Agents of pollination

Botany

Pollination is effected by many agents like wind, water, insects etc. On the basis of the agents that bring about pollination, the mode of pollination is divided into abiotic and biotic.

The latter type is used by majority of plants.

Abiotic agents

1. Anemophily – pollination by Wind
2. Hydrophily – pollination by Water

Biotic agents

3. Zoophily – Zoophily refers to pollination through animals
and pollination through insects is called Entomophily.

1. Anemophily: Pollination by wind.

The wind pollinated flowers are called anemophilous.The wind pollinated plants are generally
situated in wind exposed regions. Anemophily is a chance event.Therefore, the pollen may
not reach the target flower effectively and are wasted during the transit from one flower to another. The common examples of wind
pollinated flowers are – grasses, sugarcane,bamboo, coconut, palm, maize etc.,

Anemophilous plants have the following characteristic features:

• The flowers are produced in pendulous,catkin-like or spike inflorescence.
• The axis of inflorescence elongates so that the flowers are brought well above the leaves.The perianth is absent or highly reduced.
• The flowers are small,inconspicuous,colourless, not scented, do not secrete nectar.
• The stamens are numerous, filaments are long, exerted and versatile.
• Anthers produce enormous quantities of pollen grains compared to number of ovules available for pollination. They are minute, light and dry so that they can be carried to
long distances by wind.
• In some plants anthers burst violently and release the pollen into the air. Example: Urtica.
• Stigmas are comparatively large, protruding, sometimes branched and feathery, adapted to catch the pollen grains. Generally single ovule is present.
• Plant produces flowers before the new leaves appear, so the pollen can be carried without hindrance of leaves.

Pollination in Maize (Zea mays):

The maize is monoecious and unisexual. The male inflorescence (tassel) is borne terminally and
female inflorescence (cob) laterally at lower levels. Maize pollens are large and heavy and cannot be carried by light breeze. However,the mild wind shakes the male inflorescence to
release the pollen which falls vertically below.The female inflorescence has long stigma (silk)
measuring upto 23 cm in length, which projects beyond leaves. The pollens drop from the tassel is caught by the stigma

2. Hydrophily:

Pollination by water is called hydrophily and the flowers pollinated by water are said to be hydrophilous (Example: Vallisneria,Hydrilla). Though there are a number of aquatic plants, only in few plants pollination takes place by water. The floral envelop of hydrophilous plants are reduced or absent. In water plants like Eichhornia and water lilly pollination takes place through wind or by insects. There are two types of hydrophily, Epihydrophily and
Hypohydrophily. In most of the hydrophilous flowers, the pollen grains possesses mucilage covering which protects them from wetting.

a. Epihydrophily:

Pollination occurs at the water level. Examples: Vallisneria spiralis, Elodea.
Pollination in Vallisneria spiralis: It is a dioecious, submerged and rooted hydrophyte.The female plant bears solitary flowers which rise to the surface of water level using a long coiled stalk at the time of pollination. A small cup shaped depression is formed around the female flower on the surface of the water. The male plant produces male flowers which get detached and float on the surface of the water. As soon as a male flowers comes in contact with the female flower and pollination takes place, Stalk of the female flower coils and goes under water where
fruits are produced

b. Hypohydrophily:

Pollination occurs inside the water. Examples: Zostera marina and
Ceratophyllum

3. Zoophily:

Pollination by the agency of animals
is called zoophily and flowers are said to be zoophilous. Animals that bring about pollination may be birds, bats, snails and insects. Of these, insects are well adapted to bring pollination.
Larger animals like primates (lemurs), arboreal rodents, reptiles (gecko lizard and garden lizard)
have also been reported as pollinators.

A. Ornithophily:

Pollination by birds is called Ornithophily. Some common plants that are pollinated by birds are Erythrina, Bombax, Syzygium, Bignonia, Sterlitzia etc., Humming
birds, sun birds, and honey eaters are some of the birds which regularly visit flowers and bring about pollination.

The ornithophilous flowers have the following characteristic features:

• The flowers are usually large in size.
• The flowers are tubular, cup shaped or urn-shaped.
• The flowers are brightly coloured, red,scarlet, pink, orange, blue and yellow which attracts the birds.
• The flowers are scentless and produce nectar in large quantities. Pollen and nectar form the floral rewards for the birds visiting the
flowers.
• The floral parts are tough and leathery to withstand the powerful impact of the visitors.

B. Cheiropterophily:

Pollination carried out by bats is called cheiropterophily. Some of the
common cheiropterophilous plants are Kigelia africana, Adansonia digitata, etc.,

C. Malacophily:

Pollination by slugs and snails is called malacophily. Some plants of Araceae are pollinated by snails. Water snails crawling among Lemna pollinate them.

D. Entomophily:

Pollination by insects is called Entomophily. Pollination by ant is called myrmecophily. Insects that are well adapted to bring pollination are bees, moths, butterflies, flies, wasps and beetles. Of the insects, bees are the main flower visitors and dominant pollinators. Insects are chief pollinating agents and majority of angiosperms are adapted for
insect pollination. It is the most common type of pollination.

The characteristic features of entomophilous flowers are as follows:

• Flowers are generally large or if small they are aggregated in dense inflorescence. Example: Asteraceae flowers.
• Flowers are brightly coloured. The adjacent parts of the flowers may also be brightly coloured to attract insect. For example in Poinsettia and Bougainvillea the bracts become coloured.
• Flowers are scented and produce nectar.
• Flowers in which there is no secretion of nectar, the pollen is either consumed as food or used in building up of its hive by the honeybees. Pollen and nectar are the floral rewards for the visitors.

• Flowers pollinated by flies and beetles produce foul odour to attract pollinators.
• In some flowers juicy cells are present which are pierced and the contents are sucked by the insects.

Pollination in Salvia (Lever mechanism):

The flower is protandrous and the corolla is bilabiate with 2 stamens. A lever mechanism helps in pollination. Each anther has an upper fertile lobe and lower sterile lobe which is separated by a long connective which helps the anthers to swing freely. When a bee visits a flower, it sits on the lower lip which acts as a platform. It enters the flower to suck the nectar
by pushing its head into the corolla. During the entry of the bee into the flower the body strikes against the sterile end of the connective.
This makes the fertile part of the stamen to descend and strike at the back of the bee.The pollen gets deposited on the back of the
bee. When it visits another flower, the pollen gets rubbed against the stigma and completes the act of pollination in Salvia. Some of the other interesting pollination mechanisms found in plants are a) Trap mechanism (Aristolochia);Pit fall mechanism (Arum);Clip or translator mechanism

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