Female Reproductive Part - Gynoecium

The gynoecium is the collective term for one or more pistils of a flower, each pistil (derived from a single carpel) being made up of an ovary (the basal, swollen part containing the ovules on its internal placenta), a style (the slender part connecting ovary to stigma) and a stigma (the receptive landing platform for pollen at the tip). The gynoecium first appears as a small papillate outgrowth of meristematic tissue at the tip of the floral primordium, which actively grows and differentiates into ovary, style and stigma; the ovules (also called megasporangia) themselves arise from the placenta inside the ovary, and their number per ovary can be as few as one (paddy, wheat, mango) or very many (papaya, watermelon, orchids). Structurally, a mature ovule (megasporangium) has a stalk and a body, protected by one or two coverings called integuments. The stalk, or funiculus/funicle, attaches the ovule to the placenta at its base; the point where the funicle meets the ovule body is the hilum, and in an inverted ovule the funicle fuses along the body to form a ridge called the raphe. The body itself is a central mass of food-rich parenchymatous tissue called the nucellus, enveloped by one integument (making the ovule unitegmic) or two (bitegmic); the integument encloses the nucellus completely except at the very tip, where it leaves a pore called the micropyle, and the basal region where nucellus, integument and funicle all merge is the chalaza. In a small group of species, the innermost integument layer becomes specialised into a nutritive endothelium (integumentary tapetum), as in the Asteraceae. Ovules are also classified by the position of the sporogenous cell: a hypodermal sporogenous cell surrounded by only a single layer of nucellar tissue defines the tenuinucellate type, which normally has a very small nucellus, while a subhypodermal sporogenous cell defines the crassinucellate type, which normally has a fairly large nucellus. A group of cells at the base of the ovule, between the chalaza and the embryo sac, is called the hypostase, and thick-walled cells above the micropylar end of the embryo sac form the epistase. Towards the micropylar end of the nucellus sits a large, oval, sac-like structure - the embryo sac, or female gametophyte - which develops from the functional megaspore produced inside the nucellus. Based on orientation, form and the position of the micropyle relative to the funicle and chalaza, ovules fall into six recognised types: orthotropous (micropyle, funicle and chalaza lie in one straight vertical line, e.g. Piperaceae, Polygonaceae), anatropous (the body is completely inverted so the micropyle lies close to the funiculus - the commonest type in both dicots and monocots), hemianatropous (the body lies transversely, at right angles to the funicle, e.g. Primulaceae), campylotropous (the body is curved and bean-shaped at the micropylar end, with the hilum, micropyle and chalaza all adjacent and the micropyle oriented towards the placenta, e.g. Leguminosae), amphitropous (hilum and chalaza lie close together and curvature gives the nucellus a horse-shoe shape, e.g. some Alismataceae) and circinotropous (an unusually long funiculus coils right around the ovule, e.g. Cactaceae). Megasporogenesis is the process by which a haploid megaspore develops from a diploid megaspore mother cell. As the ovule develops, a single hypodermal cell of the nucellus enlarges to become the archesporium; in some plants this cell directly functions as the megaspore mother cell, while in others it first divides transversely into an outer primary parietal cell (which may stay undivided or divide further to bury the sporogenous cell deeper in the nucellus) and an inner primary sporogenous cell, which then becomes the megaspore mother cell. The megaspore mother cell undergoes meiosis to give four haploid megaspores, usually arranged in a linear tetrad. Depending on how many of these four megaspores actually contribute nuclei to the mature embryo sac, three basic patterns of development are recognised: monosporic development, in which only the chalazal megaspore survives and the other three degenerate (e.g. Polygonum); bisporic development, in which two megaspores jointly form the embryo sac (e.g. Allium); and tetrasporic development, in which all four contribute (e.g. Peperomia) - an ovule normally forms just a single embryo sac regardless of which pattern it follows. The commonest, simplest pattern - monosporic (Polygonum-type) development - proceeds as follows. The single surviving functional megaspore, the first cell of the embryo sac, elongates along the micropylar-chalazal axis and its nucleus divides mitotically; no wall forms after this division, and instead a large central vacuole appears between the two daughter nuclei and expands, pushing them to opposite poles. Both nuclei then divide twice more mitotically (all without wall formation - a 'free nuclear' division), producing four nuclei at each pole and eight nuclei total sharing one common cytoplasm; after this last division the whole cell elongates markedly into a sac-like shape, and only then does cellular organisation follow. At the micropylar end, three of the four nuclei organise …