Pollination is a wonderful mechanism which provides food, shelter etc., for the pollinating animals. Many plants are pollinated by a particular animal species and the flowers are modified accordingly and thus there exists a co-evolution between plants and animals.
Let us imagine if pollination fails. Do you think there will be any seed and fruit formation? If not what happens to pollinating organisms and those that depend on these pollinating organism for the food?
Here lies the significance of the process of pollination.The pollen grains produced in the anther will germinate only when they reach the stigma of the pistil. The reproductive organs, stamens and pistil of the flower are spatially separated, a mechanism which is essential for pollen grains to reach the stigma is needed.
This process of transfer of pollen grains from the anther to a stigma of a flower is called pollination.Pollination is a characteristic feature of spermatophyte (Gymnosperms and Angiosperms).
Pollination in gymnosperms is said to be direct as the pollens are deposited directly on the exposed ovules, whereas in angiosperms it is said to be indirect, as the pollens are deposited on the stigma of the pistil. In majority of angiosperms, the flower opens and exposes its mature anthers and stigma for pollination.
Such flowers are called chasmogamous and the phenomenon is chasmogamy. In other plants, pollination occurs without opening and exposing their sex organs. Such flowers are called cleistogamous and the phenomenon is cleistogamy.Based upon the flower on which the pollen of a flower reaches, the pollination is classified into two kinds, namely, self-pollination (Autogamy) and cross-pollination(Allogamy).
A. Self-pollination or Autogamy
(Greek Auto = self, gamos = marriage): According to a majority of Botanists, the transfer of pollen on the stigma of the same flower is called self-pollination or Autogamy.
Self-pollination is possible only in those plants which bear bisexual flowers. In order to promote self- pollination the flowers of the plants have several adaptations or mechanisms.
1. Cleistogamy: In cleistogamy (Greek Kleisto = closed. Gamos = marriage) flowers never open and expose the reproductive organs and thus the pollination is carried out within the closed flower. Commelina, Viola, Oxalis are some examples for cleistogamous flowers. In Commelina benghalensis, two types of flowers are produced- aerial and underground flowers. The aerial flowers are brightly coloured, chasmogamous and insect pollinated. The underground flowers are borne on the subterranean branches of the rhizome that are dull, cleistogamous and self pollinated and are not dependent on pollinators for pollination.(Figure ).
2. Homogamy: When the stamens and stigma of a flower mature at the same time it is said to be homogamy. It favours self- pollination to occur. Example: Mirabilis jalapa, Catharanthus roseus
3. Incomplete dichogamy: In dichogamous flowers the stamen and stigma of a flower mature at different time. Sometimes , the time of maturation of these essential organs overlap so that it becomes favourable for self-pollination.
B. Cross – pollination
It refers to the transfer of pollens on the stigma of another flower. The cross-pollination is of two types:i.
Geitonogamy: When the pollen deposits on another flower of the same individual plant, it is said to be geitonogamy. It usually occurs in plants which show monoecious condition. It is functionally cross-pollination but is generally similar to autogamy because the pollen comes from same plant.ii.
Xenogamy: When the pollen (genetically different) deposits on another flower of a different plant of the same species , it is called as xenogamy.
Contrivances of cross-pollination
The flowers have several mechanisms that promote cross-pollination which are also called contrivances of cross-pollination or outbreeding devices. It includes the following
1. Dicliny or Unisexuality
When the flowers are unisexual only cross- pollination is possible. There are two types.
Male and female flowers on the same plant. Coconut, Bitter gourd. In plants like castor and maize, autogamy is prevented but geitonogamy takes place.
ii. Dioecious :
Male and female flowers on different plants. Borassus, Carica and phoenix. Here both autogamy and geitonogamy are prevented.
2. Monocliny or Bisexuality
Flowers are bisexual and the special adaptation of the flowers prevents self-pollination.
In bisexual flowers anthers and stigmas mature at different times, thus checking self-pollination. It is of two types.
The stamens mature earlier than the stigmas of the flowers.
Examples: Helianthus, Clerodendrum (Figure ).
The stigmas mature earlier than the stamens of the flower. Examples: Scrophularia nodosa and Aristolochia bracteata
In bisexual flowers the essential organs, the stamens and stigmas, are arranged in such a way that self-pollination becomes impossible. For example in Gloriosa superba, the style is reflexed away from the stamens and in Hibiscus the stigmas project far above the stamens
Some plants produce two or three different forms of flowers that are different in their length of stamens and style. Pollination will take place only between organs of the same length.
The plant produces two forms
of flowers, Pin or long style, long stigmatic papillae, short stamens and small pollen grains; Thrum-eyed or short style, small stigmatic
papillae, long stamens and large pollen grains. Example: Primula
The stigma of the Thrum-eyed flowers and the anther of the
pin lie in same level to bring out pollination. Similarly the anther of Thrum-eyed and stigma of pin ones is found in same height. This helps
in effective pollination
The plant produces three kinds of flowers, with respect to the length of the style and stamens. Here,the pollen from flowers of one type can pollinate only the other two types but not their own type.Example : Lythrum (Figure) .
iv. Self sterility/ Self- incompatibility:
In some plants, when the pollen grain of a flower reaches the stigma of the same, it is unable to germinate or prevented to germinate on its own stigma. Examples: Abutilon, Passiflora. It is a genetic mechanism.