the micropyle is present a small depression representing the hilum. The raphe extends beyond the hilum as a small ridge on the testa. The embryo consists of two circular yellowish cotyledons. Both the cotyledons are attached to the embryonic axis opposite to each other.
The plumule lies immediately above the point of attachment of the cotyledons. Below the point of attachment of the cotyledons lies the hypocotyl with radicle at its base. In gram seeds, there is no endosperm and entire food is stored in the cotyledons. Thus, it is non-endospermic or exalbuminous.
Structure of corn grain (Zea mays):
Corn is a one-seeded fruit, called caryopsis in which the fruit wall or the pericarp is fused with the seed coat. The corn grain is a flat oblong structure. It shows externally a large yellowish upper area which marks the position of endosperm and on the lower side, there is a small whitish area which contains the embryo. Seed coat is not distinct and it is fused with the fruit wall or pericarp. The micropyle and the hilum are also indistinguishable.
The longitudinal section of the grain shows two distinct regions – the upper large region, called the endosperm and the lower small region, called the embryo. The endosperm is surrounded by a special one-celled thick layer called the aleurone layer. The aleurone layer is filled with the aleurone grains which are proteinous in nature.
The embryo consists of a single cotyledon, the plumule and the radicle. The shield-shaped cotyledon in the grain is known as scutellum. It is a flat structure closely pressed against the endosperm. The scutellum absorbs food from the endosperm and transfers it to the growing parts of the embryo.
The plumule consists of growing tip of the shoot along with few young leaf primordia. It is covered by a sheath called coleoptile. The radicle which lies at the base of the grain is covered by a sheath, called coleorrhiza. The hypocotyl is very short.