Seed is defined as a mature,integumented megasporangium. Seed is a miniature plant. A typical seed has the following parts:
Parts of Seed
1. Seed Coat :
It is protective covering of the seed and is made up of two layes: (a) outer-called Testa which is usually hard, and (b) inner-called Tegmen which is thin and papery. There is a small opening at one end of the seed coat, called micropyle through which water enters the seed. The stalk of the seed with which the seed is attached to fruit wall is called funiculus. A large scar is located near the middle of one edge, where the seed breaks from the stalk of funiculus, this is called hilum. There is a ridge beyond the hilum opposite the micropyle. It represents the base of the funiculus which is fused with the integuments and is called raphe.
2. Embryo :
Embryo is a young plant enclosed in a seed coat. It has two parts:
(a) Cotyledons : These are leaves of embryo. Their number is either one or two. Sometimes they store food materials and become fleshy. When they do not store food they remain thin and papery. The cotyledons are hinged to an axis (tigellum) at a point called cotyledonary node and open out like a book.
(b) Tigellum : It is the main axis of the embryo. One end of the tigellum is pointed and protrudes out of cotyledons. This lies next to micropyle and is called radicle (rudimentary root). The other end of tigellum is the feathery plumule (first apical bud of shoot). The portion of the axis above the point of attachment of cotyledons is called epicotyl and that below the cotyledonary node is called hypocotyl.
3. Endosperm : It is a food-laden tissue, either present on one side of the embryo or surrounding the embryo on all sides. In some seeds the endosperm is present until maturity. Such seeds are called endospermic (albuminous) seeds. In some seeds it is consumed in young stages by the developing cotyledons and such seeds do not have endosperm at maturity. Such seeds are called nonendospermic (ex-albuminous) seeds. All parts enclosed by seed coat are called kernel.
Types of Seeds
a) Exalbuminous Seeds (Non-endospermic seeds) :
Pea seed (Pisum Sativum) : Peas seeds are round. Each seed is attached to the placental tissue of the fruit by a stalk called funicle. The point of attachment of funicle to seed is called hilum. Near the hilum is an opening of the size of a pinhole, called micropyle. The seed is covered by seed coats of light colour. The outer tough seed coat is called testa and the inner thinner and delicate called tegmen is adherent to the inner wall of testa, (tegmen is not distinguishable in the mature seed).
Enclosed by the seed coat is present kernel. The kernel includes two thick cotyledons with stored foods. The two cotyledons are hinged to an axis (tigellum) so that they open like a book. One end of the tigellum is pointed and protrudes out of the cotyledons and lies next to the micropyle. It is radicle or rudimentary root. The protruding radicle lies under the pouchlike expansion of seed coat. The other end of the tigellum is the feathery plumule which develops into shoot. The plumule lies enclosed by two cotyledon.
The portion of the tigellum just below the cotyledonary node (between radicle and node) is called hypocotyl and the portion just above (between plumule and node) is called epicotyl.
Gram (Cicer arietinum), bean (Hindi-sem) (Dolichos lablab), tamarind (Tamarindus indica), Cucurbits-gourd, cucumber etc. are almost similar in organization.
b) Albuminous Seeds :
In these seeds food is not stored in two cotyledons but endosperm is present even in mature seed and the cotyledons are thin acting only as food sucking organs e.g., Castor (Ricinus communis), Papaya (Caricapapaya), cotton.
1. Castor Seed : The seed coat is hard shell of a mottled black or brown colour. The hilum is almost hidden by an outgrowth the caruncle. Caruncle is a proliferation of the tip of outer integument. It is spongy and absorbs water readily. There is a distinct raphe running longitudinally down the seed from hilum. Inside the hard seed coat there is a white papery membrane which is remanent of the nucellus and is called perisperm. Inside the perisperm is present a fleshy, slightly flattened, oily, oval mass of tissue the endosperm. The endosperm encloses the embryo. The embryonic axis is called tigellum, to central part of which are attached two thin papery cotyledons. One end of the tigellum called plumule is enclosed by two cotyledons and another end called radicle protrudes out of the cotyledons.
(a) Albuminous (Endospermic ) Seeds :
Most of the common monocotyledonous seeds are albuminous e.g., Maize, Wheat, Rice. In all these plants, the pericarp and testa are fused and the grains of these plants are actually fruits (Caryopsis). The micropyle and hilum are not seen because of the pericarp. The bulk of the grain is filled with endosperm and the embryo occupies a comparatively small space on one side of the grain.
1. Wheat (Triticum species) : The grain is oval and there is longitudinal groove along the ventral side. The wall (pericarp and testa) is yellowish light brown. If the grain is cut longitudinally into two, the larger portion is found to have white endosperm (starchy) and a smaller portion of the embryo. On the one side of the embryo is the shield-shaped structure called scutellum (single cotyledon of monocots). Scutellum is attached to the middle part of the axis on one lateral side. Opposite to the scutellum a tongue-like outgrowth called epiblast.
2. Maize (Zea mays) : Maize grains are more or less oblong and flattened. On the flat face embryo can be seen (externally represented by whitish area). Pericarp and testa which are fused are yellow brown coloured. In a vertical section of grain two regions are well differentiated, endosperm and embryo. On the upper and lateral part the embryo is a shield shaped structure called scutellum (single cotyledon). The tissue of the scutellum adjacent to the endosperm form the epithelium (glandular tissue secreting enzymes, which digest food from endosperm). Lateral to the scutellum is the short axis, the lower part of which is radiacle covered by a sheath called coleorhiza and upper part of it is plumule (showing a number of young leaves) sheathed by coleoptile.
(b) Exalbuminous Seeds:
Examples are Amorphophallus,Vallisneria,Alisma etc.