Pollination and its types

 

        Pollination:-

       Transfer of pollen grains from anther to the stigma of a pistil, either of same or another flower is called as pollination.

       Plants are said to be self-pollinated when the pollen is transferred from an anther to a stigma of the same flower or to the stigma of another flower on the same plant.

       The transfer of pollen from the anther of a flower to the stigma of a flower of another plant is called cross-pollination.

      

       Significance of pollination:

       Male and female gametes in angiosperms are produced in the pollen grain and embryo sac respectively. As both male and female gametes are non-metals, they have to be brought together for fertilization to take place. This aim is achieved by process of pollination.

Cross-pollination or xenogamy or allogamy:

        Cross pollination can also be defined as the migration of pollen grains from one flower to stigma of genetically different flower. Cross-pollination is also called xenogamy (allogamy).

       This is performed with the help of external agencies like wind (anemophily), water (hydrophily) and animals (zoophily) e.g. by birds (ornithophily), by insects (entomophily) and by bats (chiropterophily).

      

       Details of different agencies helping in cross pollination are as under:

       a) Anemophily:

       Anemophilous (wind pollinated plants) e.g. coconut palm, date palm, Cannabis (Bhang), Zea mays (Maize), grasses etc. exhibit the following characters which favour with pollination.

       Wind pollinated flowers are inconspicuous and not showy. They are devoid of scent, nectar, etc. They produce a very large quantity of dusty pollens. The pollen grains are dry and unwettable. The anemophilous plants grow in large groups to ensure pollination. To catch the pollen grains, the stigmas become sticky, hairy, feathery or branched. Wind pollenated flowers bear well exposed stamens (so they can be easily dispersed by wind currents). Anemophilous flowers usually bear single ovule in each ovary. Numerous flowers are packed to form inflorescence e.g. in corn-cob–the tassels which we see are stigma and style which wave in wind to catch the pollen grains.

b) Hydrophily:

       Pollination brought about through the agency of water in plants especially submerged plants is termed Hydrophily. It has been reported only in about 30 genera mostly monocotyledons. Hydrophily is further of two types:

       In hypohydrophily, pollination takes place below the surface of water e.g. Zostera and Ceratophyllum.

       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 same specific gravity as that of water. The pollen grains, therefore, can float below the surface of water.

          Ceratophyllum: - A submerged fresh water hydrophyte bears both male and female flowers on the same plant. 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 germinate and as they sink in water affect pollination of female flowers. Pollination is further helped by swaying movements of plants in water, which help the stigma in catching the pollen grains in water.

     In epihydrogamous types, pollination occurs over the surface of water e.g. Vallisneria (dioecious plant).

       The stalk bearing the female flower straightens up and takes the female flower to the surface. Male flowers open on the surface with three perianth leaves widely exposing the two stamens vertically. Two of the perianth leaves are in contact with the water surface and the third smaller tepal remains upwardly curved to act as a sail. Some of the male flowers are drawn into the depression of water surface surrounding the female flowers. The anthers burst, sticky pollens get attached with the stigma. After pollination, long stalk of female flower coils and brings it back to lower level of water where the fruit is formed.

       Few interesting characters in hydrophilous plants are:

       The pollen grains are light but covered with wax. Stigma is sticky but unwettable. Scent, colour and nectar absent. Flowers are small and inconspicuous.Sepals, petals or perianth if present are unwettable, because they have a waxy coating.

c) Zoophily:-

       Many animals act as pollination agents. Bees, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) and bats are some of the examples. Zoophily is of following types:

       i) Entomophily:

          Insect pollinated flowers (entomorphilous) possess the following characteristic features:

       The insect pollinated flowers are fragrant and omit scent and odour e.g. Cestrum, Jasmine, etc. Rafflesia gives out obnoxious and repelling odours. However some flies are attracted by such odours. Most of the night blooming flowers are fragrant. At night, when the colour fails, scent is particularly useful in directing the insects to the flowers.

       The insect pollinated flowers are beautifully coloured. In flowers, mostly petals are brightly coloured. Sometimes, when flowers themselves are not conspicuous, other parts may become coloured and showy to attract the insects e.g. bracts in Bougainvillea, leaves in Euphorbia pulcherrima, one sepal in Mussaenda etc. The nectariferous glands produce nectar for feeding the visiting insects. In entomophilous plants, the pollen grains become sticky or spiny so that they may easily get attached with the body of visiting insects. To catch the pollen grains, the stigma become sticky. Flowers become more conspicuous by grouping.   In plants like Papaver, Rosa, Clematis, etc. edible pollen grains are produced. Some of the pollen grains stick to the body of insects while feeding on the edible pollen grains.

