What is THE SEED?
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THE SEED
Seed is a ripened ovule which contains an embryo or tiny
plant with sufficient reserve food for the development Of
embryo.
The ovules after fertilisation develop into seeds. A seed is
made up of seed coats and an embryo. The embryo is made
up of a radicle, an embryonal axis and one (wheat an
maize) or two cotyledons (gram and pea).
Types of Seeds
Seeds can be classified into two different types based on the
number of cotyledons and presence or absence of endosperms,
i.e., dicotyledonous and monocotyledonous seed.
i. Dicotyledonous Seed
Gram seed is a dicot seed formed in a small pod or legume. The
outermost covering is the seed coat. An endosperm is absent.
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 s
tegmen and testa are fused.
The seed coat encloses the embryo, which is differentiated into
a 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 storing tissue. In
plants like bean, gram and pea, the endosperm is not present in
mature seeds (i.e., non-endospermous seeds).
ii. 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 is capable of
growth. It helps the future shoot in passing through the soil
during germination, e.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.