The branch of science which deals with the development of embryo from zygote is called embryology.
The sporophyte and gametophyte are two stages in the plant kingdom.
Sporophyte : This is deploid stage. It is developed from zygote. Both the vegetative and reproductive organs (flowers) develop on this plant.
Gametophyte : This is reduced haploid stage. The pollen grain is called male gametophyte and the embryo sac is called female gametophyte.
Stamen : This is also called microsporophyll. Each microspore mother cell undergoes meiosis and produces four haploid microspores.
Anther : Anther is the fertile portion of stamen.
Pollen tissue : The newly formed pollen grains are haploid. Each pollen grain acts as the first cell of malegametophyte.
Male Gametophyte
First cell of male gametophyte is pollen grain. It produces two unequal cells by periclinical division. The largest cell is vegetative cell. Smaller cell is generative cell. On division the generative nucleus forms two sperm cells which act as male gametes. At this stage the pollen tube has two male gametes and one vegetative cell. Before fertilization the vegetative nucleus disappears. The pollen tube travels through the style and reaches the ovule.
Megasporangium or Ovule
The megasporangium integuments is called ovule. Ovules develop into seeds after fertilization. Ovules are filled with tissues called ‘nucellus’. This tissue is covered by two protective membranes. These are called integuments. The nucellus of ovule is not covered by integuments. These are called ategmic ovules. The unitegmic ovules are covered by a single integument. The nucleus of bitegmic ovules is covered by two integument.
Types of Ovules
Ovules are of six types, namely,
(a) Orthotropous Ovule : In this micropyle, chalaza and funicle are arranged on the same vertical line.
(b) Anatropous Ovule : In this the micropyle comes nearer to the funicle.
(c) Campylotropous Ovule : In this the body of the ovule is at right angles to the funicle. It becomes horse-shoe during development but the embryo sac is straight.
(d) Hemi-anatropous Ovule : The body of the ovule is at right angle to the funicle.
(e) Amphitropous Ovule : The larger curvature of the Ovule, the micropyle and chalaza are brought side by side. Shape of embryo sac is horse-shoe.
(f) Circinotropous Ovule : Around the anatropous ovule the funicle coils similar to a watch spring.
Embryo Sac
Embryo sac is the female gemetophyte. It has seven cells and eight nuclei. It has three main parts—
(a) Egg apparatus : This has three nuclei. Egg apparatus has a central cell, which is big enough and acts as a egg cell. The remaining two cells are on either side.
(b) Antipodals : At the bottom of the sac, there are three cells. These are the vegetative cells of the embryo.
(c) Central Cell : The central cell is the largest cell of the embryo sac.
Secondary Nucleus
A diploid nucleus called secondary nucleus has two polar nuclei.
Fertilization
Fertilization is defined as the fusion of male and female gametes.
Pollen tube enters into the ovule by three modes : (a) Pollen tube enters into the ovule through the micropyle. This is called porogamy.
(b) Pollen tube enters into the ovule through chalaza. This is called chalazogamy.
(c) The pollen tube enters into the ovule through the integuments of funicle. This is called mesogamy.
After fertilization the following changes occur in different parts of the ovule and embryo.
(i) Funicle
(ii) Outer integument
(iii) Inner integument
(iv) Micropyle
(v) Zygote
(vi) Synergids
(vii) Antipodals
(viii) Primary endosperm nucleus
(ix) Scar of the ovule.
Embryo
The zygote developed inside the embryo sac divides mitotically to form the embryo.
Dicot embryo : The embryo has an axis and two cotyledons are on either side in opposite direction. At the tip of the axis, the apical bud is present and at the base of the axis the root tip is present. The plumule develops into shoot system and the radicle develops into root system.
Monocot embryo : In monocot embryo only one cotyledon and a primary axis are present. The cotyledon is present at the tip of the axis and the plumule is on its lateral side. Embryo is curved.
Endosperm
Endosperm is in triploid (3x) condition. But in gymnosperms it remains in haploid condition.
Non-endospermic seeds : In some plants embryo tares food in its developmental stages from the endosperm and completely utilizes it by the time the ovule is converted into seed. At that time the ovule contains only embryo without any endosperm. Such seeds are called non-endospermic seeds, e.g. Dolichos, Cicer and Capsella.
Endospermic seeds : In some plants the embryo does not utilize the endosperm completely. Such seeds are called endospermic seeds, e.g. Cocos, Ricinus and Datura.
Perisperm : In the seeds of some plants nucelus is left out and it is called perisperm, e.g. Piper nigrum.
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