Cross pollination is the transfer of pollen grain, from an anther of a flower to the stigma of another flower on the different plant. The agencies which transfer pollen grains from anther of one flower to the stigma of different flowers are:-
- Wind ( Anemophily )
- Water ( Hydrophily )
- Insects ( Entomophily )
- Birds ( Ornithophily )
- Bats ( Chiropterophily or Cheiropterophily)
- Snails ( Malacophily )
The flower can be of 3 type, on the basis of kind of pollinator visit to the flower: –
- Polyphilic Flowers – When many types of pollinators visit the flower.
- Oligophilic Flowers – when a few related species visit the flower.
- Euphilic Flower – When the flower is visited by one closely related species of pollinator.
Anemophily ( Wind Pollination )
In anemophily, the pollen grains are light and non-sticky so that they can easily transport in wind currents. Plants involved in wind pollination have versatile anthers that oscillate in all direction at the tip of filament so that wind currents easily disperse pollens.
Characters which favor wind pollination:-
- Wind-pollinated flowers are without scent and nectar, etc.
- They produce a huge quantity of dusty pollens. For example – A single flower of Cannabis produces over 500000 pollen grains.
- The movement of pollen is non-directional and may be taken to long distances. Winged pollen grains of Pinus have been found at hundreds of kilometers away from nearest pine forests and are described as sulfur shower.
- The pollen grains are dry and unwettable.
- The anemophilous plants grow in large groups to ensure pollination.
- In some deciduous anemophilous trees like Bombax, the flowers appear before leaves.
- In plants like grasses and wheat anthers are versatile, which swing freely with a slight breeze.
- Some wind pollinated flowers like Urtica dioica show an explosive mechanism by which the anthers burst and surcharge the air with pollen
Example – Coconut Palm, grass, Maize, Chenopodium, Amaranthus, Cannabis.
Hydrophily ( Water Pollination )
Hydrophily is pollination brought about through the agency of water. It is of 2 types:-
- Hypohydrophily – It includes plants which are pollinated inside the water. Example – Ceratophyllum Najas, Zostera.
- Epihydrophily – It includes plants which are pollinated on the surface of the water. Example – Vallisneria spiralis ( Ribbon weed ), Lemma.
Zostera, a submerged marine plant bears elongated, needle-like pollen grains without an exine. When they reach the stigma, they coil around it and germinate. Pollen grains have the sam specific gravity as that of water. The pollen grains, therefore, can float below the surface of water.
Ceratophyllum demersum, a submerged fresh water hydrophyte bears both male and female flowers on the same plant. The male flowers bear 30-45 stamens. The anthers abscise at the base, float to the surface of water and dehisce there. Pollen grains are produced in large numbers. Pollen grains are round and without outer covering or exine. The stigmas are long and sticky. The liberated pollen germinates and as they sink in water affect pollination of female flowers. Pollination is further helped by swaying movements of the plant in water, which help the stigma in catching the pollen grains in water.
Some interesting characters in hydrophilous plants:-
- The pollen grains are light in weight but covered with wax.
- Stigma is sticky but unwettable.
- The scent, color, and nectar absent.
- Flowers are small and inconspicuous.
- Sepals, petals or perianth if present are unwettable because they have a waxy coating.
Entomophily (Pollination by Insect)
Entomophily is the most common and specialized type of pollination brought about through the agency of insects.
The insects which help in pollination are – Bees ( carry out about 80% of pollination done by insects ), flies, wasps, moths, and beetles. Bee-pollinated flower are colored, possess special smell and produce nectar. The flowers are strong enough to bear the weight the weight of visiting insects and the visiting insects are fed by either nectar or edible pollen.Pollen grains have a sticky surface due to pollen kitt and stigma are also sticky.
Examples – Pollination in Salvia, Rafflesia, Stapelia, Arum and Amorphophallus.
Ornithophily (Pollination by Birds)
Ornithophilous (bird pollinated) flowrs differ in number of respects from entomophilous (insect pollinated) flowers. The flowers are usually large in size. They are also common colors liked by birds. Insects can not easily distinguish the colors excepts honey bees. Another interesting character is the provision of a copious and muclilaginous nectar. This nectar is more important to bird as a drink tha food. The pollens are sticky and adhere to the body of the bird. The nectar is chiefly composed of sugar.Humming bied can consumes half its body of nectarin a single day. Vivid colors attracts the birds from long distances. It has been reported that more than 100 species of Austrialian plants are bird pollinated.
Birds like hummingbirds and honey thrushes feed on the nectar of flower like Bigmonia, Erythrina sterlitzia (Bird of Paradise).
Ornithophilous flowers are large and strong with abundant nectar or edible part. Example – Bombax, Agave, Grevillea, Callistemon
Chiropterophily ( Pollination by Bats )
It is allogamous pollination performed by bats.
Flowers in chiropterophily are larger sized, scented, open at night and produce a significant amount of nectar. The bats hold on to the freely exposed, large and relatively tough flower, which open in the evening or night. Bats are nocturnal animals and transport pollen over long distances (upto 30 km). They usually give off a strong scent like that of rotting fruits. Bats also feed on the copious nectar and pollen which is produced in plenty as compared to bird pollinated flowers. In the flowers of Adansonia (Baobab tree) number of atamens ranges from 1500-2000 to produce polen in large number.
Example – Kigelia pinnatea ( Sausage tree ), Bauhinia megalandra of Java and Anthocephalus ( Kadamba ).
Malacophily ( Pollination by Snails )
Malacophily is cross-pollination brought about by the agency of snails.
Example – Arisaema.