Structure of ovule

 CHAPTER 1. REPRODUCTION IN LOWER AND HIGHER PLANTS

Structure of angiospermic ovule:-

The ovule in which micropyle, chalaza and funiculus are not in one straight line and funicle runs parallel to integuments is called as anatropus ovule or inverted ovule.

E.g. Hibiscus, Jowar etc.

Structurally ovule shows two main parts;

A)  Funicle: It is short stalk which connects body of ovule with the placenta. The ovule with funicle is said to be funiculate ovule and the ovule without funicle is called as sessile ovule. Funicle maintains vascular supply to the ovule.

B)  Body: It shows following parts;

(a) Nucellus:-It is a mass of compactly arranged parenchymatous cells. It is also called as megasporangium or sporogenous tissue.     The cells are diploid in nature.

(b) Integuments: These are two thick fleshy structures surrounding the nucellus namely outer integument and inner integument. Due to the presence of two integuments, the angiospermic ovule is called as bitegmic ovule. (Gymnosperms show unitegmic ovule).

(c) Chalaza:  It represents the basal part of ovule which produces integuments.

(d) Micropyle. The small space or cleft left behind by two integuments near the tip of ovule is called as micropyle. It allows entry of pollen tube at the time of fertilization.

e) Embryo sac: It consists of following parts;

I)    Egg apparatus; It is located near micropyler end. It shows presence of central large egg cell, haploid in nature. On either sides of egg two haploid synergids are present.

II)   Secondary nucleus:    It is formed by the fusion of two haploid polar nuclei. Hence, it is diploid in nature.

III) Antipodal cells. It is a group of three cells present towards chalazal end and are haploid in nature.

Functions of various parts of ovule.

a)   Funicle: It attaches body of ovule with the placenta and also maintains vascular supply to various parts of ovule.

b)  Nucellus: At maturity it produces female gametophyte or embryo sack.      It provides nourishment to the developing embryo sack.

c)   Chalaza:  It produces two thick fleshy integuments which protect megasporangium.

d) Integuments:      It gives protection to nucellus. After fertilization integuments develop into seed coat, outer integument develops into testa and inner integument develops tegmen.

e)   Micropyle. It shows porogamy i.e. entry of pollen tube inside the ovule at the time of fertilization.

f)    Embryo sac:

I)    Egg apparatus: After fertilization egg cell develops into zygote (2n) which further develops into embryo. Synergids provide site for attachment of pollen tube at the time of fertilization, through filiform apparatus.

II) Secondary nucleus: It fuses with one of the male gametes producing triploid primary endospermic nucleus.

III) Antipodal Cells: After fertilization these three cells degenerate and probably form food material.

 

Development of female gametophyte in angiosperms:-

The development of female gametophyte takes place inside the ovule from the nucellus.

One of the Nucellar cells towards micropyler end enlarges in size and behaves as megaspore mother cell (2n).The megaspore mother cell (2n) undergoes reduction division or meiosis producing megaspore tetrad (n). Out of these four megaspores,  degenerate and only one persists called as functional megaspore (n).Functional megaspore undergoes three successive mitotic divisions producing 2, 4 nucleates stages and finally 8 nucleates stage.     After this 8 – nucleate stage undergoes rearrangement and develops into female gametophyte or embryo sac. One of the nuclei from micropyler end and one from chalazal end move to the center of embryo sac and form two polar nuclei haploid in nature. The remaining three nuclei towards chalazal end develop as antipodal cells which are haploid in nature.  The three nuclei towards micropyler end develop egg apparatus. It consists of central large haploid egg cell and two haploid synergies on either sides of it. Before fertilization the embryo sac or female gametophyte is 8 – celled – 8 nucleated. At the time of fertilization it becomes 7 celled 8 nucleated structures due to fusion between two polar nuclei (n) forming diploid secondary nucleus or definitive nucleus. The female gametophyte develops from single functional megaspore. Hence the development is referred as monosporic type.

It was studied for the first time in plants like Polygonum and Oenothera. Hence it is called as Polygonum type.

Embryo sac or female gametophyte

Embryo sac is also known as female gametophyte. It develops from haploid functional megaspore present in the nucellus. Embryo sac is haploid in nature.

Embryo sac consists of following parts:

A)  Egg apparatus: It shows central large egg cell, haploid in nature and two haploid synergids on either sides of it. Synergids provide surface for attachment of pollen tube at the time of fertilization. Egg cell (n) after fertilization develops into diploid zygote and then into embryo.

B)  Secondary nucleus: It is formed by the fusion of two haploid polar nuclei. It fuses with one of the male gametes and develops triploid primary endospermic nucleus. It is also called as definitive nucleus.

C)  Antipodal Cells:-It is a group of three haploid cells present towards chalazal end.

After fertilization these three cells degenerate and probably produce food material.

Prior to fertilization embryo sac is 8 celled – 8 nucleated structure but of the time of fertilization it becomes 7 celled – 8 nucleated.