What is a seed? Describe the structure of dicotyledonous and monocotyledonous seeds with a diagram.

(N/A) $\rightarrow$ Seed: The ovules after fertilisation develop into seeds.
$\rightarrow$ $A$ seed is made up of a seed coat and an embryo.
$\rightarrow$ The embryo is made up of a radicle,an embryonal axis,and one (as in wheat,maize) or two cotyledons (as in gram and pea).
$(a)$ Structure of a Dicotyledonous seed:
$\rightarrow$ The outermost covering of a seed is the seed coat.
$\rightarrow$ The seed coat has two layers,the outer testa and the inner tegmen.
$\rightarrow$ The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit.
$\rightarrow$ Above the hilum is a small pore called the micropyle.
$\rightarrow$ Within the seed coat is the embryo,consisting of an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food materials.
$\rightarrow$ At the two ends of the embryonal axis are present the radicle and the plumule.
$\rightarrow$ In some seeds such as castor,the endosperm formed as a result of double fertilisation is a food-storing tissue.
$\rightarrow$ In plants such as bean,gram,and pea,the endosperm is not present in mature seeds and such seeds are called non-endospermous.
$(b)$ Structure of a Monocotyledonous seed:
$\rightarrow$ Generally,monocotyledonous seeds are endospermic,but some,as in orchids,are non-endospermic.
$\rightarrow$ In the seeds of cereals such as maize,the seed coat is membranous and generally fused with the fruit wall.
$\rightarrow$ The endosperm is bulky and stores food. The outer covering of the endosperm separates the embryo by a proteinaceous layer called the aleurone layer.
$\rightarrow$ 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 the scutellum and a short axis with a plumule and a radicle.
$\rightarrow$ The plumule and radicle are enclosed in sheaths which are called coleoptile and coleorhiza,respectively.