Cross Pollination involves the transfer of pollen grains from the flower of one plant to the stigama of the flower of another plant. It is also called Xenogamy.
(Gk. Xenos = Strange, gamos = marriage) or allogamy (gk, allos = other, gamos = marriage. The main floral characteristics which facilitate cross pollination are
Flowers are bisexual but the essential organs, the stamens and stigmas, are arranged in the flower, in such a way that self pollination becomes mechanically impossible. eg., Hibiscus sps; Gloriosa superba, etc.
Hibiscus : The stigmas project far above the stamens.
Gloriosa superba : The style is reflexed away from the stamens.
Pollen and stigma of the flower mature at different times to avoid self-pollination. It is of two types.
Gynoecium matures earlier than androecium eg. Bajra, Aristolochia etc.
Androecium matures and shed pollen gynoecium eg. maize.
In some plants, when the mature pollen fall on the receptive stigma of the same flower, it fails to bring about self pollination. It is called self incompatibility. Under such conditions, the cross pollination is the only option.
The pollen grains of some plants are not functional. Such plants set seeds only after cross pollination.
Cross pollination always occurs when the plants are unisexual and dioecious. i.e. male and female flowers occur on separate plants. eg., Papaya, some cucurbits etc.
The flowers of some plants have different lengths of stamens and styles so that self pollination is not possible eg. Primula, Linum, etc.
Pollination is effected by many agents like wind, water, insects etc. On the basis of the agents that bring pollination, the mode of pollination is as follows :
1. Anemophily (Wind)
2. Hydrophily (Water)
3. Entomophily (Insects)
4. Ornithophily (Birds)
5. Chiropterophily (Bats)
6. Myrmecophily (Ants)
It is a mode of pollination or transfer of pollengrains from anther to stigma through the agency of wind. The flowers which are wind pollinated are called anemophilous. The anemophilous flowers are characterized by the following adaptations :
(i) Flowers are small colourless, inconspicuous, odourless and nectarless.
(ii) Calyx and corolla are either reduced or absent. Anthers are usually versatile.
(iii) When flowers are unisexual, male flowers are more abundant than female flowers. In bisexual flowers, the stamens are generally numerous.
(iv) Pollen grains are small, light, dry, dusty and sometimes winged (or saccate) so that they are easily blown away to long distances (upto 1300 km).
(v) Pollen grains of anemophilous flowers are produced in huge quantities. For example, a single flower of Cannabis produces, 5,00,000 pollengrains.
(vi) The flowers are well exposed in the air. In certain plants, they are produced above the foliage before the appearance of new leaves.
(vii) The stigmas are large, well-exposed, hairy, feathery or branched to catch the air-borne pollen grains.
(viii) In some plants, (eg., Urtica), the anthers burst suddenly to throw the pollen grains into the air (gun-powder mechanism)
The common examples of wind pollinated flowers are - grasses, sugarcance, bamboo, coconut, palm, date palm, cannabis (bhang), maize etc. fig.
Hydrophily occurs only in a few aquatic plants. Hydrophily is of two types, hypohydrophily and epihydrophily.
Hypohydrophily is true hydrophily which occurs below the surface of water. It occurs in totally submerged plants and their pollen grains are water borne. eg., Zostera marina, Ceratophyllum, etc.
In Ceratophyllum desnersum (a submerged fresh water plant), the male flower bears 30-45 stamens. The mature anthers break at the base, rise to the surface of water and dehisce there. The liberated pollen germinate and sink in water. While sinking, they come in contact with stigma of female flowers to effect pollination.
b. Zostera marina
In Zostera marina, the pollen grains are elongated (upto 2,500 mm), needle like and without exine. They have the same specific gravity as that of water, therefore float below the surface of water. When they reach the stigma, they oil around it and germinate.
The important pollinating insects are bees, butterflies, moths, wasps, bettles etc. The insects visit the flowers for nectar, edible pollen grains or shelter. Bees are the chief visitors of flowers and they obtain both nectar and pollen from the flowers. They havepollen sacs or pollen baskets for collecting pollen. The important characteristics of insect pollinated flowers are as under.
