Seeds are an essential element of sexual reproduction in flowering plants (angiosperms). Seeds are developed as the end product of the sexual reproduction in plants. They are the matured ovules developed within the fruits (matured ovary) after fertilization. They consist of an embryo which is enclosed in a protective outer covering called seed coat.Some seeds consist of a triploid endosperm as well. An embryo is composed of three parts- a radicle, an embryo axis, and cotyledons. Depending on the number cotyledons, seeds have been classified into two- monocotyledonous and dicotyledonous seeds. Let’s see the structures of dicotyledonous and monocotyledonous seeds.
Structure of a Dicotyledonous Seed
Peas and grams are examples of dicotyledons. In dicotyledons, the embryo consists of an embryo axis and two cotyledons. Cotyledons are generally swollen part which acts as a food reserve for the developing seedling. The embryo axis has two ends. The one which forms the shoot tip is called plumule and the portion at the lower end which forms the root tip is called radicle. The whole content is enclosed within a protective cover, seed coat. The seed coat is made up of an outer layer called testa and an inner layer called tegmen. And a seed is attached to the fruit through a structure called hilum.
Endosperm is the third component of the seeds, others being embryo and seed coat. It is formed by the double fertilization. It consists of reserved food materials. In some plants, as the seeds mature, the endosperm is fully used by the embryo. Such seeds are devoid of endosperm and are called non-endospermous seeds; also called non-albuminous seeds (e.g. peas, beans, and groundnut).
Structure of a Monocotyledonous Seed
Embryos of a monocotyledonous seed possess only one large cotyledon called scutellum. The scutellum is generally shield-shaped and is located laterally towards a side of the embryo axis. As in dicotyledons, the embryo axis of monocotyledons possesses a shoot tip, plumule, enclosed in a sheath called coleoptile and a root tip, radicle, enclosed in coleorhiza. In a monocotyledonous seed, the endosperm is covered by a proteinous layer called aleurone layer.
The majority of the monocotyledonous seeds are albuminous seeds i.e., they have thick, swollen endosperms for nourishment. The endosperm is not completely consumed during embryo development. However, some monocotyledons like orchids show exceptions.
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