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| Pollination |
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| The pollen grains which produce the male gametes and the ovules which bear the female gametes are borne on different structures. It therefore becomes necessary that for sexual union to occur, the pollen grains must be transferred to the stigma. The transfer and deposition of pollen grains from the anther to the stigmatic surface of the flower is called pollination. |
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| Pollination is of 2 types |
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 Self Pollination |
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 Cross Pollination |
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| Significance of Pollination |
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| Pollination leads to fertilisation, resulting in the production of seeds and fruits, thus ensuring continuity of life. |
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| Summary of Self and Cross Pollination |
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| It is the transference of the pollen grains from the anther to the stigma of either the same or of another flower borne on the same plant. |
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| Autogamy |
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| The pollen grains are transferred from the anther to the stigma of the same flower. It occurs in bisexual flowers. |
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| Geitonogamy |
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| Flower is pollinated by pollen from another flower on the same plant. |
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| It is the transference of the pollen grains from the anther of one flower to the stigma of another flower borne on a different plant of the same species. It is also known as allogamy or xenogamy. |
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| Cross pollination requires the help of abiotic or biotic agencies such as wind, water, insects, birds, bats, snails and other animals. |
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| Anemophily or Wind Pollination |
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| It is the simplest form of pollination, through the agency of wind. The wind pollinated flowers show the following characteristics. |
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| a) The flowers are unisexual |
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| Example: Poplars |
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| b) Stamens are freely exposed with versatile anthers |
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| Example: Cereals |
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| c) The pollen grains are light, smooth, dry and not easily wetted by rain. |
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| d) The pollen grains are produced in enormous quantities. |
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| Example: A single flower of Cannabis produces over 5,00,000 pollen grains. |
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| e) The stigmas are large, feathery and well exposed to catch the pollen grains. |
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| f) The flowers are small, inconspicuous with no colour, odour or nectar |
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| Examples: Coconut, palm, maize, grasses etc. |
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| Hydrophily or Water Pollination |
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| It is the mode of pollination involving the transfer of pollen grains through the agency of water. Hydrophily does not occur in all aquatic plants. Many aquatic plants bear their flowers above the surface of water and are pollinated by wind or insects. |
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| Hypohydrophily is the true hydrophily and occurs in such plants where the pollen grains are water borne. In Zostera marina, the pollen grains are elongated and lack an exine. The pollen grains float below the surface of water, and on reaching the stigma, coil around it and germinate. |
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| Epihydrophily takes place over the surface of water in plants like Vallisneria. |
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| Pollination in Vallisneria |
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| The plant is dioecious. On maturity the male flowers get detached from the parent plant and float up and come to the surface of water. At the same time the female flowers also rise upto the surface of water by straightening of the coiled stalk. The detached male flowers cluster around a floating female flower and dehisce thereby performing pollination. Soon after the pollination, the long stalks of the female flower begin to coil down to bud level where the fruit ripens. |
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| Entomophily or Insect Pollination |
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| It is the most sure and least wasteful method. Insects, visit the flowers not to effect pollination but to collect nectar, edible pollen or for shelter. As the insect visits a flower, its body gets dusted with pollen grains. When the loaded insect visits another flower, its body brushes against the stigma and inadvertently transfers the pollen to it bringing about pollination. |
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| E.g., bees, moths, butterflies |
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| Some of the characteristic features of insect pollinated flowers are |
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 The flowers are large, conspicuous and brightly coloured. |
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 When flowers are small, they aggregate in the form of inflorescence. |
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 The flowers have a pleasant fragrance and sweet nectar. |
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 Pollen grains are usually rough and sticky and often show spinous outgrowths. |
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| Scanning electron micrograph of pollen grains on the stigma of a flower. |
| The spiked surface of the grains is typical of insect-pollinated flowers |
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 The stigma is sticky and rough. |
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 The pollen are edible in many cases. |
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| Example: Rose and poppy |
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| Ornithophily or Bird Pollination |
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| The common bird pollinators are humming birds, sun - birds and honey - eaters. Birds can obtain only one staple food from flowers and that is nectar. The bird pollinated flowers have funnel - shaped or tubular corollas which are brightly coloured. The floral parts are commonly leathery and they produce copious amount of nectar and large quantities of pollen which are sticky. |
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| Common bird - pollinated plants are Bombax (red silk cotton), Erythrina (coral tree) Bigonia, Lobelia, Agave. |
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| Chiropterophily or Bat Pollination |
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| Bats bring about pollination only in the tropics. Bats are nocturnal animals and the flowers they visit are large and emit a strong odour. They move very fast and transport pollen to long distances. Bat pollinated flowers generally produce more nectar than ornithophilous flowers. They also have a large number of stamens. |
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| Examples: |
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| The Sausage tree - Kigella pinnata |
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| The Baobab tree - Adansonia |
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| Anthocephalus - Kadamb tree |
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| Malacophily or Snail Pollination |
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| Snails perform pollination in cobra plant. Arisaema and some arum bulbs. |
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| a) It brings about genetic recombination and introduces variation in the offspring |
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| b) New and improved varieties of plants are produced. |
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| c) The plants are more resistant to diseases. |
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| d) The seeds produced are usually better, larger, healthy and more vigorous and numerous in number. |
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| Nature has evolved certain methods which self-pollination is prevented and cross-pollination is favoured. Given below are a few such adaptations. |
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| Unisexuality |
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| If the flowers are unisexual and present on different plants, only cross pollination is possible. |
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| Dichogamy |
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| In bisexual flowers, anthers and stigma mature at different times. |
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| This prevents self polination. |
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| Example: Sunflower and salvia |
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| Self-sterility |
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| Pollen grains of a flower are not capable of effecting fertilization even if they reach the stigma of the same flower. |
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| Examples: Potato, tobacco. |
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| This is also known as self-incompatibility. |
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| Heterostyly |
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| In certain plants, 2 or mote types of flowers having different lengths of styles and stamens are produced. The stigma and stamens are not in the same level, so that self-pollination is prevented. Example: Primrose |
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| Herkogamy |
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| In some flowers, there are some physical barriers between anther and stigma to prevent self-pollination. Example: Calotropis |
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| Incompatibility |
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| Incompatibility is the inability of functional male and female gametes to effect fertilization in certain plants. |
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| Incompatibility may be between 2 different species of the same genus, when it is called Interspecific incompatibility. If it is within the same species, it is called Intraspecific incompatibility or self sterility or self - incompatibility. |
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| In the former, fertilization between gametes produced from 2 unrelated plants is prevented. In the latter, fertilization between gametes produced by the same plant or by another plant of the same species is prevented. |
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| Intra specific incompatibility is also called self-incompatibility and is reported in abort 300 species of angiosperms. It may be sporophytic incompatibility (if incompatibility is due to the genotype of the stigma) or gametophytic incompatibility (if incompatibility is due to the genetype of the pollen) |
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| Significance of Self-incompatibility |
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| a) Pollen stigma interaction |
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| b) Pollen tube style interaction |
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| c) Pollen tube ovule interaction |
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| In self |
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| Incompatible fruit trees, it is necessary to plant two cross compatible varieties to ensure fruit formation. |
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| Self |
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| Incompatibility can be used in hybrid seed production. |
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| The germination of the pollen grain starts before it is shed from the anther. During this process, the nucleus of the pollen grain divides by mitosis into a vegetative or tube nucleus and a smaller generative nucleus. The pollen grains are generally transferred to the stigma at this two - celled stage. However in some plants e.g., cereals, the development continues and the generative cell forms 2 male gametes before the pollination takes place. |
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