POLLINATION - POLLEN TRANSFER

                                            POLLINATION

Based on the destination of pollen grains, two types of pollination are recognized .they are

1. Self pollination
2. Cross pollination

 Self-pollination or Autogamy

(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.

They are:

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 producedaerial 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.

2. Homogamy: When the stamens and stigma of a flower mature at the same time it is said to be homogamy. It favours selfpollination 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.

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 to 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 crosspollination is possible. There are two types.

i. Monoecious: 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.

i. Dichogamy: In bisexual flowers anthers and stigmas mature at different times, thus checking self-pollination. It is of two types.

a. Protandry: The stamens mature earlier than the stigmas of the flowers. Examples: Helianthus, Clerodendrum

b. Protogyny: The stigmas mature earlier than the stamens of the flower. Examples: Scrophularia nodosa and Aristolochia bracteata

ii. Herkogamy: 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

Heterostyly: 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.

a. Distyly: 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.

b. Tristyly: 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

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.

AGENTS OF POLLINATION :

In angiosperms, pollens are immotile and thus, have to be carried to the stigma by external pollinating agents. Depending upon their nature the agents may be 1. Biotic agents: Insects, Birds, Snails etc. 2. Abiotic agents; wind, water.

BIOTIC AGENTS :

i. Entomophily: It refers to the pollination brought about by insects. Examples: Salvia, Yucca, Ficus.

ii. Ornithophily: It refers to the pollination brought about by birds. Examples: Salmalia, Erythrina, Callistemon.

iii. Cheiropterophily: It refers to the pollination brought about by bats. Examples: Adansonia, Eperua falcata.

iv. Malacophily: It refers to the pollination brought about by snails & slugs. Examples: Alocasia, Lemna, Colocasia. 

v. Myrmecophily: It refers to the pollination brought about by ants. Examples: Medicago sativa, Melitotus officinalis.

vi. zoophily: It refers to the pollination brought about by animals. Examples: Arctium, Galium aparine.

 

ABIOTIC AGENTS:

 i. Anemophily: It refers to the pollination brought about by winds. Examples: Wheat, maize, coconut.

ii. Hydrophily: It refers to the pollination brought about by water. It is of two types; 1. Ephydrophily: Pollination occuring on the water surface. Examples: Elodes, Hydrilla. 2. Hyphydrophily: Pollination occuring beneath the water surface. Examples: Najas, Ceratophyllum.

REFERENCE :

 

THE EMBROLOGY OF ANGISPERMS 6th EDITION Author; SS BHOJWANI, SP BHATNAGAR ,PK DANTU.