Here is your short essay on Pollination

Pollination is defined as the process of transfer of pollen grains, “from an anther to the stigma of the same flower or of different flower.” If the pollen grains are transferred from an anther to the stigma of the same flower, the process is called self pollination or autogamy. If these are transferred to the stigma of different flower on the same plant or different flower of different plant or the same species or different flower of a different plant of different species, it is called cross pollination or allogamy.

(i) Self Pollination:

Contrivances for self pollination for self pollination, following adaptations are found in plants:

(1) Bisexuality (Hermaphrodites):

It is only in bisexual flowers that self pollination can take place.

(2) Homogamy:

Both sex organs of a flower should mature at the same time.

(3) Cleistogamy:

In some plants e.g. Commelina, Viola, Impatiens, Archis, Oxalis, the anthers mature and burst liberating their pollen grains in a closed flower. Such plants usually have two types of flowers (a) underground closed flowers and (b) aerial flowers which open like normal flowers. Cleistogamous (closed) flowers are very small, colourless, odourless, and without nectar. Aerial flowers are larger, brightly coloured and scented and in these flowers cross pollination takes place.

In some plants, e.g. Helianthus, if cross pollination fails, selfing is sure. The flower is protandrous i.e., anther mature first. Stigma is bilobed which is hidden in tube formed by anthers.

At maturity, pollen grains are discharged in the anther tube but self pollination does not occur as the stigma is not mature mid receptive. At maturity, the stigma grows out of the anther tube, both stimatic lobes open, thus facilitating cross pollination. If, however, cross pollination fails, to occur, the two stigmatic lobes, curve downwards and they come in contact with pollen grains of the same flowers.

(ii) Cross Pollination:

Contrivances for cross pollination are:

(1) Dicliny or unisexuality:

In unisexual flowers, stamens and carpels are found in different flowers. Unisexuality can be of two types:

(A) Monoecious plants:

When male and female flowers are borne on the same plant, e.g. Maize, cucurbits, cast on

(B) Dioecious plants:

When male and female flowers are borne on different plants, e.g. Casica papaya, Cannalis.

(2) Self sterility or incompatibility:

In this condition, the pollen grains of the same flower cannot fertilize their own ovules as they do not germinate on stigma of same flowers, e.g. Passiflora, Malva, Potato.

(3) Dichogamy:

In many hermaphrodite flowers, stamens and carpels of a flower donot mature at the same time. Dichogamy is of two types.

(A) Protandry:

The anthers of a flower mature earlier than carpels, e.g. Jasminum, Foeniculum, Tagetus, and Sunflower.

(B) Protogyny:

The carpels of a flower mature earlier than stamens e.g. Ficus benghalensis, Polyat.iea, Anona, Michelia, Rose.

(4) Heterostyly:

The flowers may be of two or more types with regard to the length of the style and lengths of stamens and pollens from a flower cannot bring about effective pollination in flowers of its own type, e.g. Oxalis, Lythrum, Primula.

(5) Herkogamy:

In some homogamous flowers, there are some adaptations due to which self pollination cannot occur. These adoptations are as follows: In caryophyllaceous flower, the stigma projects beyond the stamens so that pollen donot fall on it. Conversely in Gloriosa they dehisce at a distance so that stigma is out of reach to its own pollen.

Modes of Cross Pollination:

The agencies which transfer pollen grains from anthers of one flower to the stigma of a different flowers are as follows: WIND (Anemophily), WATER (Hydrophily), INSECTS (Entomophily), BIRDS (Ornithophily)’ and BATS (Cheiropterophily).

(1) Anemophily:

Anemophilous plants produce enormous amount of.pollen grains: A single plant of Mercurialis annually has been estimated to produce 1,352,000,000 pollen grains. Anemophilous plants bear small and inconspicuous flower. The pollen grains are small, light, smooth and dry. Pollen of some plants are said to be blown to 1,300 km. In some plants as Pinus, pollen grains are winged.

The flowers are usually unisexual in some plants e.g. Mulberry is borne in independent catkins which can sway freely and shake off their pollen in air. The flowers may be borne on long axis (as in grasses) much above the leaves.

The anther is versatile so as to oscillate in all directions at the tip of filament. In Urticaceae filaments are very long. Anempohilous flowers have adequate devices to catch the air-borne-pollen grains with utmost efficiency. For this the stigma is usually large and feathery (as in grasses) and brush like as in Typha.

(2) Hydrophily:

It is of two types:

(a) Hypohydrogamy:

Includes plants which are pollinated inside the water, e.g. Ceratophyllum, Najas.

(b) Epihydrogamy:

Vallisneria spiralis (ribbon weed) is a submerged dioecious plant. The flowers are borne under water. When mature, the male flower get detached from the parent plant and float on the surface of water. The pistillate flowers also develop under water, at the time of pollination, they are brought to the surface by their long and slender stalks. As it arrives on the surface it forms a cuplike depression. If male flowers floating on water get lodged into the depression, the pollination takes place. After pollination, the stalk of the pistillate flower undergoes spiral torsion bringing the pollinated flower under water once more.

(3) Entomophily:

Some of the insects which help in pollination are bees, flies, wasps, moths and beetles. Bees, flies and beetles visit flowers which open after sunset. Bees probably carry out 80% of all pollination done by insects. Bee pollinated flowers are coloured, possess special smell and/or produce nectar. Pollen grains are sticky or with spinous exine. Also the stigma is sticky and bees are colour blind for red.

(4) Ornithophily:

Tiny birds like humming birds and honey thrushes (hardly 1 inch long) feeds on the nectar of flower like Bignonia, Erythrina is visited by crows.

(5) Chiropteriphily:

Bauhinia megalandra of Java and Anthocephalus are pollinated by bats.

(6) Malcophily:

Many aroids which are usually pollinated by Diptera are also pollinated by snails.

Prints 3, 4, 5 & 6 given above are included in Zoophily.