Question

Explain types of seed on the basis of numbers of cotyeledons. Explain structure of dicot seed.

Answer 1

1. Seed is a ripened ovule which is the result of fertilization.
2. Presence of seed is character of all spermatophyta, i.e. Gyumospenns and Angiosperm.
3. It contains the embryo or the miniature of plant body.
4. In gyumospenns no ovary exists so seeds are exposed while in angiosperms it is enclosed hi fruit.
5. Seeds of different plants vary in their size and shape. However, the general plan of structural organization remains almost the same.
6. A mature seed consists of two essential parts- the Seed coat and the embryo.
7. Every embryo has one embryonal axis and one (Wheat and Maize) or two (Gram, Pea) cotyledons.
8. Parts of a Seed: A mature seed consists of two essential parts- the Seed coat and the embryo

Seed Coat:

1. The outer covering of the seed is known a seed coat.
2. It develops from integuments of the ovule. Seed coat is made of two layers.
3. The outer layer is called testa and the inner layer is called tegmen.
4. The testa is hard and leathery whereas tegmen is thin and membranous.
5. The seed remains attached to the pericarp (ovary wall or fruit wall) by a short stalk called seed stalk or funiculus.
6. The mature seed shows a scar or mark in place of funiculus which is called hilum.
7. There is a small pore, called micropyle, which represents the micropyle of ovule.

Embryo:

1. The seed coat encloses an embryo which is differentiated into radicle, plumule and cotyledons.The radicle,
2. when elongates, gives rise to primary root whereas the plumule gives rise to aerial shoot.
3. The number of cotyledons or seed leaves may be one (as in monocotyledons) or two (as in dicotyledons).
4. Usually the cotyledonary’ leaves are different than the foliage leaves.
5. They store reserve food materials (e.g., Gram, Pea, Almond, Cashewnut, etc.) or serve as photosynthetic organs in young seedling,
6. The part of embryonic axis between the radicle and the point of attachment of cotyledons is called hypocotyl.
7. The part of embryonic axis between the plumule and the point of the attachment of cotyledons is called epicotyl. On the basis of number of cotyledons seed are of two type:

   Before Fertilization	After Fertilization
   1.Ovule	Seed
   2. Outer Seed coat	Testa
   3. Inner Seed coat	Tegmen
   4. Funicle	Destroy
   5. Nucellus	Destroy or forms Perisperm
   6. Egg cell	Embryo
   7. Synergids	Destroy
   8. Antipodal cells	Destroy

(a) Dicotyledonous seed
(b) Monocotyledonous seed

Dicotyledonous seed:

Gram, pea, castor and beans are main dicotyledon seeds, contains two cotyledons as store food which requires during germination of seed.
A typical dicotyledon seed has following structures in it:

1. Seed coat
2. Embryo
3. Endosperm

Seed coat: Seed coat of a seed has two sub layers. The outer one is though, hard which may be of different colours called as testa. Inner layer of seed coat is called as tegmen which is comparatively thin and white in colour. Every seed has a small pore opening in it which called as micropyle, near to micropyle a scar is also observed which is called as hilum. Hilum is the place where a seed remain attach with fruit.

2. Embryo: Every dicotyledon seed has a embryonal axis beneath seed coat. One end of embryonal axis is called as Plumule and other end is called Radicle. Plumule forms shoot while radicle forms root during germination of seed.

3. Endosprem: Endosperm is food storage site which forms after double fertilization of central cell of ovule
When endosperm is consumed during development of zygote into the embryo then in seed endosperm is absent but when endosperm not consumed completely then it is present in seed also.

In gram, pea and beans endospenn is present in initial stage of seed but later on it is consumed so mature seeds do not have endosperm in it. These seed are non endospermic seeds but in castor seed fleshy endosperm is present in mature seed so it is called as endospermic seed.