       Salvia bears bilipped brightly coloured flowers. It shows a special type of turn-pipe floral mechanism. Insects attracted by the inflorescence, colour and nectary glands on the lower lip. There are two stamens in the flowers each with a lever mechanism. Each stamen has a short epipetalous filament. One of the anther’s lobe is sterile. Two anther lobes are separated by a long connective. The flowers are protandrous. An insect moves towards nectary, its head strikes with sterile anther lobe, thereby back of insect gets the blow of fertile anther lobe. Pollen covered insects, when the flowers with older anthers withered (being protandrous), pollinate the flowers from elongated style and get the nectar from nectary lying below the ovary.

 

ii) Ornithophily:

       Ornithophilous (bird pollinated) flowers differ in number of respects from entomophilous (insect pollinated) flowers. The flowers are generally scentless. The flowers are usually large in size. They are also beautifully coloured with colour contrasts and are common colours liked by birds. Another interesting character is the provision of a copious and mucilaginous nectar. This nectar is more important to bird as a drink than food. The pollens are sticky and adhere to the body of the bird. Some examples of bird pollinated plants are coral tree, bottle brush, Butea monsperma and silk cotton tree.

        iii) Chiropterophily:

       The flowers of Kigellia pinnata, Duria, Anthocephalus are pollinated by bats. 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 copious nectar and pollen which is produced in plenty as compared to bird pollinated flowers.

Adaptation/ Devices / Contrivances promoting self and cross pollination:

      

       Devices favoring self-pollination:-

       Homogamy: -

       It is the condition in which anthers and stigmas in bisexual flower attain maturity at the same time.

       Cleistogamy:-

        In cleistogamy, flowers never open to expose their sex organs and the pollens fall on the stigma of same flower.

       The cleistogamous flowers are invariably autogamous, as there is no chance of cross pollination. Pollination are not required in such flowers.

 

        Chasmogamous flowers are with exposed anthers and stigma. In a normal flower which opens and exposes the anthers and stigma, complete autogamy is rather rare. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity. For autogamy in such flowers, anthers and stigma should lie close to each other so that self-pollination can occur.

Devices favoring cross-pollination:-

       i) Dicliny /Unisexuality:

       The flowers are unisexual having one sex only. These unisexual flowers are present on different plants thus promoting the cross-pollination.

 

ii) Dichogamy:

       In bisexual flowers, anthers and stigma mature at different times. Dichogamy is of two types: Protandry and Protogyny. In protandry the anthers mature but the stigma of the same flower is not receptive at that time (Sunflower, Cotton and Salvia). In protogyny the stigma of the carpel matures earlier than the anthers of the same flower e.g. Ranunculus and Mirabilis Jalapa (Four o’clock plant).

iii) Suppression of one sex:-

        In certain bisexual flowers one sex organ, either stamen or carpel is completely suppressed and becomes sterile. Anther does not form pollen grains and carpel does not produce ovule. Thus facilitating cross-pollination.

      

       iv) Prepotency:

       In many plants the pollen grains form a flower when present on the stigma of another genetically different flower germinate more quickly as compared to the pollen grains of the same flower. Thus promoting cross pollination e.g. Grape, Pear and Apple.

 

       v) Self-sterility or self-incompatibility:

       Pollen grains of a flower are incapable of effecting fertilization even if they are placed on the stigma of the same flower due to mutual inhibition e.g. Potato, Tobacco and Petunia axillaris.

 

       vi) Heterostyly:

       The occurrence of two or more types of flowers having different length of styles and stamens. In Primrose, there are two types of flowers while in Oxalis three types, based on the length of styles and stamens. An insect visiting a flower will get dusted with pollen grains on a particular area of its body depending upon the height of anther. Now this insect will pollinate only that flower in which stigma lies at the level of anther of the first flower. This process involves pollens and stigma from two or more flowers having the similar height.

vii) Herkogamy:

       In some flowers there may be some physical barriers between the anther and the stigma so that pollination between them becomes difficult or even impossible.

Pollen pistil interaction:-