- 1 What is Pollination in Flower? – Agents, Advantages, and Disadvantages of Pollination
- 1.1 Introduction of Pollination
- 1.2 Types of Pollination
- 1.3 Method of Pollination
- 1.4 Mechanics of the Pollination
- 1.5 Agents of Pollination
- 1.6 Flower-Specialization and Pollination
- 1.7 Flower-Pollinator Relationships
- 1.8 Advantages and Disadvantages of Self and Cross-Pollination
- 1.9 Evolution of Plant/Pollinator Interactions
- 1.10 Significance of Pollination
One of the most fascinating Biology Topics is the study of genetics and how traits are passed down through generations.
What is Pollination in Flower? – Agents, Advantages, and Disadvantages of Pollination
Introduction of Pollination
The transference of pollen grain from the anther of the stamen to the stigma of the carpel of the same flower or of different flowers of the same plant or different plants of the same species is called pollination.
Pollination in angiosperms is the process that transfers the pollen grains, which contain the male gametes i.e., sperm to where the female gametes are contained within the carpel. The receptive part of the carpel is called stigma. The study of pollination brings together many disciplines, such as botany, horticulture, entomology, and ecology. The pollination process as an interaction between the flower and vector was first addressed in the 18th century by Christian Konrad Sprengel.
Pollination is a necessary step in the sexual reproduction of flowering plants, resulting in the production of offspring that are genetically diverse. It is important in horticulture and agriculture because fruiting is dependent on fertilization, which is the end result of pollination. Pollination is the process by which pollen is transferred in the reproduction of plants, thereby enabling fertilization and sexual reproduction.
Types of Pollination
Pollination is of two types Self-pollination or Autogamy and Cross-pollination or Allogamy.
The transference of pollen grains from the anther of a flower to the stigma of the same flower or different flowers of the same plant is called self-pollination, e.g., Dolichos lablab, Mirabilis jalapa, etc.
2. Cross Pollination:
The transference of pollen grains from the anther of one flower to the stigma of another flower borne on a different plant of the same species is called cross-pollination, e.g., Borassus flabellifer, Cucurbita maxima, etc.
Recently depending on the source of pollen, pollination has been divided into three types: autogamy, geitonogamy, and cross-pollination.
On the basis of abiotic and biotic components, pollination can be divided into two i.e., abiotic pollination and biotic pollination.
1. Abiotic Pollination
Abiotic pollination refers to situations where pollination is mediated without the involvement of other organisms; these abiotic factors that achieve pollination include wind and gravity. Only 10% of flowering plants are pollinated without animal assistance. The most common form, hemophilia, is pollination by wind. This form of pollination is predominant in grasses, most conifers, and many deciduous trees. Hydrophily is pollination by water and occurs in aquatic plants which release their pollen directly into the surrounding water. About 80% of all plant pollination is biotic. Of the 20% of abiotically pollinated species, 98% is by wind and 2% by water.
2. Biotic Pollination
Biotic pollination is the more common process of pollination which requires biotic pollinators: organisms that carry or move the pollen grains from the anther to the receptive part of the carpel or pistil. This is “biotic pollination”. The various flower traits (and combinations thereof) that differentially attract one type of pollinator or anther are known as pollination syndromes.
There are roughly 2,00,000 varieties of animal pollinators in the wild, most of which are insects. “Entomophily”, pollination by insects, often occurs on plants that have developed coloured petals and a strong scent to attract insects such as bees, wasps, and occasionally ants (Hymenoptera), beetles (Coleoptera), moths, and butterflies (Lepidoptera), and flies (Diptera). In “Zoophily”, pollination is conducted by vertebrates such as birds and bats, particularly, hummingbirds, sunbirds, spiderhunters, honeyeaters, and fruit Bats. Plants adapted to using bats or moths as pollinators typically have white petals and a strong scent, while plants that use birds as pollinators tend to develop red petals and rarely develop a scent (few birds have a sense of smell).
