Pollination

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Introduction
The pollination involves the transfer of pollen from male part of a plant, called stamen to the female part of the plant, known as carpel. This enables fertilization and sexual reproduction to occur. Pollen grains are contained within the pollen while the carpel contains the female gamete. In gymnosperms the pollen is directly applied to the ovule. The receptive part of the carpel is called a stigma in the flowers of angiosperms while the receptive part of the gymnosperm ovule is called the micropyle. Pollination is essential in the reproduction of flowering plants to produce offspring that shows genetic variation. Pollination involves wide field of studies such as botany, horticulture, entomology, and ecology. In 18th century, the pollination process as an interaction between flower and vector was first addressed by Christian Konrad Sprengel. It is important in horticulture and agriculture because fertilization as the end result of pollination is the key factor to fruiting.

Male and Female Gamates in Plants
What is a Pollen?
Pollen is a substance produced by the anthers of seed-bearing plants that consists of numerous fine pollen grains containing male gametes. This means that pollen carries the cells which enable fertilization of plants so that plants may reproduce. The pollen wall protects the sperm nucleus while the pollen grain is moving from the anther to the stigma, it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins. When pollen lands on a compatible pistil of flowering plants, it germinates and produces a pollen tube that transfers the sperm to the ovule of a receptive ovary. The outer pollen wall is composed of two layers, namely tectum and the foot layer. The tectum and foot layer are separated by the columella which is composed of strengthening rods. Pollen itself is not the male gamete, but each pollen grain contains vegetative cells and a generative cell containing two nuclei which is a tube nucleus that produces the pollen tube and a generative nucleus that divides to form the two sperm cells. The outer wall is constructed with a resistant biopolymer called sporopollenin. Pollen is produced in the microsporangium that is contained in the anther of an angiosperm flower, male cone of a coniferous plant or male cone of other seed plants. Pollen grains come in a wide variety of shapes,mostly spherical, sizes, and surface markings characteristic of the species. Formation of pollen grains in the anther

In angiosperms, during flower development four groups of sporogenous cells form with in the anther, the fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac, some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. Four haploid microspores are produced from each diploid sporogenous cell called a microsporocyte, after meiotic division. After the formation of the four microspores, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. Pollen apertures are any modification of the wall of the pollen grain. These modifications include thinning, ridges and pores. They serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. Furrows in the pollen grain which along with the pores are called colpi and are a chief criteria for the identifying pollen classes. Pollen grains may have furrows, the orientation which classify the pollen as colpate or sulcate. The number of furrows or pores helps to classify the flowering...
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