Pollination – Types, Agents, Mechanisms, Advantages and Disadvantages

What is Pollination

Pollination is the transference of pollen grains from the anther to the stigma whether of the same flower or of a different flower.

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Types of Pollination:

Pollination is of two types, viz :

(a) Self-pollination or Autogamy

In this type the transference of pollen grains from the anther of a flower to the stigma of the same flower takes place. Bisexual flowers are generally self-pollinated.

Sometimes self-pollination is also applicable in the transference of pollen from the anther of male or of bisexual flower to the stigma of female or another bisexual flower occurring on the same plant. This process is called geitonogamy. According to this concept, only one parent plant is concerned in self-pollination.

(b) Cross-pollination or Allogamy

It is the transference of pollens from the anther of one flower to the stigma of another flower borne by a different plant of the same or different species. All unisexual flowers are generally cross-pollinated.

Cross-pollination when takes place within the same species is called xenogamy and between two different species is called hybridism; hybrids are produced as a result of hybridism.

Contrivances or Devices for Cross-Pollination:

There are many devices for ensuring cross-pollination and hindering self-pollination, such as —

(1) DICLINY OR UNISEXUALITY

In dioecious plants of Trichosanthes sp. (Cucurbitaceae), Croton sp. (Euphorbiaceae) self-pollination is impossible as the flower are unisexual and contain only one essential member i.e. either stamen or carpel, in such plants cross-pollination becomes obligatory. In case of monoecious plants, however geitonogamy may take place.

(2) SELF-STERILITY

Sometimes self-pollination is hindered due to the sterile condition of the pollen of the bisexual flower, as a result in such cases a flower cannot be fertilised by the pollen of the same flower, e.g. species of Solanum (Solanaceae) — hence cross-pollination is obligatory in those plants. In Petunia axillaris, growth of the pollen tube (formed upon self pollination) is inhibited in the mid-part of the style—as a result cross pollination is obligatory.

(3) DICHOGAMY

This refers to maturation of stamens and stigma of a bisexual flower at different times. In dichogamous flowers it is natural that self-pollination cannot take place on account of different times of maturity of stamens and carpels. It is of two types —

(a) Protandry

This is a condition when anthers mature earlier than stigma. So in immature stigma pollen of the same flower cannot germinate. Examples of protandrous flower are found in Helianthus sp. (Compositae), Hibiscus sp. (Malvaceae), Leonurus sp. (Labiatae).

(b) Protogyny

Here stigma matures earlier than anthers. In Michelia champaca, species of Magnolia (Magnoliaceae), Annona, Polyalthia (Annonaceae), species of Brassica and Rosaceae, etc. mature stigma is pollinated by the pollen of other flowers due to immature condition of their own anthers.

(4) HERKOGAMY

When self-pollination becomes impossible due to some physical barrier between the anther and the style. This is met within Iris sp. (Iridaceae). Here the large outer perianth leaf acts as a landing stage for insects, the extrorse anthers are concealed by the overarching wide petaloid style — thus a barrier is created between the anther and the style, as a result cross-pollination is favored by insects. In Gloriosa sp. the anthers dehisce at a distance so that the stigma is out of reach of its own pollen.

(5) HETEROMORPHISM

This is the phenomenon of the occurrence of two (dimorphic) or three (trimorphic) forms of flowers with anthers and stigmas at different level, i.e. flowers may have short styles with long filaments and vice versa in the same flower; due to this condition self-pollination is not effected, so cross-pollination takes place between stamens and carpels of same length.

In Primula sp. there are two types of flower e.g. flower with (1) long style, short stamens and small pollen grains., and (2) with short style, long stamens and large pollen grains. Stigma in the short-styled flowers is at the level of anthers in long styled flowers and vice-versa.

As a result pollens from short-styled flower can bring about pollination only in long-styled flowers and similarly pollens from long-styled flowers can bring about pollination in short-styled flowers, but not in their own flowers. This is seen also in species of Oxalis (Oxalidaceae), Polygonum (Polygonaceae), Linum (Liliaceae), Lythrum (Lythraceae), etc.

Contrivances for Self-pollination :

(1) CLEISTOGAMY

Various types of adaptation in cleistogamous flowers belonging to different families are met with. In Viola sp. (Violaceae) there are apetalous flower or flowers having small petals in which anthers are closely applied to the stigma by bending of filaments.

Cleistogamy is also observed in the underground flowers of Commelina benghalensis (Commelinaceae) — these flowers are minute and inconspicuous, they never open from bud condition, so there is no chance of transference of pollens from the anther of other flower to the stigma of closed flower.

But brightly coloured (blue or violet) aerial flowers of Commelina benghalensis are insect-pollinated (chasmogamous — during anthesis the flowers open normally). Cleistogamy often occurs in species of Impatiens (Balsaminaceae), Cardamine (Cruciferae), Halenia (Gentianaceae), etc.

(2) HOMOGAMY

It is the condition of the maturity of anthers and stigmas at the same time, so that self-pollination is effected, although not always. Sometimes pollen grains are carried to the stigma by some agents like wind, insects etc.