Method of Pollination
A. Self Pollination
In self-pollinated plants, the transfer of pollen grains from the anther to the stigma of the same flower is facilitated mainly by means of wind and insects. When the flowers are swayed by air currents, the dry pollen grains are transferred from the anther to the sticky stigma. In the case of bright-colored and sweet-scented flowers, pollination is affected by insects. When an insect enters the flower in search of nectar, the spiny (ornamented) pollen grains adhere firmly to its back. As the insect comes out of the flowers or moves inside the flower, the pollen grains are transferred to the sticky stigma as it rubs against the insect’s surface. It is of two types-
- Autogamy: In autogamy, pollen grains are transferred from another to the stigma of the same flower. This pollination occurs usually in bisexual flowers. Autogamy is followed by these methods.
- Geitonogamy: In geitonogamy, pollen grains of one flower are transferred to the stigma of another flower of the same plant.
Special Types (Process Helping Self-Pollination):
- Homogamy: Self-pollination occurs usually in the bisexual flower in which the anthers and the stigmas mature at the same time, a condition known as Homogamy, e.g., Argemone, Opuntia, Ixora, Mimbilis, Cichorium, etc.
- Cleistogamy: Self-pollination also takes place in those bisexual flowers which never open, e.g., Commelina benghalensis, etc.
Cross-pollination is the rule in unisexual flowers. In bisexual flowers, cross-pollination occurs through some adaptations of flowers. When cross-pollination takes place between two flowers of two different plants of the same species.
Process Helping Cross-Pollination
(i) Unisexuality or Dicliny: Here the flowers are unisexual, therefore cross-pollination occurs through some adaptations in flowers, e.g., Croton sp.
(ii) Self Sterility: Here pollen of a flower has no fertilization effect on the stigma of the same flower, e.g., Solanum sp., Petunia axillaris.
(iii) Dichogamy: Here either male parts (Protandry when anther matures earlier so that its stigma is not really to receive pollen from its anther, e.g., Hibiscus rosa sinensis) or female parts (Protogyny when stigma matures earlier, e.g., Fig, Magnolia sp.) mature first.
(iv) Herkogamy: Where adaptive barrier between male and female parts prevents self-pollination, e.g., Iris sp., Gloriosa sp., etc. Some physical barriers are-
- A fcap covers the stigma, e.g. Pansy.
- Corolla forms pockets, e.g. Kalima.
- Extrose dehiscence of anthers.
- In Gloriosa, anthers dehisce at a long distance that stigma does not reach out.
(v) Heteromorphism: Flowers having long or short male and female parts, e.g., Primula, Linum, Oxalis, etc. Heterostyly is seen in bisexual flowers when the flowers have a long style and short stamen is known as pin-eyed di heterostyly flowers. On the other hand, the short style and the long stamen are known as thrum-eyed di heterostyly flowers (e.g., Primula). When the flower bears long, short, and medium height style is known as tri heterostyly (e.g., Oxalis).
Merits and Demerits of Self and Cross Pollination:
|1. Necessity of Agents||Merits:
|2. Emergence of new characters in the offspring||Merits:
It maintains the parental characters or purity of the race indefinitely.
Mechanics of the Pollination
Pollination also requires consideration of pollenizers. The terms “pollinator” and “pollenizer” are often confused, a pollinator is an agent that moves the pollen, whether it be bees, flies, bats, moths, or birds; a pollenizer is a plant that serves as the pollen source for other plants. Some plants are self-fertile or self-compatible and can pollinate themselves (e.g., they act as their own pollenizer). Other plants have chemical or physical barriers to self-pollination and need to be cross-pollinated: with these self-infertile plants, not only pollinators must be considered but pollenizers as well. In pollination management, an efficient pollenizer is a plant that provides compatible, viable, and plentiful pollen and blooms at the same time as the plant that is to be pollinated.
1. Pollination can be cross-pollination with a pollinator and an external pollenizer, self-polarization with a pollinator, or self-pollination without any pollinator: Cross-pollination (syngamy): pollen is delivered to a flower of a different plant. Plants adapted to outcross or cross-pollinate have taller stamens than carpels to better spread pollen to other flowers.
2. In self-pollination (autogamy) pollen moves to the female part of the same flower, or to another flower on the same individual plant. This is sometimes referred to as self-pollination, but this is not synonymous with autogamy. Clarity requires that the term “self-pollination” be restricted to those plants that accomplish pollination without an external pollinator (for example the stamens actually grow into contact with the pistil to transfer the pollen). Most peach varieties are autogamous, but not truly self-pollinated, as it is generally an insect pollinator that moves the pollen from anther to stigma. Plants adapted to self-fertilize have similar stamen and carpel lengths.
