Seed and parts of seed
Aug 09, 2022, 16:45 IST
Seed is defined as the ripened and fertilized ovule which contains an active embryo, reserve food and protective covering. The angiospermic seed may have either one (monocotyledons) or two (dicotyledons) cotyledons.
The seed may also be defined as the integumented megasporangium containing a single megaspore which after germination and fertilization gives rise to embryo while in situ.
PARTS OF SEEDS
It is the outer covering of the seed. It develops from integuments of the ovule. In most of the seeds, the seed coat is made of two layers. The outer layer is called testa and the inner layer is called tegmen. Seeds developing from the unitegmic ovules (i.e., ovules with one integument only) have only single layered seed coat. The function of seed coat is to protect the delicate embryo within.
The seed remains attached to the pericarp (fruit wall) by a short stalk called seed stalk or funiculus. In a mature seed the position of seed stalk is represented by a small oval depression called hilum.
The young or miniature plant, enclosed within the seed coat is called as embryo. It develops from the fertilized egg. Embryo has two parts, cotyledon and embryonal axis (tigellum).
The point on embryonal axis where cotyledons get attached is called as cotyledonary node. The part of the embryonal axis lying immediately above the cotyledonary node is known as epicotyl and at the tip of the epicotyl lies the plumule. It gives feathery appearance due to presence of one or more leaf primordia i.e., young leaves at its apex.The part of the embryonal axis below the cotyledonary node is known as hypocotyl. It represents the root-stem transition region i.e., the part where stem changes into root.The radicle is the basal tip of hypocotyl. Externally the radicle may or may not be differentiated from the hypocotyl but internally these two parts are quite different. In seeds, food is stored either in the cotyledons or in a special food storage tissue (endosperm). In legumes, for example, the food is stored chiefly in the cotyledons and there is no endosperm. Absence of endosperm in seeds indicates that it (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 in monocotyledons e.g., cereals and grasses, food is stored mainly in the endosperm and such seeds are known as endospermic or albuminous.
GERMINATION OF SEEDS
The process by which the dormant embryo of the seed resumes active growth and forms a seedling is known as germination.
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