Agents of Pollination-digieduco

The agents of pollination are : (a) wind, (b) water current and (c) animals which include various types of insects, snails, birds and bats. The structure of flowers and pollens are not the same in all types of flowers pollinated by different agents. With regard to the pollinating agents there are different types of pollination, viz :

(1) ANEMOPHILY-When pollination is affected with the help of wind, it is called anemophily. In wind pollinated or anemaphilous flowers the adaptation by means of wind is not less important. Wind pollinated flowers are found among the members of the families Gramineae, Cyperaceae, Salicaceae, Betulaceae etc. Anemophilous plants bear small and inconspicuous flowers. The perianth lobes of flowers are quite reduced or absent. The pollen grains are smooth-walled, small, light, dry and produced in profuse quantities. The anther is versatile that can freely oscillate in all direction at the tip of filament. The stigma is large feathery (in grasses) or brush-like (seen in Typha sp.) and projects beyond the floral envelopes, so that pollen grains floating on air are easily caught by such stigmas. In anemophilous flowers pollen grains are produced in profuse quantities for rendering pollen freely accessible to wind.

(2) HYDROPHILY-When pollination is affected with the help of water, it is called hydrophily. The flowers pollinated by water current are known as hydrophilous. Typical hydrophilous flowers are found in many submerged aquatic monocotyledonous plants like species of Vallisneria, Hydrilla, Najas, Zostera, etc. Flowers are small and inconspicuous. Floral envelopes are highly reduced or absent. Hydrophily are of two types as follows :

(a) Hyphydrophily-It includes plants which are pollinated completely under Water e.g. Najas, Ceratophyllum, Zostera, etc. The pollen is very long (upto 250 m), niddle-like and without exine. Specific gravity of these pollens is almost the same as that of water. a result, they can float freely in water at any depth. The stigma is very long in such plants. When the pollen comes in contact with the stigma, it coils round the latter. Hyphydrophily is known as hypohydrogamous.

(b) Ephydrophily or Epihydrogamous-Here the pollination takes place on the surface of water e.g. species of Vallisneria, Hydrilla (Hydrocharitaceae) etc. Vallisneria spiralis is typical example of this type ; it is a submerged dioecious plant. The staminate flowers on maturity get detached from male plant and float on the surface of water ; they are carried to the pistillate flower (which by elongation of its spirally coiled stalk rises above the surface of water), the stigma comes in contact with the anthers of the floating staminate flowers, after pollination the female flower is dragged below by recoiling of the stalk.

(3) ZOOPHILY-when pollination is affected by animals, it is called zoophily. Animal. pollinated flowers are called zoophilous flowers. Zoophilous flowers are sub-divided into (a) entomophilous (insect-pollinated), (b) ornithophilous (bird-pollinated), (c) malacophilous (snail and slug-pollinated), (d) chiropteriphilous (bat-pollinated) etc.

(a) Entomophily-When pollination is affected by insects, it is called entomophily, The entomophilous flowers are showy, the corolla are brightly coloured i.e. acting as flag apparatus for attracting insects. In addition, there are other devices for attracting insects which visit flowers after flowers-there are honey glands, nectar glands etc. situated at the base of petals or sepal glands secreting nectar, sweet scent of flowers etc. The pollen grains of entomophilous flowers are larger, the exine is pitted, tuberculed, spiny etc. so that they can adhere firmly on the sticky stigma. There are various adaptations or devices in the organisation of flowers, e.g. Helianthus annuus (Compositae), Bougainvillea Spectabilis (Nyctaginaceae ), orchids, flowers of Papilionaceae etc.

Of various insects, bees are the main flower visitors that carry out about 80 percent of all pollination done by insects. Bees visit flowers to collect pollens and nectar as their food, and act as instrument in bringing pollination.The flowers of Ophrys Sp. (Orchidaceae) are pollinated by hairy wasp (Colpa aurea). Such bees and wasp pollinated flowers are called hymenopterous flowers.

Fly pollinated flowers are called diptera flowers. Flies are attracted by the unpleasant smell of flowers such as Arum, Aristolochia, Rafflesia, etc. Those flowers are provided with “fly-trap mechanism” for ensuring pollination.

(b) Omithophily-When pollination is affected by birds, it is called omithophily and the flowers pollinated by birds are called ornithophilous flowers. Birds are more important pollinators than insects. Ornithophilous flowers are characterised by their tubular or urn shaped or cup-shaped form, bright colour, plenty of nectar and large quantities of pollen. Their stamens and carpels usually project beyond the perianth lobes. Ornithophilous flowers are pollinated by humming-birds, sun-birds and honey-eaters. Humming birds are very small and they feed on the nectar of flowers of Bignonia sp. and thereby pollinate them.

A remarkable adaptation in ornithophily exists in Strelitzia regineae (Musaceae). The flower is large and very showy having its three outer perianth leaves (calyx) bright orange-yellow, the azure blue lip corresponds to inner lateral perianth members forming arrow-shaped structure including five stamens ; the posterior petal is small, the style lies in the furrow of the same. The stigma projects freely from the furrow enclosing the stamens and style. The similarly coloured sun-bird (Nectarina afra) flies first to the stamens and peck them-so touches them, then secure pollens from the stamens, next in visiting another flower deposit the pollens on the stigma of that flower. Examples are also found in Bombax ceiba(Bombacaceae), Butea monosperma (Papilionaceae), Oroxylon sp. (Bignoniaceae) etc.

(c) Malacophily-Pollination by other animals like slugs, snails, squirrels etc. Some flowers of aroids are pollinated by snails. Large number of trees are visited by squirrels and they have some role in pollination of those trees.

(d) Chiropteriphily-Pollination by bats has been noted in Some plants growing in tropics. Chiropteriphilous plants have flowers with very long stalk énd they occur in clusters or singly and quite away from branches and leaves. The pollinating bats have long and slender muzzle and an extensive tongue. Bats clasp the flowers of Adansonia digitata (Bombacaceae) with their claws and suck nectar. Kigelia africana (Bignoniaceae) is also bat-pollinated tree. Bat collects nectar from the base of the long floral tube and at the same time gets its back dusted with pollens.