The Pistil, Megasporangium (ovule) and Embryo Sac
$\displaystyle \small \bullet$ Pistil is the female unit of flower.
$\displaystyle \small \bullet$ Pistil consists of one carpel (monocarpellary) or more than one (polycarpellary).
$\displaystyle \small \bullet$ When there are more than one carpel, pistils may be fused together (syncarpous) or free (apocarpus).
$\displaystyle \small \bullet$ A carpel has three parts
1. Stigma: Topmost part of carpel, attached to style. It receives the pollen grains.
2. Style: Neck of the carpel, to which stigma is attached.
3. Ovary: Swollen basal part of carpel. Inside the ovary is the ovarian cavity (locule) that contains ovules. Each ovule encloses an embryo-sac.
Placenta
$\displaystyle \small \bullet$ A special tissue that forms the inner wall of ovary bearing one or more ovules is called placenta.
$\displaystyle \small \bullet$ Number of ovules in an ovary may be one(ex: wheat, paddy, mango) or many (ex: papaya, watermelon).
$\displaystyle \small \bullet$ The ovules are also called megasporangia.
$\displaystyle \small \bullet$ Ovule is attached to placenta by a stalk called funicle.
$\displaystyle \small \bullet$ Placentation: It refers to the arrangement of placenta in the ovary.
Structure of Megasporangium
$\displaystyle \small \bullet$ An ovule develops as tiny knob in the ovary.
$\displaystyle \small \bullet$ As it develops, it gets raised from the ovary wall by a short stalk called funicle through which food and water are transported to the ovule.
$\displaystyle \small \bullet$ One end of the funicle is attached to placenta and the other end to the body of the ovule.
$\displaystyle \small \bullet$ The point of attachment of funicle with the body is called hilum.
$\displaystyle \small \bullet$ Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe.
The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end.
$\displaystyle \small \bullet$ The multilayered body of ovule is called nucellus and it is enclosed in one or two protective layers called integuments.
$\displaystyle \small \bullet$ These leave an opening at the top of the ovule called micropyle.
$\displaystyle \small \bullet$ Megaspore mother cell or an embryo-sac mother cell is located in the nucellus.
Megasporogenesis
$\displaystyle \small \bullet$ The process of formation of megaspores from the megaspore mother cell (MMC) by meiotic division is called megasporogenesis.
$\displaystyle \small \bullet$ MMC is a large cell containing dense cytoplasm and prominent nucleus.
$\displaystyle \small \bullet$ A MMC is a large diploid (2n) cell. it undergoes meiotic division and give rise to four haploid cells or megaspores.
$\displaystyle \small \bullet$ In a majority of flowering plants, only one of the megaspore is functional and the rest three degenerate.
(i) Monosporic type (Polygonum): In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac.
(ii) Bisporic type: In this type two megaspore nuclei take part in embryo sac formation.
(iii) Tetrasporic type: This type of embryo sac develops from four megaspore nuclei.
Female gametophyte: The functional megaspore develops into the female gametophyte (embryo sac).
Development of an Embryo-sac
$\displaystyle \small \bullet$ The nucleus of functional megaspore divides mitotically to form 2 nuclei which move to opposite poles to form 2-nucleate embryo sac.
$\displaystyle \small \bullet$ Two more sequential mitotic division results in formation of 4-nucleate and later 8-nucleate embryo sac.
$\displaystyle \small \bullet$ Six of the eight nuclei surrounded by cell wall and remaining two nuclei (polar nuclei) are situated below the egg apparatus.
$\displaystyle \small \bullet$ Three cells are grouped at micropylar end to constitute egg apparatus and three cells at chalazal end forms antipodal cells.
Before fertilization, a mature embryo-sac contains 8-nucleate and is 7 celled.
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