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Pollination is the mechanism that allows the transport of pollen from the anthers to the stigma and constitutes the necessary prelude for fertilization and seed production.
In most plants (allogamouspollination occurs between different individuals (cross-pollination) and only in some plants (self-pollinating) the pollen is transferred to the same flower or to flowers of the same plant (self-pollination). A very narrow type of self-pollination (cleistogamy) occurs in the wheat, when the spikelet has not yet opened.
To avoid self-pollination, pollen and eggs from the same individual ripen at different times in many flowers, or the stamens are placed lower than the stigma. In certain species, as occurs for example for dioecious species, cross pollination is the only possible.
The transfer of pollen can take place by wind, water or animals.
Anemophilous pollination (anemogama)
It is the simplest and, perhaps, the most primitive form of pollen transport. Anemophilous pollination flowers are normally small and often belonging to the less evolved phylogenetically plants (ex. Gimnosperme).
Since the wind does not guarantee that pollen always arrives at its destination, the anemophilous species entrust the success of pollination to a high production of pollen. To meet this need, anemophilous plants develop in many cases an evident sexual dimorphism, with male organs suitable for producing a large quantity of pollen granules and with female ones shaped so as to catch them easily.
The pollen grains of the anemophilous species are light, small in size (and therefore often allergenic) and sometimes equipped with devices that facilitate their suspension in the air (for example, air sacs in the pollen of conifers).
In addition, to improve the efficiency of pollen distribution, anemophilous species have developed several adaptations:
- the flowers lack all those parts that serve as a lure for pollinating animals (colored petals, nectar production, etc.);
- very often the plants bloom before the leaves emerge, in order to eliminate all obstacles to the spread of pollen
Hydrophilic pollination (hydrogama)
It is a method of transporting pollen from the male to the female part of a flower that is not widespread and limited to some aquatic plants: the pollen of the water lilies is released under the surface of the water, that of the elodea floats on the surface, but in both cases they are the currents aquatic animals that carry it until it reaches the stigma of another flower of the same species.
Zoophilic pollination (zoogama)
And pollination by animals such as small birds (hummingbirds) in tropical areas, bats and especially insects belonging to various orders: lepidoptera, beetles, diptera and hymenoptera. The impollizazione by insects is called entomophilous or entomogamous. Flowers with pollination entomophilous need to attract insects and do it with bright colors and intense aroma, are equipped with large corollas and calyxes, they produce a lot of nectar.
They produce little pollen, often sticky, and bloom in seasonal periods or at times of the day favorable to specific pollinators.
Foraging bee (photo Romeo Caruceru)
Bees and pollination
Bees and bumblebees play a fundamental role in promoting pollination and are certainly among the most important pollinating insects. The morphology of the bee is closely linked to pollination: the insect is covered with hair which, during visits to the flowers, is completely filled with pollen grains thus transporting the male gametes to the pistils and ensuring the reproduction of life and biodiversity. Pollination by pollinators is an indispensable factor for many crops. Once the pollen granule has reached the stigma, it is retained on its surface by the papillae covering it and by any exudate present. On the part of the pollen grain in contact with the stigma then a swelling is formed which, developing, gives rise to the pollen tube; this extends along the entire stylus until it reaches the ovum by fertilizing it. Each forager visits thousands of flowers a day by collecting a lot of pollen for the brood. For thousands of years man has learned to exploit the work of bees, starting from prehistoric times to collect honey in naturewww.agraria.org/apicoltura.htm