Structure of Seed
Structure of Seed
An angiospermic plant can be divided into two types based on the type of seeds.The two types are :-
Dicotyledonous Seed :-
Structure of Dicotyledonous Seed : - Seed can be studied under two heads, i.e., external structure and internal structure. It is light or dark brown in colour. Its surface may be smooth or wrinkled. A small oval scar present at the side called hilum. It is the point where the stalk or funicle of the seed is attached to it. A narrow ridge called raphae runs from hilum to chalaza inside the furrow. A small pore called micropyle present between the hilum and pointed end. The outermost covering of the seed is seed coat. The outer hard and leathery layer of the coat is called testa and the inner thin and membranous layer is the tegmen. In some seeds, the tegmen and testa are fused.
The seed coat encloses the embryo, which is differentiated into radicle, a plumule and cotyledons. The radicle develops into root and plumule into shoot. Cotyledons may be one or two to serve as reserve food. Hypocotyl is a part present between the point of attachment of cotyledon and radicle. Epicotyle is present between point of attachment of cotyledons and plumule. In some seeds, such as castor seeds, the endosperm is formed as a result of double fertilisation, which is a food storing tissue. In plants like bean, gram and pea, the endosperm is not present in mature seeds (i.e., non-endospermous seeds).
Monocotyledonous Seed : -
The monocotyledonous seeds are endospermic, but some as in orchids are non-endospermic. In the cereals, such as maize, the seed coat is membranous and generally fused with the fruit wall.
Structure of Monocotyledonous Seed : - The endosperm is bulky and stores food. The outer covering of endosperm separates the embryo by a proteinous layer called aleurone layer. The embryo is small and situated in a groove at one end of the endosperm. It consists of one large 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 coleorhiza, respectively.
Coleoptile has a terminal pore for the emergence of first leaf during germination. The sheath is capable of growth. It helps the future shoot in passing through the soil during germination, c.g., maize grain is whitish, yellow, violet or red in colour. It has smooth or shiny surface.
Its grain is covered with a single, thin hard covering. It is formed by the fusion of seed coat or testa and the fruit wall or pericarp.
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