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The process of reproduction in which two parents are involved to produce new generation by the fusion of meiotically formed male and female gametes is called sexual reproduction.
In higher plants, sexual reproduction takes two steps:
Male gametes are formed inside anther which follows two steps: microsporogenesis and micro gametogenesis.
The structure of anther is discussed below:
•An anther consists outer wall and inner sporogenous tissue. Anther wall consists of Epidermis, Endothecium, Middle layers and Tapetum.
•Epidermis is single layered protective in function thick outermost layer.
•Second layer present below epidermis is endothecium in which cells are radially elongated and have fibrous thickenings of cellulose, little amount of suberin and lignin. The fibrous thickening makes it hygroscopic in nature which helps in dehiscence of pollen grains. The point of dehiscence is called stomium.
•Middle layers are of two to four layers. Cells of this layer are ephemeral and degenerates at maturity.
•Tapetum is the innermost layer. It is nutritive in function. Cells of this layer contain large nucleus and dense cytoplasm. The cells of tapetum are large, multinucleated, and have the ability to show polyploidy due to endomitosis.
•The sporogenous tissue cells also go mitosis to form microspore mother cell which later divides meiotically to from haploid tetrad that ultimately forms pollen grains.
Microsporogenesis is the process of development of microspores or pollen grains from microspore mother cell inside the microsporangium/pollen sac of anther.
Stamen is the male sex organ of a flower which has three parts- Anther, Connective, and Filament. The fertile part of the stamen is anther. On either side of connective, there is an anther lobe. Each anther lobe contains two chambers or pollen savs called microsporangia. Therefore, there are four microsporangia per anther i:e tetrasporangiate. Such anthers are called dithecous. In Malvaceae, anther is bisporangiate and monthecous.
Each microsporangium/pollen sac develops from a group of archesporial cells. In immature cells of the anther, one cell gets distinct and contains abundant cytoplasm, called archesporial cell. The archesporial cell then divides periclinally to form the outer primary parietal cell and inner primary sporogenous cell. The parietal cell further divides to give rise to outer layers of anther and sporogenous cell enlarges and behaves as microspore mother cell. Microspore mother cell then divides meiotically to form microspores or pollen tetrad.
The process of formation of male gametophyte from functional pollen grain is called micro gametogenesis. The study of the pollen grain is called palynology. Pollen is haploid structure. Each pollen is globose, uninucleated, dust-like yellowish minute structure. A pollen grain has two layered wall- an outer exine and inner intine. The exine is composed of sporopollenin ( hardest part in plant tissue) and cutin. It is derived from tapetum. Inner to it, then and the elastic layer made of cellulose and pectin is present, called intine. It is derived from haploid, vegetative cell and protoplast of the pollen grain. At some places, exine is absent and called germ pore from where emerges a pollen tube.
Pollen grain divides mitotically to form two unequal cells- small generative and larger vegetative cell. At this two-celled stage, pollination takes place. The generative cell gives rise to two male gametes while the vegetative cell forms pollen tube which is made from intine.
Female gametes are formed inside ovules which follows two steps: megasporogenesis and megagametogenesis.
The ovules are borne on a cushion like tissue called placenta. Each ovule is an ovoid structure which is attached to the placenta by means of a stalk called funicle. The point of attachment of the funicle with the ovule is called hilum. The funicle may get united with the ovule for some distance beyond the hilum. It then appears as a ridge called raphe. The ovule contains a mass of thin-walled parenchymatous cells called nucellus. The nucellus is covered by one or two multicellular coats, called integuments. The integument does not cover nucellus completely. A narrow passage called micropyle is left at the tip of the ovule. The base of nucellus from where the integuments arise is called chalaza.
The process of development of megaspore inside megasporangium from megaspore mother cell is called megasporogenesis.
The gynoecium is composed of the carpel. Each carpel bears a stigma, style, and an ovary. The ovary contains ovules and arrangement of ovules inside ovary are called placentation.
In young ovule, a hypodermal cell in nucellus becomes large to form the archesporial cell. The archesporial cell divides periclinally to form the outer primary parietal cell and inner primary sporogenous cell. A primary parietal cell divides anticlinally many times to form the wall of the embryo sac. The primary sporogenous cell enlarges itself to form megaspore mother cell. Megaspore mother cell divides meiotically to form four haploid megaspores. Out of four megaspores, three towards micropyle degenerate and only one towards chalaza remain functional.
The process of formation of female gamete from functional megaspore is called megagametogenesis. Functional megaspore undergoes three times mitosis to form embryo sac with 8 nuclei. Initially, there are 4 nuclei at each micropyle and chalaza end but one of them from each pole move towards center and fuse together at the time of fertilization to from a diploid secondary nucleus. Three cells at chalaza end are called antipodal cells which later degenerate. Out of 3 cells of the micropylar end, one enlarges in size and is called egg cell. Other two cells are called as synergids which secrete chemicals to guide pollen tube.
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