3. Cleistogamy is self-pollination that occurs before the flower opens. The pollen is released from the anther within the flower or the pollen on the anther grows a tube down the style to the ovules. It is a type of sexual breeding, in contrast to asexual systems such as apomixis. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are then pollinated. Cleistogamous flowers by necessity are self-compatible or self-fertile plants. Many plants are self-incompatible, and these two conditions are endpoints on a continuum.
It is effective pollination between flowers of different species of the same genus, or even between flowers of different genera (as in the case of several orchids).
Agents of Pollination
Cross-pollination as a rule is brought about by external agents which carry the pollen grains from one flower to the stigma of another flower. The agents are wind, water, insects (bees, flies, moths, etc.), and some animals (birds, snails, bats, etc.). According to the nature of the pollinating agents, cross-pollination is of different types, e.g., Anemophily (wind-pollinated), Hydrophily (water-pollinated), Entomophily (insect-pollinated), Ornithophily (bird-pollinated), Malacophily (snail-pollinated) and Chiropterophily (bat-pollinated). A brief account of the different types of pollinating methods along with the characteristics of flowers is given below:
1. Anemophily (anemos-wind, philein-to love)
The wind transfer of pollen grains is employed by the plants of the Grass family which are commonly grown for fodder, e.g., Typha sp., Maize, Grass, etc.
Characteristics of Wind-Pollinated Flowers
- The flowers are aggregated in a long-stalked inflorescence well above the vegetative portion.
- The flowers are never colored or showy.
- The flowers never emit any fragrant odour.
- The perianth is usually small and inconspicuous and does not cover the sexual parts at maturity.
- The sex organs are exposed and hang outside the perianth at maturity.
- The stamens are long filaments that swing easily in the wind.
- The anthers bear large quantities of pollen grains.
- The pollen grains are small, smooth, light, and dry. They are easily carried by the wind and are distributed over a wide area.
- The style is long protruding, and the stigmas are outside the flower.
- The stigmas are large and feathery or bristly and can catch easily any pollen grain floating in the air.
Method of Pollination: In this type, as the anthers of one flower burst, the dry powdery light pollen grains are carried by air currents. The powdery pollen grains floating in the air are caught by the long, hanging, feathery styles of another flower. In this way, pollination is brought about by the wind.
2. Hydrophily (Hydro-water; Philein-to love)
In aquatic or water plants, pollination is entirely brought about through the agency of water, e.g., Vallisneria, Hydrilla, Elodea, Naias, and Ceratophyllum. The water-pollinated flowers or hydrophilous flowers are variously adapted for the effective transference of pollen grains from anther to the stigma. Water-pollinated plants are of two types-those in which pollination takes place completely underwater are termed Hypohydrogamous, e.g., Naias, Ceratophyllum, etc., and those where pollination takes place on the water surface are termed Epihydrogamous, e.g., Vallisneria, Hydrilla, etc.
Characteristics of Water-Pollinated Flowers
- Flowers are very small, light in weight, and colourless.
- Floral parts are always covered by a waxy layer, to prevent the flowers from getting wet.
- Accessory floral parts are always smaller than the essential floral parts, therefore the essential parts remain always exposed and are not covered by the accessory parts.
- Matured male and female parts are always protruded out of the flower at maturity.
- Anthers dehisce violently scattering the pollen grains to a considerable distance.
- Small and lightweight pollen grains can easily float and are transferred over a long distance.
- Stigmas are with bristles and can hold pollen grains easily.
Method of Pollination: In hypohydrogamous plants, pollination takes place underwater. In epihydrogamous plants, like Elodea, flowers are always at the surface of the water. Here the pollen grains are liberated at the water surface and are carried to the stigma by the water current. In dioecious Vallisneria, the flowers are submerged. Here the minute male flowers are released from the spadix and float on the surface of the water. The female flower comes to the surface of the water by uncoil¬ing its long stalk. After pollination, which takes place on the water surface by water current, the stalk of the female flower recoils, and it again goes down into the water.
