बीज, What is a Seed and it's Types ? बीज और उसके प्रकार ?

'Seed'

A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seedis part of the proces of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosperm plants. 

They are as follows:

Dicotyledonous Seeds: Two embryonic leaves or cotyledons.

Monocotyledonous Seeds: One embryonic leaves or cotyledons

Structure of a Monocotyledonous Seed

A Monocotyledonous seed, as the name suggests, has only one cotyledon. There is only one outer layering of the seed coat. A seed has the following parts:

Seed Coat: In the seed of cereals such as maize, the seed coat is membranous and generally fused with the fruit wall, called Hull.

Endosperm: The endosperm is bulky and stores food. Generally, monocotyledonous seeds are endospermic but some as in orchids are non-endospermic.

Aleuron layer: The outer covering of endosperm separates the embryo by a proteinous layer called aleurone layer.

Embryo: The embryo is small and situated in a groove at one end of the endosperm.

Scutellum: This is one large and shield-shaped cotyledon.

Embryonal axis: Plumule and radicle are the two ends.

Coleoptile and coleorhiza: The plumule and radicle are enclosed in sheaths. They are coleoptile and coleorhiza.

Structure of a Dicotyledonous Seed

Unlike monocotyledonous seed, a dicotyledonous seed, as the name suggests, has two cotyledons. It has the following parts:

Seed coat: This is the outermost covering of a seed. The seed coat has two layers, the outer testa and the inner tegmen.

Hilum: The hilum is a scar on the seed coat through which the developing seed was attached to the fruit.

Micropyle: It is a small pore present above the hilum.

Embryo: It consists of an embryonal axis and two cotyledons.

Cotyledons: These are often fleshy and full of reserve food materials.

Radicle and plumule: They are present at the two ends of the embryonal axis.

Endosperm: In some seeds such as castor, the endosperm formed as a result of double fertilisation, is a food storing tissue. In plants such as bean, gram and pea, the endosperm is not present in the matured seed. They are known as non-endospermous. 

Difference Between Mono and Di-cotyledons

● Seed germination

Germination: The process in which seed embryo starts growing, which leads to the development of seedling.

➢ Stages of seed germination: The process of seed germination involves several consecutive but overlapping events like i. absorption of water, ii. initiation of cell enlargement and division, iii. increased enzymatic activity, iv. food translocation to growing embryo,

v. increase in respiration and assimilation, vi. increase in cell division and enlargement and vii. differentiation of cells into tissue and organs of a seedling.

➢ Types of seed germination:

The radicle, the growing point of root emerges from the base of the embryo axis and the plumule, the growing point of shoot is at the upper end of embryo axis, above the cotyledons.

The section of seedling stem above the cotyledons is called as epicotyl and below the cotyledons is called as hypocotyl.

Two types of germination are commonly found in cultivated plants.

1.               Epigeal germination: Seed germination in dicots in which the cotyledons come above the soil surface.

In this type, the hypocotyl elongates and raises the cotyledons above the ground surface, it is called as epigeous or epigeal germination. This type of germination is very common in beans, gourds, castor, tamarind and onion etc.

2.               Hypogeal germination: Seed germination in dicots in which the cotyledons remain below the soil surface.

In this type, the epicotyl elongates and the hypocotyl does not raise the cotyledons above ground, which is called as hypogeous or hypogeal germination.

This type of germination is common in mango, custard apple, pea, gram, lotus and maize etc.

1.     SEED DORMANCY

Seed dormancy can be defined as the state or a condition in which seeds are prevented from germinating even under the favourable environmental conditions for germination including, temperature, water, light, gas, seed coats, and other mechanical restrictions.

Many species seeds are incapable of germinating because of some inhibitory factor(s). Such seeds even placed under suitable environments of germination are unable to germinate. Sometimes the seeds are quiescent. Such seeds do not germinate because of the lack of favourable environmental conditions.

• Types of Seed Dormancy

The seed dormancy is of following types:

a) Innate dormancy: A seed which in innately dormant is one which is

incapable of germination when freshly dispersed even if conditions suitable for seedling growth are supplied. This inability to germinate may be due in certain species to the embryo being immature at the time of dispersal. Innate dormancy may be imposed chemically by the presence of inhibitory compounds either in the seed coat or in the embryo. The presence of innate dormancy can usually be interpreted as an adaptation either for staggering germination. or for delaying germination until the appearance of ) Enforced dormancymost favourable season.

b)enforced dormancy: It is the condition of seeds which is incapable of germination due to an environmental restraint which includes, an adequate amount of moisture, oxygen, light and a suitable temperature. No special physiological mechanism is involved here, and the seeds might more properly be considered merely quiescent. Seeds lying deep in the soil are probably prevented from germination by a lack of oxygen.

c) Induced dormancy: It is caused by the unfavourable conditions. In many species newly dispersed seeds have no innate dormancy, but if they fail to meet suitable conditions for germination, they acquire an induced (or secondary) dormancy.