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| Cross Pollination or Allogamy |
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| Cross pollination is defined as the transference of pollen grains from the anther of a flower to the stigma of another flower of a different plant belonging to the same species. |
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| Hybridisation is a special type of cross pollination between two flowers borne by two plants of different varieties or species. |
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| Significance of cross pollination |
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| Nature favours cross pollination. The seed produced as a result of cross pollination are usually bigger, more in number and germinate into better plants compared to those produced by self pollination. Moreover, due to allogamy there are chances of mixing of characters of different plants. This leads to variation and speciation. Frequent self pollination results in gradual deterioration of the plants. |
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| Types of Cross Pollination |
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| In order to bring about cross pollination the pollen grains will have to be carried from the anther of one flower to the stigma of another flower. This is brought about through external agents like insects, birds, wind and water. |
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| Depending upon the agents, cross pollination is different types, namely: |
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| Hydrophily: pollination by water |
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| Anemophily: pollination by wind |
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| Zoophily: pollination by animals. |
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| Zoophily is further divided into |
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| Entomophily: pollination by insects |
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| Ornithophily: pollination by birds |
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| Chiropterophily: pollination by bats |
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| Malocophily: pollination by snails |
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| Mermecophily: pollination by ants |
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| Entomophily |
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| Cross pollination brought about by the agency of insects is called entomophily. Entomophilous flowers develop several types of modifications to attract insects. In addition to some remarkable adaptations in some flowers, the main adaptations are colour, nectar and scent. |
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| a. Colour: The petals are brightly coloured to attract insects. But in some plants the flowers are very small and inconspicuous. In such flowers other parts may become prominent and brightly coloured. In Mussaenda, the flowers are small but one of the sepals becomes brightly coloured. In Bougainvillea the bracts become brightly coloured. In the spadix inflorescence, very often the spathe becomes brightly coloured. |
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| b. Nectar: Nectar is a sweet juice secreted by glands called nectaries. They are generally situated at the base of one of the floral whorls. When insects like bees visits flowers to collect nectar, cross pollination is brought about. |
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| c. Scent: Many of the flowers emit sweet smell to attract insects. It is an excellent adaptation especially in nocturnal flowers. Because of scent even in the absence of light, insects can reach the flowers. e.g., Cestrum (night queen). Bees and butterflies are attracted by sweet smell. But the inflorescence in Amorphophallus emits a stinking smell. Some flies are attracted to this smell . |
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| The pollen grains and nectar also serve as an excellent source of food. Sometimes the insects visit flowers in search of shelter, to get protection from sunlight, rain and enemies. |
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| d. Special adaptations: There are a very large number of excellent special adaptations in flowers for entomophily, many such adaptations are seen in Orchids, Ficus, Snapdragon, Salvia, etc. |
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| Lever Mechanism |
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| As a typical example the mechanism in Salvia is described. The flower has a bilipped corolla. There are two stamens and each stamen has two anther lobes which are separated from one another by a long connective. One anther lobe is fertile and the other is sterile. The insect lands on the lower lip of corolla and enters the flower to collect nectar. While doing so it pushes the lower sterile anther lobe. The connective swings like a lever and the upper fertile lobe strikes the back to the insect and dusts it with pollen grains. After the insect leaves the flower, the stigma matures and bends down to receive the pollen grains from the back of another insect which has brought them from another flower. |
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| Hydrophily |
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| The pollination brought about by the mediation of water is called hydrophily. This is seen in aquatic plants particularly the submerged ones. e.g. Vallisneria, Hydrilla, etc. |
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| Vallisneria: It is submerged and dioecious. In the male plant the spadix is submerged but the minute male flowers get detached and float on water. Each flower has three tepals and two stamens. The tepals curve back and form a sort of tripod supporting the two stamens. The female flowers are solitary and are provided with long stalks, which bring them to the surface of water. There is a depression on the surface of water around each female flower. When male flowers come to this depression it moves towards the female flower. The anthers burst and the pollen grains are deposited on the stigma of the female flower. After pollination the stalk of the female flower coils spirally and draws it under water and the fruit develops under water. |
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| In some hydrophytes, the pollen which are very long, slender and thread-like long are deposited into the water and transported (e.g. Zostera). |
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| fig. 27.67: Vallisneria |
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| Anemophily |
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| Cross pollination brought about by wind is called anemophily. Some of the common features met within anemophilous flowers may be listed as follows: |
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| a. They are generally small, inconspicuous and are not brightly coloured. |
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| b. They produce a very large quantity of pollen grains. |
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| c. The pollen grains produced by them are dry, smooth, loose and |
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| powdery, so that they are easily carried by the wind. |
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| d. The filaments of the stamens are generally long and exerted with versatile anthers. |
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| e. The stigmas are usually long and feathery so that they can catch pollen floating in air. |
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| f. In some anemophilous trees, the flowers are produced after all the leaves are shed so that the pollen are easily carried away by wind without any interference. |
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| In anemophilous flowers, an enormous quantity of pollen is wasted. The direction of the wind is also an important criterion. Anemophilous flowers are seen in maize, grasses, sugarcane, coconut, etc. |
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| Zoophily |
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| Pollination brought about by the mediation of animals like birds, snails etc., is called Zoophily. e.g., Anthocephalus, Arisaema etc. |
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| Ornithophily |
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| Pollination brought about by the mediation of birds is called ornithophily. e.g., Erythrina, Bombax, etc. |
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| Contrivances for cross pollination: As nature prefers cross pollination to self pollination, the flowers developed some modifications which prevent self pollinations but increase the chances for cross pollination. These are known as contrivances for cross pollination. |
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| a. Unisexuality or dicliny: This is the production of unisexual flowers. If the plant becomes dioecious cross pollination becomes indespensible e.g., Mulberry. |
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| b. Dichogamy: The maturation of anthers and stigma at different times is called dichogamy. When anthers mature earlier than the stigma of the same flower, the condition is called protandry. e.g., Helianthus, Leucas. But when stigma matures earlier than the anthers of the same flower, the condition is called protogyny. E.g., Ficus, Mirabilis. |
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| c. Self incompatibility: In some plants even if self pollination takes place, fertilisation is prevented as the pollen fail to germinate. Such plants can produce seeds only after cross pollination. E.g., Brassica |
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| d. Heterostyly: It is a condition where some plants produce two types of flowers (dimorphic flowers) In one type the stamens are long and style is short but in the second type, stamens are short and style is long. This also increases the chances for cross pollination. e.g., Oxalis, Jasminum, Biophytum, etc. |
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| e. Herkogamy: It is a condition where some sort of a barrier is formed between the stigma and the stamens of the same flower. The barrier may be in the form of a hood covering the stigma or anthers, and stigma may lie at some distance from the anther or the stigma might become sticky and receptive only on the lower side. E.g., Vinca, Iris, Clerodendrum thampsonii, etc. |
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