3. Zoophily (Zoo-animal; Philein-to love)
The animal pollinated flowers or zoophilous flowers are variously adapted to attract animals as pollinators. In zoophilous flowers, the principal adaptations are nectar and scent, e.g., Begonia, Silk cotton (Bombax), Rose apple (Syzygium), Anthocephalus cadamba, etc.
Different Types of Zoophily
Types of zoophily are discussed below:
- Entomophily: Pollination takes place by insects.
- Anthropophily: Pollination takes place by humans.
- Ornithophiiy: Pollination takes place by the birds.
- Myrmecophiiy: Pollination takes place by the ants.
- Phalenophily: Pollination takes place by the moths.
- Chiropterophily: Pollination takes place by bats.
- Hymenopterophily: Pollination takes place by the bees.
- Malacophily: Pollination takes place by the snails.
- Psychophiiy: Pollination takes place by the butterflies.
- Cantharophiiy: Pollination takes place by the beetles.
Zoophilous flowers, on the nature of the animal, are again divided into four types: Entomophilous (Insect-Pol-limited), Ornithophilous (bird-pollinated), Chiropterophilous (bat-pollinated), and Malacophilous (slug and snail-pollinated).
Characteristics of Animal-Pollinated Flowers
- Flowers are brightly coloured.
- Flowers are sweet-scented.
- Flowers are with nectar glands.
- Pollen grains are sweet tasting.
- Pollen grains are ornamented and sticky.
- Sticky and rough stigma.
Method of Pollination: In animal-pollinated flowers tiny birds like hummingbirds and the honey-thrushers visit the flower in search of nectar and thereby pollinate them. In some trees (Erythrina), flowers are often visited by crows and mynahs which also play some part in pollination.
Several Types of Zoophily are discussed below:
1. Entomophily (entomon-an insect):
The insect-pollinated flowers or entomophilous flowers are variously adapted to attract insects as pollinators. In entomophilous flowers, the principal adaptations are colour, nectar, and scent with some special devices in the organisation of the flowers, e.g., Crotalaria, Sunflower, Salvia, Bougainvillea, etc.
Characteristics of Insect-Pollinated Flowers:
- The flowers are brightly coloured and showy to attract insects. The coloured corollas (e.g., Crotalaria), sepals (e.g., Mussaenda), bracts (e.g., Bougainvillea), etc., serve as an ‘advertisement flag’ to attract insects.
- The anthers are large and swollen. The filament is basifixed or dorsifixed, helping easy adherence of pollen to the back of insects.
- The honey glands or nectar glands occurring at the base of the floral parts secrete sweet tasty fluid.
- Nocturnal flowers emit, at night, a sweet scent to attract insects, e.g., Jasmine, Rangoon creeper, etc.
- The pollen grains are large with rough sticky surfaces so that it may stick easily to the body of an insect.
- Pollen grains are sweet to taste. The insects visit the flowers for the sweet-tasted pollen grains.
- The surface of the stigma is rough and sticky so that the pollens may adhere firmly.
- The flowers may have special shapes to make it easy for the insect to visit, e.g., the standard of Crotolaria, the lower lip of Salvia forms a landing stage for insects, the pollinia of Calotropis; Orchid, etc.
Method of Pollination: In insect-pollinated flowers, a pollinator has to visit first a flower in which the male part (stamen) is mature. As the pollinator backs out of the flower, it carries the pollen grains stuck to its body surface. This insect then must visit a second flower in which the female part (carpel) becomes mature. While visiting the second flower, the pollen grains are trans¬ferred to the stigma as it rubs against the body surface of the pollinator.
