Polllen grains of Angiosperms germinate only when they are transferred to the stigma. The process of transfer of pollen grains from anthers to the stigma is called pollination.
Types of Pollination:
(I) Self pollination:
Transfer of pollen grains from an anther to the stigma of the same flower or to a flower on the same plant is known as self-pollination. It can occur in bisexual flowers as well in unisexual flowers which occur on the same plant (monoecious condition).
In this type of self- pollination pollen grains of an anther are transferred to the stigma of the same flower. In other words , autogamy means , pollination of a flower by its own pollens.
When pollens of a flower pollinate any other flower present on the same plant, it is called geitonogamy. Thus this type of self-pollination occurs between two different flowers present on the same plant.
(i) Contrivances or adaptations for self-pollination:
It is the condition in which anthers and stigma of a flower mature at the same time. Thus the stigma is receptive at the time when anthers shed their pollens as in Catharanthus roseus. Mirabilis.
Production of flowers which never open is known as cleistogamy. In such flowers self- pollination is carried out within closed buds. Viola, Oxalis , Juncus , Saxifraga are the common examples of cleistogamous flowers.
(II) Cross-pollination :
Transfer of pollen grains from the anther of the flower on one plant to the stigma of the flower on another plant is called cross-pollination.
(i) Contrivances and adaptations for cross â€“pollination :
Flowers show a number of adaptations which induce cross â€“pollination where self-pollination might otherwise be possible. Some of such adaptations are as follows:
(a) Self- sterility or self incompatibility:
It is a condition where pollens of a flower do not grow or grow very slowly if they fall on the stigma of the same flower, so as to prevent self-fertilization. In Malva the pollen grains do not germinate at all on the stigma of the same flower.
In some plants the stigma may become receptive before the pollens of the same flower are mature or the anthers shed their pollens before the stigma of the same flower becomes receptive. This condition is called dichogamy.
(c ) Hekogamy:
In some bisexual flowers the structure of anthers and styles is such that autogamy is mechanically impossible . This condition is known as herkogamy. In Caryophyllaceae e.g. the style is much longer than the stamens , due to this the pollens of the flower fail to reach the stigma.
Agents and Types of Cross Pollination:
Pollen grains generally do not have any structural speciality to facilitate pollination and as such pollination is dependent upon external agencies. Usually, only pollens are transferred from one flower to the other but in Vallisneria entire male flower moves to the female flower. The various agencies helpful in cross-pollination are calssified into two broad categories- Biotic and abiotic.
(I) Anemophily (pollination by wind) :-
Transfer of pollen grains through wind is known as anemophily The flowers are inconspicuous, usually not bright coloured or scented. The petals are either small and green or absent. The male flowers are usually more numerous than the female flowers. The anthers are versatile so that they swing freely by air currents.
(II) Hydrophily (pollination by water ):
Aquatic plants make excellent use of water for pollination. Generally, those plants whose flowers are permanently submerged in water (e.g. Ceratophyllum , Zostera) are adapted to hydrophily.
Hydrophilous flowers are usually small and inconspicuous like anemophilous flowers. Hydrophily is of the following two types.
(a) Hypo- hydrophily:-
It is the pollination of flower below water level and is found in submerged plants like Najas, Ceratophyllum.
It is the pollination of flower at the surface of water. Vallisneria spiralis, a dioecious plant , is the common example of epi-hydrophily.
(III) Entomophily (pollination by insects):-
Angiosperms as a whole are probably more dependent on insects than any other agency for pollination. Insects can carry a small amount of pollen grains from anthers of one flower and deposit it precisely on the stigma of another flower. Insects visit the flowers to secure food in the form of pollen , sap and nectar , to deposit their eggs , for shelter or for unknown reasons. In order to attract insects , flowers develop large and brightly coloured petals, or if the flowers are small , they are grouped into conspicuous inflorescences. Certain flowers like Orchis , Verbascum , Salix secrete juice or sticky substances which cause pollens to stick to the body of the insect.
(IV) Ornithophily(pollination by birds):-
A large number of tropical plants are pollinated by some small birds known as humming birds and honey thrushes. These flowers are generally tubular, cup- shaped or urn- shaped and mostly of scarlet colour. In order to attract birds they secrete large quantities of nectar.
(V) Cheiropteriphily (pollination by bats ):
Some tropical flowers like Anthocephalus cadamba , Bauhinia megalander are pollinated by bats. These have large flowers / inflorescences with long pedicels. The bat visits flowers to eat floral parts or in search of moths which are attracted towards flowers by their white colour.
In nature desirable characters are not restricted to only one plant but are distributed amongst a wide variety of plants. Plant breeders , with an objective of developing new improved varieties , transfer one or more desired characters into a variety from other varieties. This is achieved by hybridization which consists of mating or crossing of two plants of dissimilar genotypes. The pollen grains from a plant with desired characters are transferred to the stigmas of female flowers of other plant. In hybridization special care is taken to prevent self- pollination and pollination by pollen from other undesired plants.
In dioecious plants cross pollination is prevented by enclosing female flowers in suitable paper or cloth bags of appropriate size. But in bisexual flowers, to prevent self- pollination , stamens or anthers are removed in bud condition, a technique known as emasculation.
Author:- Dr Anupama Goyal
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