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| Pollination |
Pollination involves the transfer of pollen from anther to stigma in flowering plants, or from male cone to ovules in gymnosperms. There are two different types of pollination: self-pollination and cross-pollination.
Pollination is the process, in sexually reproducing plants (both angiosperms and gymnosperms), whereby the male sperm and female egg are joined via transfer of pollen (malemicrospore).
If the anthers and stigmas of the plants involved have the same genetic makeup or they are produced on the same plant, the type of pollination is called self-pollination. If anthers and stigmas are from plants with different geneticmakeups, the type of pollination is called cross-pollination.
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Self-pollination is efficient because pollen from the anther of a flower can be transferred easily onto the stigma of the same flower, owing to the proximity of the two parts.
On the other hand, cross-pollination is risky because the transfer of pollen involves long distances and precise destinations, both of which depend on animal pollinators.
In areas with few animal pollinators, the opportunities for cross-pollination may be greatly reduced(one of the many reasons that preserving biological diversity is an important ecological issue).
In spite of the risk associated with cross-pollination, most flowers have mechanisms that promote this kind of pollination. Cross-pollination increases the likelihood that offspring are vigorous, healthy, fertile, and able to survive even if the environment changes.
Self-pollination leads to off-spring that are less vigorous, less productive, and more subject to inbreeding depression (weakening of the offspring as a result of inbreeding).
When certain consumers forage among plants for food, they often come in contact with flowers. Many insects and other animals become dusted with pollen, and in the course of their travel they unintentionally but effectively bring about pollination.
Throughout the evolutionary history of flowering plants, many pollinators have coevolved with plants. Coevolution occurs when the floral parts of a plant and the body parts and behavior of the pollinators become mutually adapted to each other, thereby increasing the effectiveness of their interaction.
In many instances, the relationship between the plant and pollinator has become highly specialized, resulting in mutualism, which is interaction where both organisms benefit from each other.
In the case of pollination by animals, the pollinator receives a reward from the flower in the form of food. When the pollinator moves on, the plant’s pollen is transferred to another plant.
The adaptations between the flower and its pollinators can be intricate and precise and may even involve force, drugs, deception, or sexual enticement. In flowering plants, pollination is mostly due to insects or wind, but birds, bats, and rodents also act as pollinators for a number of plants.
Insects
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| Pollination by insect |
Insect pollination occurs in the majority of flowering plants. There is no single set of characteristics for insect-pollinated flowers, because insects are a large and diverse group of animals.
Rather each plant may have a set of reproductive features that attracts mostly a specific species of insect. The principal pollinating insects are bees, although many other kinds of insects act as pollinators, including wasps, flies, moths, butterflies, ants, and beetles.
Bees have body parts suitable for collecting and carrying nectar and pollen. Their chief source of nourishment is nectar, but they also collect pollen for their larvae.
The flowers that bees visit are generally brightly colored and predominantly blue or yellow—rarely pure red, because red appears black to bees. The flowers they visit often have distinctive markings that function as guides that lead them to the nectar.
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Bees can perceive ultraviolet (UV) light (a part of the spectrum not visible to humans), and some flower markings are visible only in UV light, making patterns perceived by bees sometimes different from those seen by humans. Many bee-pollinated flowers are delicately sweet and fragrant.
Moth- and butterfly-pollinated flowers are similar to bee-pollinated flowers in that they frequently have sweet fragrances. Some butterflies can detect red colors, and so red flowers are sometimes pollinated by them.
Many moths forage only at night; the flowers they visit are usually white or cream-colored because these colors stand out against dark backgrounds in starlight or moonlight.
With their long mouth parts, moths and butterflies are well adapted for securing nectar from flowers with long, tube-shaped corollas (the petals collectively), such as larkspur, nasturtium, tobacco, evening primrose, and amaryllis.
The flowers pollinated by beetles tend to have strong, yeasty, spicy, or fruity odors. They are typically white or dull in color, in keeping with the diminished visual sense of their pollinators. Although some beetle-pollinated flowers do not secrete nectar, they furnish pollen or other foods which are available on the petals in special storage cells.