Differences between Wind Pollinated and Insect Pollinated Flowers:
|Features||Wind Pollinated Flowers||Insect Pollinated Flowers|
|1. Petals||Small inconspicuous, sometimes absent. If present, not brightly coloured.||Large, brightly coloured, conspicuous, and attractive to insects.|
|2. Scent||None||Often Scented|
|4. Pollen||Produced in large quantities, light, smooth pollen grains.||Less produced, pollen grains larger, sculptured walls to add an attachment to insects and to stigma.|
|5. Anthers||Move freely, so pollen is easily dispersed.||Fixed to filaments and positioned to come into contact with visiting insects.|
|6. Stigma||Large often branched and feathery, hanging outside the flower to trap pollen.||Small enclosed within the flower, positioned to come into contact with visiting insects.|
2. Ornithophily or Bird Pollination:
The common bird pollinators are hummingbirds, sunbirds, and honey-eaters. Birds can obtain only one staple food from flowers and that is nectar. The bird-pollinated flowers have funnel-shaped or tubular corollas which are brightly coloured. The floral parts are commonly leathery and they produce copious amounts of nectar and larger quantities of pollen which are sticky.
Common bird: Normal pollinated plants are Bombax (red silk cotton). Erythrina (coral tree), Begonia, Lobelia, Agave.
Ornithophily or bird pollination is the pollination of flowering plants by birds. This co-evolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics and on some island chains.
3. Chiropterophiiy or Bat Pollination:
Bats bring about pollination only in the tropics. Bats are nocturnal animals and the flowers they visit are large and emit a strong odour. They move very fast and transport pollen over long distances. Bat-pollinated flowers generally produce more nectar than ornithophilous flowers. They also have a large number of stamens.
Examples: The Sausage tree-Kigella pinnate; The Baobab tree Adansonia and Kadamb tree-Anthocephalus.
4. Malacophily or Snail Pollination:
Snails perform pollination in cobra plants, Arisaema, and Arum bulbs.
Flower-Specialization and Pollination
Flowering plants usually face selective pressure to optimize the transfer of their pollen, and this is typically reflected in the morphology of the flowers and the behaviour of the plants. Pollen may be transferred between plants via a number of ‘vectors’. Some plants make use of abiotic vectors- namely wind (anemophily) or, much less commonly, water (hydrophily). Others use biotic vectors including insects (entomophily), birds (ornithophily), bats (chiropterophiiy), or other animals. Some plants make use of multiple vectors, but many are highly specialized. Cleistogamous flowers are self-pollinated, after which they may or may not open. Many Viola and some Salvia species are known to have these types of flowers.
The flowers of plants that make use of biotic pol¬len vectors commonly have glands called nectaries that act as an incentive for animals to visit the flower. Some flowers have patterns, called nectar guides, that show pollinators where to look for nectar. Flowers also attract pollinators by scent and colour. Still, other flowers use mimicry to attract pollinators. Some species of orchids, for example, produce flowers resembling female bees in colour, shape, and scent. Flowers are also specialized in shape and have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant (such as nectar, pollen, or a mate). In pursuing this attractant from many flowers of the same species, the pollinator transfers pollen to the stigmas, arranged with equally pointed precision, to all of the flowers it visits.
Anemophilous flowers use the wind to move pollen from one flower to the next. Examples include grasses, birch trees, ragweed, and maples. They have no need to attract pollinators and therefore tend not to be “showy” flowers. Male and female reproductive organs are generally found in separate flowers, the male flowers having a number of long filaments terminating in exposed stamens, and the female flowers having long, feather-like stigmas. Whereas the pollen of animal-pollinated flowers, tends to be large-grained, sticky, and rich in protein (another ‘reward’ for pollinators) anemophilous flower pollen is usually small-grained, very light, and of little nutritional value to animals.
Many flowers have close relationships with one or a few specific pollinating organisms. Many flowers, for example, attract only one specific species of insect and therefore rely on that insect for successful reproduction. This close relationship is often given as an example of coevolution, as the flower and pollinator are thought to have developed together over a long period of time to match each other’s needs. This close relationship compounds the negative effects of extinction. The extinction of either member in such a relationship would mean almost certain extinction of the other member as well. Some endangered plant species are so, because of shrinking pollinator populations.
Advantages and Disadvantages of Self and Cross-Pollination
Advantages of Self Pollination
- It is a certain process.
- Pollen grains are more utilized.
- Independent of agencies.
- The purity of the species is maintained.
Disadvantages of Self Pollination
- It results in weaker progeny.
- The viability of seeds is less.
- New varieties are not formed.
- Reduction in excellence takes place in progeny.
- Adaptability in progeny decreases.
- Gradually becomes extinct.
Advantages of Cross-Pollination
- Formation of healthier offspring.
