The seeds develop from ovules inside the ovary after fertilization. A seed is generally made up of seed coat and enclosed inner parts like cotyledons, plumule and radicle. The cotyledons store reserve food, to be used during resting period and germination. The plumule gives rise to shoot and the radicle to root system.
On the basis of the number of cotyledons, the seeds are divided into two types, viz monocotyledonous and dicotyledonous. The dicot seeds are further divided into endospermic and non-endospermic based on the presence or absence of endosperm (reserve food tissue). The endospermic seeds of castor and custard apple contain an outer hard, blackish and mottled shell called testa or the seed coat. There is an outgrowth at the micropyle called caruncle, which is spongy and absorbs moisture readily during seed germination. A ridge called raphe, formed by the funicle, is prominent. The perisperm, a remnant of the nucellus, is a thin white papery membrane surrounding the endosperm. Endosperm, a fleshy food storage tissue, rich in oil is present under the cover of the perisperm. Embryo lies embedded inside the endosperm, and contains two thin cotyledons hinged to an axis called tigellum. The tigellum shows a protruding radicle and the plumule hidden between the cotyledons. The non-endospermic seeds of gram, pea and bean are also covered by a seed coat. The seed coat is generally two layered. The outer one is called testa and the inner tegmen. Hilum is a point of attachment of seed with the stalk. A minute opening (micropyle) above the hilum and an outgrowth (raphe) are also distinct. The embryo consists of an axis (tigellum) and two fleshy cotyledons full of reserve food material. The lower pointed end of the axis is radicle and the upper leafy end is the plumule.
The monocotyledonous seeds are with a single cotyledon and are generally endospermic. The seed coat is membranous, and fused with the fruit wall. The main bulk of the grain is endosperm, which stores food. Endosperm is separated from the embryo by a distinct layer known as aleurone layer. The embryo is small and occurs in a groove at one end of the endosperm. It consists of one shield-shaped cotyledon known as scutellum, a short axis with plumule and radicle. The plumule is covered by a sheath called coleoptile. Similarly, the radicle is protected by coleorhiza. Some monocots are non-endospermic, e.g. Orchids and Sagittaria.