Pollination

Pollination

Pollination is a wonderful mechanism which provides food, shelter etc., for the pollinating animals. 

Many plants are pollinated by a particular animal species and the flowers are modified accordingly and thus there exists a co-evolution between plants and animals. 

Let us imagine if pollination fails. Do you think there will be any seed and fruit formation? If not what happens to pollinating organisms and those that depend on these pollinating organism for the food? Here lies the significance of the process of pollination.

The pollen grains produced in the anther will germinate only when they reach the stigma of the pistil. The reproductive organs, stamens and pistil of the flower are spatially separated, a mechanism which is essential for pollen grains to reach the stigma is needed. This process of transfer of pollen grains from the anther to a stigma of a flower is called pollination.

Pollination is a characteristic feature of spermatophyte (Gymnosperms and Angiosperms). Pollination in gymnosperms is said to be direct as the pollens are deposited directly on the exposed ovules, whereas in angiosperms it is said to be indirect, as the pollens are deposited on the stigma of the pistil. In majority of angiosperms, the flower opens and exposes its mature anthers and stigma for pollination. Such flowers are called chasmogamous and the phenomenon is chasmogamy. In other plants, pollination occurs without opening and exposing their sex organs. Such flowers are called cleistogamous and the phenomenon is cleistogamy.

Based upon the flower on which the pollen of a flower reaches, the pollination is classified into twokinds,namely,self-pollination(Autogamy) and cross-pollination(Allogamy).

A.  Self-pollination  or  Autogamy 

(Greek Auto = self, gamos = marriage):

According to a majority of Botanists, the transfer of pollen on the stigma of the same flower is called self-pollination or Autogamy. Self-pollination is possible only in those plants which bear bisexual flowers. In order to promote self- pollination the flowers of the plants have several adaptations or mechanisms. They are:

1. Cleistogamy: In cleistogamy (Greek Kleisto = closed. Gamos = marriage) flowers never open and expose the reproductive organs and thus the pollination is carried out within the closed flower. Commelina, Viola, Oxalis are some examples for cleistogamous flowers. In Commelina benghalensis, two types of flowers are produced-aerial and underground flowers. The aerial flowers are brightly coloured, chasmogamous and insect pollinated. The underground flowers are borne on the subterranean branches of the rhizome that are dull, cleistogamous and self pollinated and are not depended on pollinators for pollination.(Figure 1.11).

2.     Homogamy: When the stamens and stigma of a flower mature at the same time it is said to be homogamy. It favours self-pollination to occur. Example: Mirabilis jalapa, Catharanthus roseus

3. Incomplete dichogamy: In dichogamous flowers the stamen and stigma of a flower mature at different time. Sometimes , the time of maturation of these essential organs overlap so that it becomes favourable for self-pollination.

B. Cross – pollination

It refers to the transfer of pollens on the stigma of another flower. The cross-pollination is of two types:

i. Geitonogamy: When the pollen deposits on another flower of the same individual plant, it is said to be geitonogamy. It usually occurs in plants which show monoecious condition. It is functionally cross-pollination but is similar to autogamy because the pollen comes from same plant.

ii. Xenogamy: When the pollen (genetically different) deposits on another flower of a different plant of the same species , it is called as xenogamy.

Contrivances of cross-pollination

The flowers of the plants have also several mechanisms that promote cross-pollination which are also called contrivances of cross-pollination or outbreeding devices. It includes the following.

1. Dicliny or Unisexuality

When the flowers are unisexual only cross-pollination is possible. There are two types.

i. Monoecious: Male and female flowers on the same plant. Coconut, Bitter gourd. In plants like castor and maize, autogamy is prevented but geitonogamy takes place.

ii. Dioecious : Male and female flowers on different plants. Borassus, Carica papaya and date palm. Here both autogamy and geitonogamy are prevented.

2. Monocliny or Bisexuality

Flowers are bisexual and the special adaptation of the flowers prevents self-pollination.

i. Dichogamy: In bisexual flowers anthers and stigmas mature at different times, thus checking self-pollination. It is of two types.

a. Protandry: The stamens mature earlier than the stigmas of the flowers. Examples: Helianthus, Clerodendrum (Figure 1.12 a).

b. Protogyny: The stigmas mature earlier than the stamens of the flower. Examples: Scrophularia nodosa and Aristolochia bracteata (Figure 1.12 b).

ii. Herkogamy: In bisexual flowers the essential organs, the stamens and stigmas, are arranged in such a way that self-pollination becomes impossible. For example in Gloriosa superba, the style is reflexed away from the stamens and in Hibiscus the stigmas project far above the stamens (Figure 1.13).

iii. Heterostyly: Some plants produce two or three different forms of flowers that are different in their length of stamens and style. Pollination will take place only between organs of the same length.(Figure 1.14)

a. Distyly: The plant produces two forms of flowers, Pin or long style, long stigmatic papillae, short stamens and small pollen grains;