- Offsprings are better adapted to struggle for existence.
- Seeds produced are more viable.
- The germinating capacity of seeds is much better.
- New varieties are produced.
- Offsprings show better adaptability to their environment.
Disadvantages of Cross-Pollination
- It is an uncertain process as it is dependent on agencies.
- Wastage of pollen grains takes place.
- The purity of species is lost.
- Dependent on another plant.
- Dependent on the agency.
- Reproduction stops in the absence of the agent.
- Less economical as various devices have to be adopted to attract pollinating agents.
Differences between Self Pollination and Cross Pollination
|1. Flowers involved||Takes place between flowers of the same genotype or within a single flower.||Takes place between flowers of different genotypes.|
|2. Certainty||A certain process.||An uncertain process.|
|3. Wastage of Pollen||Pollen grains are less wasted.||Pollen grains are more wasted.|
|4. Offspring||Offsprings are less healthier.||Offsprings are much healthier.|
|5. Seed||Seeds are less viable.||Seeds are more viable.|
|6. Seed Germination||Germinating capacity is not good.||Germinating capacity is much better.|
|7. Basic feature||Basic characters are not altered.||Basic characters are altered.|
|8. New feature||New characters are not formed.||New characters are formed.|
|9. Species Formation||New species is not formed.||New species are formed.|
|10. Degeneration of Species||Degeneration of species takes place.||Gradual prominence of species takes place.|
|11. Evolution||It has no role in evolution.||It plays an important role in evolution, i.e., the struggle for existence.|
|12. Quality||Quality deteriorates through generation.||Quality improves through generation.|
Evolution of Plant/Pollinator Interactions
Pollination syndromes are flower traits that attract pollinators and can be highly specialized. Plants and their pollinators are often in coevolutionary mutualisms.
The first fossil record for abiotic pollination is from fern-like plants in the late Carboniferous period. Gymnosperms show evidence of biotic pollination as early as the Triassic period. Many fossilized pollen grains show characteristics similar to the biotically-dispersed pollen today. Furthermore, the gut contents, wing structures, and mouthpart morphologies of fossilized beetles and flies suggest that they acted as early pollinators. The association between beetles and angiosperms during the early Cretaceous period led to parallel radiations of angiosperms and insects into the late Cretaceous. The evolution of nectaries in late Cretaceous flowers signals the beginning of the mutualism between hymenopterans and angiosperms.
Environmental Impacts of Pollination
Loss of pollinators, also known as Pollinator decline (of which colony collapse disorder is perhaps the most well-known) has been noticed in recent years. Observed losses would have significant economic impacts. Possible explanations for pollinator decline include habitat destruction, excessive use of chemical herbicides or pesticides, parasitism/diseases, air pollution, and other causes.
Significance of Pollination
Pollination is essential for the sexual reproductive process known as fertilization so that plants develop fruit and seeds. The pollen grains are the male part of the plant and need to be transferred from one flower to another. This is where pollinators come into play. They can be an assistant to pollination, where improved genetic transfer happens helping to increase plant diversity. In some cases, visits from pollinators are essential to produce seeds for the continuation of the species. Other plants may produce seeds but in smaller and lesser amounts. To attract pollinators, plants produce nectar or excess pollen that pollinators collect for either immediate or later use as their main food source.
This may all sound very scientific and unrelated to the daily lives of people until we realize how much of the food we eat depends upon this process, about 1/3 of the food you eat depends upon pollination. Even then, many may wonder why they should care, after all, this process has been going on for millions of years without any problem, right? What has changed? Why should we worry about this, isn’t it someone else’s problem? Here is what you should know. Many pollinator species (honey bees, bumblebees, butterflies, hummingbirds, moths, and flies) are in decline all around the world.
The first well-publicized indication of this was the Colony Collapse Disorder, which made worldwide headlines in 2006 when the populations of tens of thousands of honey bee colonies “disappeared” in a matter of months. This could mean a drastic change in this process of pollination. People and other animals depend upon plants for their food, shelter, clothing, medicine, and aesthetic needs, all provided by the process of pollination as without that reproduction in plants would not occur. It is high time that we realize its significance and take care not to cause any hindrance in this natural process.