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Fertilization in flowering plants and how self-fertilization / pollination is avoided

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RABIA SALEEM Fertilization in flowering plants and how self-fertilization/pollination is avoided Flower is the reproductive part of all flowering plants. These flowering plants reproduce sexually. The male part of the flower is called the stamen which consist of the long filament and the anther which consists of the pollen sacs in which pollen is made. The female part is called the carpel which constitutes a sac-like structure called the ovary, the thread like style and the sticky stigma. The male gamete, pollen, comes from the anther which consists of four pollen sacs. Each pollen sac contains the pollen mother cells, each of which undergoes meiosis to form a tetrad of four haploid cells. The cells round off and are called microspores. The single nucleus divides by mitosis to give the tube nucleus and the generative nucleus. The wall thickens and forms an inner layer, the intine, and an often highly sculptured outer layer, the exine. On the other hand, the ovary consists of one or many ovules depending on the plant species. ...read more.


A sucrose solution is secreted by the epidermal cells of the stigma. This stimulates germination of the grain and possibly supplies food. A pollen tube emerges from one of the pores in the wall of the pollen grain and grows rapidly down the style to the ovary. This is because the female gamete is protected by within the carpel and the male gamete can only reach it via the pollen tube. Its growth is controlled by the tube nucleus of the pollen grain, which is found at the growing tip of the tube. The tube nucleus is only concerned with the growth of the pollen tube and plays no part in fertilization. Growth is stimulated by auxins produced by the carpel, and the pollen tube is directed towards the ovary by certain chemicals. Growth depends upon compatibility between the pollen and the style tissue. During growth of the pollen tube, the generative nucleus of the pollen grain divides by mitosis to produce two male nuclei that represent the male gametes. ...read more.


If the stigma and ovule ripen before the stamens, the flower is protogynous. * Is a plant has separate male and female flowers it is said to be monoecious. This condition clearly limits the possibility of self pollination. * Some flowers have a stigmatic flap which is exposed to the pollen on the back of a visiting insect. The insect collects pollen from the stamens and closes the flap as it withdraws from the flower, thus protecting the stigmatic surface from its own pollen. * A dioecious species is one in which some individual plants have either all male or female flowers. This greatly prevents self-pollination/fertilization. * In some flowers, the anthers hang outside the flower, thus reducing or preventing self pollination. * Sometimes, the petals almost enclose the stigma so the pollen and the stigma have a lesser chance of meeting. * The pollen grain size may differ to the size of the stigma, which would prevent it from sticking to the stigma. * Lastly, in some flowers, such as pears, the pollen only becomes functional if the stigmatic surface on which it lands has a different genetic composition. This condition is known as incompatibility. ...read more.

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