Birds
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| Pollination by bird |
Birds and the flowers that they pollinate are also adapted to each other. Birds do not have a highly developed sense of smell, but they have a keen sense of vision. Their flowers are thus frequently bright red or yellow and usually have little, if any, odor. The flowers are typically large or are part of a large inflorescence.
Birds are highly active pollinators and tend to use up their energy very rapidly. Therefore, they must feed frequently to sustain themselves. Many of the flowers they visit produce copious quantities of nectar, assuring the birds’ continued visitation.
The nectar is frequently produced in long floral tubes, which prevent most insects from gaining access to it. Examples of bird-pollinated flowers are red columbine, fuchsia, scarlet passion flower, eucalyptus, hibiscus, and poinsettia.
Bats and Rodents
Bat-pollinated flowers are found primarily in the tropics, and they open only at night, when the bats are foraging. These flowers are dull in color, and like bird-pollinated flowers, they are large enough for the pollinator to insert part of its head inside.
The plants may also consist of ball-like in florescences containing large numbers of small flowers whose stamens readily dust the visitor with pollen.
Bat-pollinated flowers include bananas, mangoes, kapok, and sisal. Like moth-pollinated flowers, flowers that attract bats and small rodents open at night. Mammal pollinated flowers are usually white and strongly scented, often with a fruity odor.
Such flowers are large, to provide the pollinators enough pollen and nectar to fulfill their energy requirements. The flowers are also sturdy, to bear the frequent and vigorous visits of these small mammals.
Orchid Pollinators
The orchid family has pollinators among bees, moths and butterflies, and beetles. Some of the adaptations between orchid flowers and their pollinators are extraordinary. Many orchids produce their pollen in little sacs called pollinia, which typically have sticky pads at the bases.
When a bee visits such a flower, the pollinia are usually deposited on its head. In some orchids, the pollinia are forcibly “slapped” on the pollinator through a trigger mechanism within the flower.
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| Orchid diagram |
In some orchids, a petal is modified so that it resembles a female wasp or bee. Male wasps or bees emerge from their pupal stage before the females and can mistake the orchids for potential mates.
They try to copulate with these flowers, and while they are doing so, pollinia are deposited on their heads. When the wasps or bees visit other flowers, the pollinia are caught in sticky stigma cavities.
When moths and butterflies pollinate orchids, the pollinia become attached to their long tongues by means of sticky clamps instead of pads. The pollinia of certain bog orchids become attached to the eyes of the female mosquitoes that pollinate them.
After a few visits, the mosquitoes are blinded and unable to continue their normal activities (a good example of a biological control within an ecosystem).
Among the most bizarre of the orchid pollination mechanisms are those whose effects are to dunk the pollinator in a pool of watery fluid secreted by the orchid itself and then permit the pollinator to escape underwater through a trap door. The route of the insect ensures contact between the pollinia and stigma surfaces.
In other orchids with powerful narcotic fragrances, pollinia are slowly attached to the drugged pollinator. When the transfer of pollinia has been completed, the fragrance abruptly fades away, and the insect recovers and flies away.
Wind and Water
Wind pollination is common in those plants with inconspicuous flowers, such as grasses, poplars, walnuts, alders, birches, oaks, and ragweeds. These plants lack odor and nectar and are, hence, unattractive to insects. Furthermore, the petals are either small or absent, and the sex organs are often separate on the same plant.
In grasses, the stigmas are feathery and expose a large surface to catch pollen, which is lightweight, dry, and easily blown by the wind. Because wind-pollinated flowers do not depend on animals to transport their pollen, they do not invest in the production of rewards for their visitors. However, they have to produce enormous quantities of pollen.
Wind pollination is not efficient because most of the pollen does not end up on the stigmas of appropriate plants but on the ground, bodies of water, and in people’s noses (a major cause of allergic reactions). Wind pollination is successful in cases where a large number of individuals of the same species grow fairly close together, as in grasslands and coniferous forests.
Water pollination is rare, simply because fewer plants have flowers that are submerged in water. Such plants include the sea grasses, which release pollen that is carried passively by water currents.
In some plants, such as the sea-nymph, pollen is threadlike, thus increasing its chances of coming in contact with stigmas. In eelgrass, the entire male flower floats.