Homogamy is seen in Argemone mexicana (Papaveraceae), Opuntia dillenii (Cactaceae), Microcos asiatica (Tiliaceae) etc. In Mirabilis sp. (Nyctaginaceae), self-pollination is favored as the filaments are brought to the contact of the stigma by curling and twining of the same.

Agents of Pollination:

The agents that bring about 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 anemophilous 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 underwater e.g. Najas, Ceratophyllum. Zostera, etc. The pollen is very long (upto 250m), Needle-like, and without exine. Specific gravity of these pollens is almost the same as that of water. As 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 also 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) Ornithophily

When pollination is affected by birds, it is called ornithophily 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 hummingbirds, sun-birds, and honey-eaters. Hummingbirds 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 and 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.

Pollination Mechanisms in (a) Salvia, (b) Calotropis, (c) Ficus, (d) Orchid, (e)Vallisneria, (f) Papilionaceous flowers etc.

(a) Salvia (Labiatae)

In Salvia sp. cross-pollination by insects (bees) is brought about by special elaborate development of morphological structures in the androecium. The stamens, numbering two are provided with long connective, the anterior end of the connective forms a knob at the end of arm—thus a lever arrangement with spring action has been elaborated; the lower lip of Salvia forms a landing stage for insects e.g. bees.

Honey is secreted in the lower side of the hypogynous disc. The single anther lobe of stamens is concealed in the head-like upper lip. A bee in search of nectar sits on the lower lip, then in attempting to get nectar it touches the knob of sterile anther, as a result of which the upper fertile anther lobe strikes on the back of the bee. The back of the bee is thus dusted with pollens. The style with forked stigmas forms a bent-structure. As the bee with the pollen powder on its back enters another mature flower, the pollen powders are caught by the forked recurved stigmas from the back of the bee. The flowers of Salvia are highly protandrous and have corolla arrangement of 2/3 type i.e. upper lip of 2 lobes and lower of 3 lobes.

(b) CALOTROPIS (Asclepiadaceae)

In Calotropis sp. too there is a highly elaborate arrangement for cross-pollination by means of insects (bees). The corona-like appendages are associated with the secretion and storing of nectar in Calotropis. The pollen is not granular but aggregated into two waxy masses—the so-called pollinia which are attached with each other by slender stalk-like processes known as translators which end in notched knoblike structure, the corpusculum.

Stamens and pistils are adherent to form gynostegium. The anthers are adherent on the lateral sides of the pentangular stigma. There are small openings or slits between the adjacent cells of anthers. A bee or other insects in search for nectar comes in contact with the notched corpusculum which comes out partially through the slits of anthers. The corpusculum is hygroscopic, it is caught by the leg of the insect firmly. The insect when retiring from the flower drags with it the whole pollinia attached to its leg and on visiting another flower the pollinia are deposited on the adhesive stigma of the same. Thus cross pollination by insect is easily brought about.

(c) FICUS (Moraceae)

In Ficus sp., the small flowers are almost enclosed by the receptacle of the hypanthodium, but there exists a small opening through which small insects (gall wasps) enter. Ecological adaptation has arisen in Ficus for ensuring cross-pollination by insects.

The hypanthodium contains three kinds of flowers—the male flowers and two kinds of female flowers—one with short styles called gall flowers which are sterile and the others with long styles which are fertile. Pollination is brought about by means of very small wasps, the so-called gall wasps (Blastofaga) which enter the hypanthodium through its small apical pore to lay their eggs in the ovaries of gall flowers. As the male flowers are there, the bodies of wasps are dusted with pollens; the wasps on entering another hypanthodium come in contact with the styles of fertile female flowers, thus cross-pollination is effected.

(d) ORCHID (Orchidaceae)

The highest adaptation for cross-pollination by insect agency is developed in the flowers of orchid. The labellum or lip acts as flag apparatus as well as landing stage for insects. Besides there are various complexity in the organisation and construction of other floral parts which play an important role in the transference of pollen. The rostellum (the posterior sterile stigmatic lobe) forms a viscid disc which secretes a sticky fluid so that entire pollinia adhere to the body of insects. The pollinia with caudicles and corpusculum are also adaptations for entomophily in Orchids.

(e) VALLISNERIA (Hydrocharitaceae)

Refer to Hydrophily article.

(f) PAPILIONACEOUS FLOWER

In Papilionaceae, the construction of the flower itself is an adaptation for cross-pollination by insects. The standard is the flag apparatus, the wing petals are the landing stage for insects. The flower is so constructed that the weight of the insect on wing petals causes the keel to come down—as a result the hidden stigma within the sheathing filament jerks out and consequently comes in contact with the pollen-dusted body of the insect.

Advantages and Disadvantages of Self- and Cross-pollination :

In self-pollination, there is least possibility of failure and an almost universal phenomenon in bisexual flowers. For self-pollination, plants are not to depend on external agents to carry pollens that is why the process is easily secured. Disadvantage of self-pollination is that if self-pollination is continued generation after generation, it results in weaker progeny.

Cross-pollination has many advantages, such as:
(a) it produces healthy progeny, (b) it results in the production of more viable seeds, (c) plants are better adapted in nature and (d) the possibility in the production of new varieties.

Most important disadvantage is that for cross-pollination, plants have to depend on various external agents and there is wastage of pollens, specially in anemophily.