What is Cross-pollination?
The transfer of pollen grains form the anther of one flower to the stigma of another flower belonging to the same species or closely allied species is called cross-pollination or Allogamy. When the pollination takes place between two flowers of the same species, it is called xenogamy, while if it occurs between two closely related species, it is called hybridism.
What are the Characteristics of cross-pollination?
Cross-pollination usually occurs in plants having unisexual flower. It may also occur in flowers showing male sterile lines e.g. maize, Solanum. Flowers showing different maturation times for stamens and carpels may also show cross-pollination, e.g. sunflower, Magnolia. It may be in bisexual flowers with differential stamens and carpels, commonly known as heteromorphisme.g. Oxalis. Some flowers, where a barrier is created between stamen nd carpel also shows cross-pollination e.g. Iris. The flowers showing different genetic makeup are showing the cross-pollination.
Adaptations for cross-pollination:
a) Dicliny or Unisexuality: In unisexual flowers, self-pollination is impossible, so cross-pollination is observed e.g. gourd.
b) Self-sterility : In certain flowers, the pollen grains are sterile in nature due to male sterility and hence they cannot fertilize the egg of the same flower, so they depend on the pollen grains of another flower coming via cross-pollination, e.g. Solanum; maize.
c) Dichogamy: The androecium and gynoecium of a bisexual flower do not mature at the sametime, so self-pollination can never take place and they are of two types: Protandry (The anther matures earlier than the stigma, e.g. sunflower) and Protogyny (The stigma matures earlier to anther, e.g. Polyalthia).
d) Herkogamy: In this case, self-pollination is impossible, because some of the floral parts act as a physical barrier between anther and stigma and thus cross-pollination is favoured. In Iris flower, the anthers are extrorse and they are concealed by overlapping or overarching of the petaloid style, which conceals the anther, making self-pollination impossible.
e) Heteromorphism: The flowers of one single species may vary on the basis of the forms of stamens and carpels and accordingly, the flowers may be dimorphic or trimorphic in nature.
f) Homomorphicself incompatibility: It is a rare type of self incompatibility, in which stamens and pistil do not vary in size, but even than, the pollen grains are rejected by the stigma of the same flower. It is found in Oenothera, Aster.
Methods of Cross-Pollination
There are various types of cross-pollination and the agents are discussed below:
When pollination is brought upon by wind, it is called anemophily and the flowers are called anemophilous, e.g. paddy, wheat, maize, grasses.
Adaptations of wind-pollinated flowers : These types of flowers show the following characteristics: -
i) The flowers are small and not easily seen.
ii) The petals are not colored and they are not scented i.e., devoid of osmophores.
iii) They are without nectaries.
iv) The flowers are aggregated on a long peduncle above the vegetative parts, which makes the process of wind-pollination easy.
v) The sepals and petals are small and not easily seen and sometimes, undifferentiated to form perianth.
vi) The accessory whorls do not cover the sexual reproductive organs.
vii) The stamens are provided with long filaments with versatile anthers, which are easily cut off by air current.
viii) The pollen grains are small, granular, light weight, dry and shaped in huge quantity.
ix) The style is also long and that helps in the protrusion of the stigma from the flower.
x) The stigma is large, feathery and branched, which helps in easy trapping of the pollen grains.
When the cross-pollination in a flower takes place with the help of water, it is called hydrophilous and the phenomenon is termed as hydrophily. The water pollinated plants are of two types :
i) Hypohydrogamous: The pollination taking place in completely submerge condition under water e.g. Ceratophyllum.
ii) Epihydrogamous: The pollination taking place along the surface of the wateje.g. Vallisneria, Hydrilla.
Adaptations for water-pollinated flowers
The hydrophilous flowers show the following characteristics :
i) The flowers are small, inconspicuous, light in weight, helping in floatation.
ii) The flowers are not showy, without coloured petals, without any fragrance.
iii) The floral parts are covered with waxy substance or cutin, which prevent them from getting damaged by water.
iv) The accessory whorls, calyx and corolla are small, so the essential floral whorls or androecium and gynoecium are always exposed in water current.
v) The dehiscence of anther is rapid and so the pollen grains are scattered in wider areas in a short time.
vi) The pollen grains are small, light in weight, impervious to water and hence carried by water to long distances.
vii) The female flowers usually have a short coiled stalk, that reach the water surface by uncoiling.
viii) The stigma is provided with bristles, which can easily trap the pollen grains floating in water.
ix) The coiled stalk of female flower may recoil again after pollination.
Method of pollination
In hypohydrogamousflowers like Ceratophyllum, the flowers never comes above the water surface, the male flowers have superior position, which drop the pollen-grains (impervious to water) on to the stigma of the female flower remaining below. The epihydrogamousflowers are always at the water surface, so pollination is always brought about on the water surface, where pollen grains are carried by the water-current from the male flower, up to the stigma. Some flowers, like that of Vallisneria, the flowers usually remain submerged. The small .male flowers on maturity get detached from the spadix and float on the water surface on a boat-like spathy bract. The coiled stalk of the female flower uncoils and the female flowers come to the water surface, their stigmas come in contact with the anthers of the male flower and pollination takes place. After pollination, the stalk of the female flowers recoil again and the flowers again go down underwater.
When the pollination is brought about by animals, the pollination is called zoophily and the flowers are called zoophilous e.g. Calotropis. The zoophilous flowers in general shows the following characteristics :
i) The flowers are brightly coloured, showy, sweet scented, i.e., with osmophores.
ii) They may contain the nectaries.
iii) The pollen grain are edible and palatable.
iv) The exines of the pollen grains are rough and sticky, which easily get adhered to the body surface of animals.
v) The stigma surface is usually sticky to trap the pollen grains after coming into its contact.
The cross-pollination brought about by insects is called entomophily and the flowers are called entomophilous e.g. Salvia, sunflower, Calotropis. The pollination affected by birds is called ornithophily and the flowers are called ornithophilous. e.g. BignoniaJSterilitzia. When the pollination is brought about by snails and slugs, it is called malacophily and the flowers are called malacophilous e.g. members of Aroideae. When the process of cross-pollination brought about by bats is called chiropteriphily and the flowers are called chiropteriphilous. e.g. silk cotton, Bauhinia.