The flower's function is to bring about sexual reproduction (the production of offspring following the fusion of male and female nuclei). The male and female nuclei are contained within the pollen grain and ovule respectively and pollination is the transfer process. Cross-pollination ensures that variation is introduced into new generations of offspring.
Self pollination occurs when pollen comes from the same flower (or a different flower on the same plant) as the ovule, common in Fabaceae (bean family).
Cross-pollination occurs when pollen comes from a flower of a different plant, with a different genetic make-up from the ovule, common in Brassicaceae (cabbage family).
Natural agents of cross-pollination are mainly wind and insects.
Wind-pollinated flowers. The characteristics of wind-pollinated flowers are their small size, their green appearance (lacking coloured petals), their absence of nectaries and scent production, and their production of large amounts of pollen which is intercepted by large stigmas. They also often have proportionally large stigmas that protrude from the flower to maximize the chances of intercepting pollen grains in the air.
Pollination is the plant process whereby a male pollen grain is transferred from the anther to the stigma of the flower, thus enabling fertilization to take place.
The commonest examples of wind-pollinated plants are the grasses, and trees with catkins such as Salix (willow), Betula (birch), Corylus (hazel), Fagus (beech), Quercus (oak). The Gymnosperma (conifers) also have wind-pollination from the small male cones.
Insect-pollinated flowers. The characteristics of insect-pollinated flowers are brightly coloured petals (and scent production) to attract insects, and the presence of nectaries to entice insects with sugary food. Insects such as bees and flies collect the pollen on their bodies as they fly in and out and carry it to other flowers. In tropical countries, slugs, birds, bats and rodents are also pollinators. Some floral mechanisms, e.g. in snapdragon and clover, physically prevent entry of smaller non-pollinating insects and open only when heavy bees land on the flower. Other plant species, such as Arum lily, trap pollinating insects for a period of time to give the best chance of successful fertilization.
Certain Primula spp. have stigma and stamens of differing lengths to encourage cross-pollination; in thrum-eyed flowers, the anthers emerge further from the flower than the stigma, so that insects rub against them when reaching into the flower tube; in pin-eyed flowers, the stigma protrudes from the flower and will catch the pollen from the same place on the insect body, so ensuring cross-pollination (see Figure 10.4).
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