Biology XII:2 Sexual rep in flowering plants(3 Pollination)

Pollination It is the process of transfer of pollen grains from the anther to the stigma. Depending on the source of pollen, pollination can be divided as follows: Autogamy − It is the transfer of pollen grains from the anther to the stigma of the same flower. Autogamy requires the anther and the stigma to lie close. It also requires synchrony in the pollen release and stigma receptivity.Plants like Viola, Oxalis, etc., produce two kinds of flowers—chasmogamous flowers (with exposed anther and stigma) and cleistogamous flowers (which do not open at all and only autogamy occurs). Geitonogamy − It is the transfer of pollens from the anther of one flower to the stigma of another flower in the same plant. Genetically, it is similar to autogamy, but it requires pollinating agents. Xenogamy − It is the transfer of pollen grains from the anther to the stigma of a different plant. Pollination causes genetically different types of pollens to be brought to a plant. Agents of Pollination Plants use air, water (abiotic agents) and animals (biotic agents) for pollination. Pollination by wind It is the most common form of abiotic pollination. Plants possess well-exposed stamens and large, feathery stigma. Pollens should be light and non-sticky to be carried easily by winds. Wind-pollinated flowers often have single ovule in the ovary and numerous flowers packed in an inflorescence. It is common in grass. Pollination by water It is rare in flowering plants, except for some aquatic plants like Vallisneria and Hydrilla. In most water-pollinated plants, the pollen grains are long and ribbon-like, and are protected from wetting by mucilaginous covering. In a majority of water plants like water hyacinth and water lily, flowers emerge above the water level and are pollinated by insects. Pollination by animals Majority of flowering plants use butterflies, bees, wasps etc., for pollination. Most of the insect-pollinated flowers are large, colourful, fragrant, and contain nectar to attract the animal pollinators. These are called floral rewards. Floral reward can be in the form of providing safe places to lay eggs (example: the tallest flower, Amorphophallus) A symbiotic relationship exists between the plant, Yucca and its pollinator moth. The moth is dependent on the plant since the moth deposits its eggs in the locule of the ovary of the plant, and in return, the plant is pollinated by the moth. The pollen grains are sticky and get stuck to the body of the pollinator. Out Breeding Devices Repeated self pollination leads to inbreeding depression. Plants have developed methods to prevent self pollination. Autogamy is prevented by following ways: Pollen release and stigma receptivity not coordinated Different positioning of the anther and the stigma Production of unisexual flowers Ways to prevent both autogamy and geitonogamy: Presence of male and female flowers on different plants, such that each plant is either male or female (dioecy). This mechanism is present in several species of papaya. Pollen−Pistil Interactions Pollination does not always ensure the transfer of compatible pollens. Hence, the pistil has the ability to recognise the right type of pollen to promote post- pollination events. If the pollen is of the wrong type, the pistil prevents pollen germination. This interaction is mediated by chemical components of the pollen and the pistil. Pollen−pistil interaction is a dynamic process involving pollen recognition, followed by promotion or inhibition of the pollen. The pollen tube reaches the ovary and enters the ovule through the micropyle. Then, through the filiform apparatus, it reaches synergids. In this way, the pollen tube grows.