The Seed: Definition, Types, Seeds and its Structure
The ovules develop into seeds after fertilization. A seed is made up of a seed coat and an embryo. The embryo is made up of an embryonal axis and one (as in wheat, maize) or two cotyledons (as in gram and pea).
Structure of Dicotyledonous Seed
I. Seed coat: Outer, protective covering of the seed is called seed coat, which develops from integuments of ovule. The seed coat has two layers, the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle.
II. Embryo: Embryo is the most important part of the seed. Embryo consists of an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food materials. At the two ends of the embryonal axis are present the radicle and the plumule.
III. Endosperm: Endosperm is formed as a result of double fertilization. In some seeds such as castor it is a food storing tissue. But in plants such as bean, gram and pea, the endosperm is not present in mature seed and such seeds are called non-endospermous.
Fig.: Structure of Dicotyledonous Seed
Structure of Monocotyledonous Seed
In the seeds of cereals such as maize the seed coat is membranous and generally fused with the fruit wall. Below the grain covering are present two structures, endosperm and embryo. The endosperm is bulky and stores food. So monocotyledonous seeds are endospermic but some as in orchids are non-endospermic. The outer covering of endosperm separates the embryo by a proteinaceous layer called aleurone layer. The embryo is small and situated in a groove at one end of the endosperm. It consists of one large and shield shaped cotyledon known as scutellum and a short axis with a plumule and a radicle. The plumule and radicle are enclosed in sheaths which are called coleoptile and coleorrhiza respectively.
Perispermic Seed
Mostly nucellus is consumed after fertilization due to absorption of food by the endosperm and embryo. Sometimes, the nucellus remains persistent in the seed and is called perisperm. Such seeds are called perispermic seeds, e.g., Piper nigrum (black pepper).