1. The flowers are large or if small they are grouped into inflorescence
2. The flowers are usually brightly coloured and have specific odour.
3. The flowers usually possess nectar or edible pollen.
4. They produce fewer pollen grains.
5. Anthers and stigmas are commonly inserted.
6. Stigmas are usually unbranched and may be flat or lobed.
A detailed study of the inter-relationship between the structure of flower and insect pollinators clearly indicates that some angiospermous plants are dependent upon a particular type of insect for pollination. Some classic examples are as follows :
a. Pollination in Salvia
The genus Salvia belongs to family Labiatae (Mint family) in which the gamopetalous corolla is two-lipped (bilabiate). The lower lip provides platform for the visiting insect and the upper lip is just like a hood which protects the floral organs. The flowers are protandrous. Each flower has two epipetalous stamens located in anterio-lateral position. Each stamen has a short filament and an elongated curved connective. The anther has two parts - one half is sterile and another half is fertile. Both the parts of anther are separated apart due to elongation of connective. The elongated connective has two unequal arms. The upper arm is long and curved. It bears the fertile lobe of anther. The lower arm of connective is short and bears the sterile lobe of anther. The two sterile lobes jointly form a sterile plate of tissue which is placed at the mouth of corolla tube and partly blocks the path of the visiting insect. The upper fertile lobes are sheltered in the upper lip of corolla. As a bee visits the young flower and moves inward in search of nectar, its head pushes the sterile plate which brings down the fertile anther lobes to strike against its back. The pollengrains are deposited upon the back of the bee. When the pollen-dusted bee visits older flower (with protruded bilipped stigma), its back rubs against the mature stigma bringing about the pollination.
Since the bisexual flowers of Salvia are protandrous, (anthers mature earlier than the gynoecium), cross pollination occurs only when pollen-dusted bee visits older flowers with mature gynoecium.
Ornithophily (Gk. Ornis = bird, philein = to love) is a mode of pollination performed by birds. The act of pollination is not performed by all the birds except a few. The most common bird pollinators are Sun bird, Humming bird, Crow, Bulbul, Parrot, Mynah, etc. The birds visit a large variety of flowers such as Bombax (Red Silk Cotton), Erythrina (Coral Tree), Callistemon (Bottle Brush), Bignonia, Agave, ete. Over about 100 species of Australian plants are pollinated by birds. Humming bird pollinates while hovering over the flowers and sucking nectar. The bird can derive about half of its body weight of nectar in a single day. The nectar is chiefly made of sugars and provides a sweetdrink to the bird.
The ornithophilous flowers are characterized by the following adaptations:
i) The flowers are usually large in size. They have tubular or funnel-shaped corollas.
ii) The flowers are brightly coloured (such as red, yellow, orange, blue, etc.) which attract the birds from long distances.
iii) The flowers produce abundant watery nectar.
It is a mode of pollination performed by bats. The bats are nocturnal flying mammals which move swiftly and transport pollen grains to long distances, sometimes over 30 kms. The flowers they visit are large, dull-coloured and have a strong scent. Chiropterophilous flowers produce abundant pollen grains and secrete more nectar than the ornithophilous flowers. Some of the common chiropterophilous plants are - Kigelia pinnata (Sausage Tree), Adansonia (Baobab Tree), Bauhinia megalandra, Anthocephalus (Kadam Tree), etc. Each flower of Adansonia (Baobab Tree) has 1500-2000 stamens.
Sometimes ants take their food or shelter on some trees such as Mango, Litchi, South American Acacia and so on. These ants act as body guards of the plants against any disturbing agent and also helps in pollination.
The pollination performed by any type of animal is called Zoophily.
i) Cross pollination brings about genetic recombination and production of new varieties (variations).
ii) Cross pollination results in healthy and stronger offsprings due to phenomenon of hybrid vigour.
iii) Several crop plants (such as Mustard, Safflower, Sunflower, Clover, Cucurbits), give significantly higher yields if bees are available and cross pollination is allowed to occur.
iv) Variations caused due to cross pollination may result in production of disease resistant plants.