Female Reproductive Unit of Flower

Female Reproductive Unit

The Pistil, Megasporangium (Ovule) and Embryo Sac are the parts of the female reproductive unit of flowers.

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The female gametophyte includes the following parts.

The pistil or gynoecium represents the female reproductive unit of a flower.
The gynoecium may have single pistil (called monocarpellary), more than two pistils (called multicarpellary), fused pistils (called syncarpous) or free pistils (called apocarpous).
Inside the ovary there is a compartment called ovarian cavity or locule separated by septum.
Placenta is the tissue in ovarian cavity from where ovule or megasporangium arise.

Parts	Functions	Description
Stigma	It receives the pollen grains.	It has a sticky surface and pollen grains get stuck to it during pollination.
Style	It holds the stigma of receiving pollen grains.	It is an elongated slender part beneath the stigma, that connects the stigma with ovary. It is through the style that the pollen tube grows and reaches the ovule.
Ovary	Contains ovules and eggs.	It is the basal swollen part of the pistil that contains ovules and eggs.

Pistil

The pistil or gynoecium represents the female reproductive part of the flower. It may consist of one pistil or carpel (monocarpellary), two carpels (bicarpellary), three carpels (tricarpellary) or many carpels (multicar Ellary).

Pistil may be syncarpous (i.e., more than one pistil is fused together) as in Papaver, Solanum, etc., or apocarpous, i.e. (carpels remain free) as in Michalia, rose, etc.

The gynoecium may have single pistil (called monocarpellary), more than two pistils (called multicarpellary), fused pistils (called syncarpous) or free pistils (called apocarpous).

A typical pistil consists of the hollow basal swollen ovary, the elongated style and the terminal stigma (serves as a landing platform for pollen grains). Inside the ovary, is the ovarian cavity (locule) in which placenta is located. Megasporangium (ovules) arises from placenta.

Structure of Megasporangium (Ovule)

The ovule is an integumental megasporangium within which meiosis and megaspore formation takes place. It is attached to the placenta by means of a stalk called funicle. It develops into a seed after fertilization. The junction between an ovule and funicle is called hilum, which later becomes a scar on the seed.

Each ovule has one or two protective envelops called integuments, which encircle the ovule except at the tip where a small opening called micropyle is located.

The basal part of an ovule just opposite to micropyle is called chalaza. Enclosed within the integuments is a mass of cells called nucellus, whose cells are rich in reserve food materials. An ovule generally has a single embryo sac or female gametophyte, developed from a megaspore through reductional division and located within the nucellus.

Megasporogenesis

The process of formation of megaspores from Megaspore Mother Cell or MMC (diploid) is called megasporogenesis.

It occurs inside the nucellus of the developing ovule of angiosperms. The process begins very early when nucellus is not completely surrounded by the integuments.

The MMC (a large cell containing dense cytoplasm and a prominent nucleus) enlarges in size and divides by meiosis. It first divides transversely into two cells called megaspore dyad.

These two cells again divide transversely forming a linear row of four haploid cells which is called megaspore tetrad or linear tetrad.

Female Gametophyte (Embryo Sac)

Å in general, the development of embryo sac is monosporic, e.g., in Polygonum. In this type of development, only one megaspore situated towards chalaza end remains functional, while the remaining three megaspores gradually degenerate and finally disappear.

Following are the different stages in development of female gametophyte.

The functional haploid megaspore is the first cell of female gametophyte of angiosperms.
Its nucleus undergoes mitotic division to form 2-nucleate that move to opposite poles forming 2-nucleate embryo sac.
The 2-nucleate embryo sac undergoes two more sequential mitotic divisions giving rise to the 4-nucleate stage and later 8-nucleate stage of embryo sac. This stage comprises of a micropylar end and a chalaza end with four nuclei at each end.
Six of the eight nuclei are surrounded by cell walls and get organized into cells. The remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.
Three cells present towards the micropylar end are grouped together and constitute the egg apparatus, i.e., two synergids and one egg cell.
Three cells of the chalaza end are called the antipodals. The large central cell is formed by the fusion of 2 polar nuclei. Thus, a typical angiosperm embryo sac or female gametophyte at maturity consists of eight nuclei and seven cells.

FAQ

What is the Female Reproductive Unit of Flower?

The Female Reproductive Unit of Flower consists of one or more carpels, each containing an ovary, style, and stigma.

How does pollination occur in the Female Reproductive Unit?

Pollination in the Female Reproductive Unit typically involves the transfer of pollen grains from the anther to the stigma, either by wind, water, insects, or other animals.

What happens after successful pollination?

After successful pollination, the pollen grain germinates on the stigma, and a pollen tube grows through the style into the ovary, where fertilization of the ovules occurs.

What is the role of the ovary in the Female Reproductive Unit?

The ovary contains ovules, which are the female reproductive cells. After fertilization, the ovules develop into seeds within the ovary.

How are seeds dispersed after fertilization?

Once fertilization occurs and seeds develop within the ovary, various mechanisms such as wind, water, animals, or explosive action help in the dispersal of seeds from the ovary to new locations for germination.

Female Reproductive Unit of Flower