Thrum-eyed or short style, small stigmatic papillae, long stamens and large pollen grains. Example: Primula (Figure 1.14a). The stigma of the Thrum-eyed flowers and the anther of the pin lie in same level to bring out pollination. Similarly the anther of Thrum-eyed and stigma of pin ones is found in same height. This helps in effective pollination.

b.     Tristyly: The plant produces three kinds of flowers, with respect to the length of the style and stamens. Here,the pollen from flowers of one type can pollinate only the other two types but not their own type. Example : Lythrum (Figure 1.14b).

iv. Self sterility/ Self- incompatibility: In some plants, when the pollen grain of a flower reaches the stigma of the same, it is unable to germinate or prevented to germinate on its own stigma. Examples: Abutilon, Passiflora. It is a genetic mechanism.

Agents of pollination

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 (Figure 1.15).

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 flower comes in closer to a female flower, it gets settled in the depression and contacts with the stigma thus bringing out pollination. Later the stalk of the female flower coils and brings back the flower from surface to under water where fruits are produced. (Figure 1.16).

b. Hypohydrophily: Pollination occurs inside the water. Examples: Zostera marina and Ceratophyllum. Zostera marina is a submerged marine sea grass and pollination takes place under water. The pollen grains are long, needle like. They are shed under water. The specific gravity of the pollen is same as that of sea water, so that, the pollen floats freely at any depth. The stigma is very large and long. The pollen comes in contact with the stigma and gets coiled around the stigma thus effecting pollination.

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., Bats are nocturnal and are attracted by the odour of the flowers that open at or after dusk. The cheiropterophilous plants have flowers borne singly or in clusters quite away from the leaves and branches either on the long peduncle or on the trunk or branches. The flowers produce large quantities of nectar.

Pollination in Adansonia digitata: In this plant, the ball of stamens and the stigma project beyond the floral envelops. The bat holds the flower by clasping the stamen ball to its breast. While taking nectar its breast becomes laden with numerous pollen grains, some of which get deposited on the stigma of the flower when it visits next.

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 of Salvia is adapted for Bee pollination. 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 (Figure 1.17a). Some of the other interesting pollination mechanisms found in plants are a) Trap mechanism (Aristolochia);Pit fall mechanism (Arum);Clip or translator mechanism (Asclepiadaceae) and Piston mechanism (Papilionaceae).

Pollination in Calotropis (Translator mechanism)

This mechanism is found in members of Asclepiadaceae( Apocynaceae as per APG system of classification). The flowers are bisexual with 5 stamens forming gynostegium(union of stigma with the androecium). The stigma is large and 5 – angled and is receptive on the underside. Each stamen at its back possesses a brightly coloured hood like outgrowth enclosing horn shaped nectar. The pollen in each anther lobe of a stamen unites into a mass, forming a pollinium. Pollinia are attached to a clamp or clip like sticky structure called corpusculum. The filamentous or thread like part arising from each pollinium is called retinaculum. The whole structure looks like inverted letter ‘Y’ and is called translator. When the insect visits the flower for nectar, the translator gets attached to the proboscis or leg of the visitor. During the visit to the next flower the pollinia come in contact with the receptive stigma carrying out pollination.

Pollination in Aristolochia (Trap mechanism)

A special mechanism to accomplish pollination called trap mechanism is found in Aristolochia. The flowers are axillary and perianth is tubular with a hood at the top. The basal region is swollen and possesses gynostegium. The gynostegium has six anthers.

The inner wall of tubular middle part of the perianth is slippery and lined with stiff hairs which are pointed downwards. The young flowers are erect. During this stage small flies enter and could not escape because they are trapped by the hairs. As soon as the anthers of the flower ripe, the hairs wither and flower bents down. The flies escape with pollen and enter another flower where it dusts pollen on the stigma bringing out pollination.

Advantages of self-pollination:

·         Pollination is almost certain in bisexual flowers.

·         When the members of the species are uncommon and are separated by large distances, the plant has to depend on self-pollination.

·         If all the chances of cross-pollination fails, self-pollination will take place and prevent the extinction of the species.

Disadvantages of self-pollination:

·         Continuous self-pollination, generation after generation results in weaker progeny.

·         Chances of producing new species and varieties are meager.

Advantages of cross-pollination:

·         It always results in bringing out much healthier offsprings.

·         Germination capacity is much better.

·         New varieties may be produced.

·         The adaptability of the plants to their environment is better.

Disadvantages of cross-pollination:

·         Depend on external agencies for the pollination and the process is uncertain.

·         Various devices have to be adopted to attract pollinating agents.

Significance of Pollination

·         Pollination is a pre-requisite for the process of fertilisation. Fertilisation helps in the formation of fruits and seeds.

·         It brings the male and female gametes closer for the process of fertilisation.

·         Cross-pollination introduces variations in plants due to the mixing up of different genes. These variations help the plants to adapt to the environment and results in speciation.