In this article we will discuss about:- 1. Meaning of Cross-Pollination 2. Advantages and Disadvantages of Cross-Pollination 3. Adaptations.
Meaning of Cross-Pollination:
If pollen grains are carried to the stigma of a flower of the same species growing on a different plant, it is cross-pollination.
All unisexual flowers are cross-pollinated and even the bisexual flowers which, in fact, form the vast majority, have special contrivances for favouring cross-pollination. Cross-pollination is secured through many agencies like insects, wind, water current and animals.
The insect-pollinated flowers are called entomophilous, (entomon = insect), wind-pollinated ones are anemophilous, (anemos = wind), water-pollinated flowers are hydrophilous, bird-pollinated flowers are ornithophilous (ornitho = bird), and animal-pollinated ones are known as zoophilous flowers. The majority of the plants growing around us bear insect-pollinated or wind-pollinated flowers.
The entomophilous flowers have many insect visitors like bees, butterflies, moths, etc. These flowers are usually very showy, having bright colours. Many of them, particularly the nocturnal flowers like Nyctanthes (B. Shieuli), jasmine lack bright colour but have very fragrant odour. Flowers of Typhonium (B. Ghet Kachu) have very disagreeable odour.
Small flowers are aggregated together into inflorescence only to make themselves attractive to their visitors. Less attractive flowers often have corona for enhancing their beauty. Nectaries and honey glands are quite common.
Insects, during their visits to the flowers in search of honey, carry the pollen grains from flowers to flowers. The pollen grains are usually sticky and provided with spiny and warty projections, so that they can adhere conveniently to the insect-bodies.
The stigmas are also sticky due to secretion of stigmatic fluid which facilitates the reception of the pollen grains. Examples—China-rose, lotus, roses, jasmine, tube-rose, Oestrum (B. Hasnahana).
The anemophilous flowers are small insignificant ones, neither brightly coloured nor sweetly scented. The stamens have usually versatile anthers which swing freely in the air and liberate pollen grains to be carried away easily even by the gentle breeze.
Pollen grains are small, light and dusty. They are abundantly produced, as there is always the risk of a huge wastage. The stigmas are generally branched or feathery which expose maximum surface for receiving the light smooth pollen grains from air.
Hydrophilous flowers are present only in the aquatic plants like Hydrilla, Vallisneria (B. Pata shaola), Ottelia, etc. Mode of pollination in Vallisneria is very interesting. It bears unisexual flowers. The pistillate flowers have long stalks to keep them at the surface of water, whereas the staminate ones have very short stalks and so remain under water.
Before pollination the male flowers get detached from the mother plant and float on the water. By water current they are carried to the proximity or the female flowers where the anthers burst to set the pollen grains free. Thus pollination takes place, after which the stalks of female flowers coil to bring the ovary under water.
Bird-pollinated or ornithophilous flowers are large, brightly coloured and showy. A large number of there usually remain aggregated together and they possess profuse quantities of honey and pollen grains. This type of flower is common in forests where birds of different kinds are present.
Silk cotton is a good example. Zoophilcus flowers are those where animals like snails, birds, etc., serve as agents for pollination. Arum, Anthocephalus (B. Kadamba), silk cotton, Butea (B. Palas) are the familial examples.
Advantages and Disadvantages of Cross-Pollination:
Cross-pollination is more advantageous than self-pollination, though the latter is a surer process. The progeny of cross-pollination is healthier and better suited for the struggle for existence, as they inherit paternal and maternal characters from two different plants.
They give better yield and many new varieties are produced. The only disadvantage of cross-pollination is that the process is dependent on so many external agencies.
Adaptations for Cross-Pollination:
Nature has some contrivances for preventing self-pollination and favouring cross-pollination:
1. Uni-Sexuality of Flowers:
In plants bearing unisexual flowers self-pollination is practically out of question and cross- pollination is inevitable.
2. Self-Sterility:
In some plants pollen grains have no effect and arc. thus, sterile on the stigma of the same flower, e.g. Orchids.
3. Dichogamy:
In some bisexual flowers stamens and carpels do not mature at the same time.
Two conditions are thus possible, viz.:
(i) Protandry, when stamens mature much earlier than the carpels, as in coriander, Carum (B. Joan) and
(ii) Protogyny, when carpels ripen before the anthers mature, as in Michelia (B. Champa). In any case self-pollination is avoided.
4. Heterostyly:
Self-pollination is rendered impossible in some flowers with peculiar forms. They bear long stamens with short styles, or short stamens with long styles. Examples—Oxalis (B. Amrul), Linum (B. Linseed).
5. Heikogamy:
Bisexual flowers with stamens and, carpels maturing at the same time often prevent self-pollination by placing barriers between the two, e.g. Aristolochia (B. Ishermul), Calotropis (B. Akanda). The only adaptation for favouring self-pollination is noticed in cleistogamous flowers which remain under the soil and thus ever-closed, e.g. Commelina (B. Kanshira).