Pollination Biology

Plant pollination is almost as diverse as the plant community itself. Self-pollination occurs in some plant species when the pollen (male part) produced by the anthers in a single flower comes in contact with the stigma (female part) of the same flower or with the stigma of another flower on the same individual. Self-pollination does not allow much modification in the genetic makeup of the plant since the seeds produced by self-pollination create plants essentially identical to the individual producing the seed. A plant population that has all individuals identical in form, size, and growth requirements has little possibility of modifications to allow for change in its environment.

Most plant species have evolved ways to ensure an appropriate degree of interchange of genetic material between individuals in the population, and cross-pollination is the normal type of pollination. In this case flowers are only pollinated effectively if the pollen comes from another plant. Plants benefit most by being pollinated by other individuals because this broadens the genetic characteristics of individual plants. As a result, they are more adaptable to necessary changes.

Fertilization takes place when the pollen comes in contact with the stigma of a flower. Pollen reacts with the stigmatic fluids and germinates, then grows as a tube through the stigma and down the style to the ovary cavity. There the sperm unites with the ovule and develops into a seed.

There are both physical and chemical or genetic barriers to fertilization. Sometimes pollen grains are inhibited from germinating by a chemical imbalance, or germination is controlled genetically. Sometimes there is no genetic barrier but the pollen is simply not placed in the proper position in the flower. This is caused by physical restraints, such as large differences in the length of the stamens and the styles. Some species of plants have long-styled forms and short-styled forms to discourage self-pollination. The shape of the corolla and the positioning of the sexual parts (style and stamens) may also ensure that only an insect of a certain size and shape can pollinate a flower. Most of the pollination syndromes mentioned next involve these features.

Wind Pollination

Perhaps the simplest form of pollination is that of wind pollination, which is common in many of the early spring-flowering trees in temperate areas. The oak (Quercus in Fagaceae), maple (Acer in Aceraceae), birch (Betula in Betulaceae), hickory (Carya in Juglandaceae), and many other trees in temperate forests are pollinated by wind-borne pollen. Air currents and moisture in early spring make this a suitably efficient method of pollination because the trees have not yet produced leaves, and flowers are exposed, often in slender catkin -type inflorescences that dangle with long stigmatic hairs capable of catching the pollen. The corn plant (Zea mays in Poaceae) is another wind-pollinated plant. Its long, silky tufts of fiber, which constitute the styles, are well suited to trapping the airborne pollen. Wind polli-nation is rare in the tropics, perhaps owing to the fact that trees are usually not leafless and wind-borne pollen would not be very efficient. Moreover, heavy daily rains common in the tropics would keep anthers too wet for effective wind pollination. Nevertheless, one type of airborne pollination in the tropics does exist. Understory shrubs in the Urticaceae have anthers that open explosively and throw the pollen into the air sufficiently far to effect at least self-pollination of other inflorescences on the plant.

Insect Pollination

Plants have coevolved with insects, and each insect pollinator group is closely associated with a particular type of plant. This is called a pollination syndrome. Without even knowing the exact insect that pollinates a plant, the type of insect that will visit the plant can be predicted because of the shape, color, size, and scent of the flower involved.

Bees.

Most bees visit flowers that are bilaterally symmetrical (zygomorphic or not round in outline) and have a landing platform on which the bee can be properly oriented for entry. An example is the ordinary household pea plant (Pisum sativa ) and most other members of the subfamily Papilionoideae of the legume family (Leguminosae). Bee flowers tend to have an aroma as well because bees have a good sense of smell. Bees are among the most prevalent of plant pollinators and are remarkably diverse in size and shape. The honeybee is the most obvious example of this pollination syndrome, and the economic importance of the honeybee to fruit and seed production is enormous. Without them and other similar bees, many of our food crops would not exist.

Bees are believed to be intelligent, and some bees return to the same plant on a regular basis (a behavior called trap lining). In such cases, the plants commonly produce just one or a few flowers each day, ensuring that all are pollinated without investing as much energy as it would in a mass-flowering species. Other species produce massive numbers of flowers so that the plant can attract large numbers of pollinators. These are two opposing strategies that accomplish the same goal: to produce seeds for reproduction.

Bees are more likely than other insects to establish a one-to-one polli-nation system. Many plants produce a special scent that attracts only one or a few different species of bees. This is especially common in orchids and aroids. Some flowers have evolved to produce a "style" that mimics the insect itself in appearance. Most orchids are so dependent on pollination by a single type of bee that they put all of their pollen in a single package (called pollinia) that is picked up by the bee. In the case of the Catasetum orchid, the sticky pollinia is forced onto the head of the bee, where it adheres until it in turn is passed onto the style of another plant. This one-chance system, though risky, ensures that all of the pollen load arrives exactly where it is most effective.

Flies.

These are less-important pollinators, but they are essential in the pollination of some temperate and many tropical flowering plants. Flies generally visit flowers that smell foul, often with scents of decaying meat or feces. Many tropical aroids (Araceae), including such mammoth plants as Amorphophallus, which often produce inflorescences, are pollinated by flies. The skunk cabbage (Symplocarpus foetidus ), another aroid and one of the earliest plants to flower in the spring (even emerging from snow banks), is pollinated by flies. Flies are seemingly less intelligent than bees and fly polli-nation syndromes often involve deceit and entrapment. Flies are attracted to foul-smelling plants because they anticipate finding a suitable substance, such as dung or decaying meat on which to lay their eggs. Once inside, however, the flies are unable to leave the inflorescence. In Aristolochia (Aristolochiaceae), the corolla tube is folded into a bend with stiff hairlike appendages at the base, orientated to allow the fly to enter easily. However, only after the insect has been inside long enough to ensure pollination do the appendages become loose enough to allow the fly to depart the lower part of the corolla. The tropical genus Dracontium (Araceae) has no real trap but instead the lower part of the spathe is white or apparently transparent, and the opening is curved so that little light enters. The not-so-intelligent fly tries repeatedly to leave through an opening that does not exist and in the process crashes against the inflorescence to deposit pollen it might be carrying from visiting other flowers.

Moths and Butterflies.

Both have the ability to unroll their long tongues and extend them into long slender flowers. Members of the Asteraceae (Compositae), such as dandelions, sunflowers, goldenrods, and other genera, are usually visited by butterflies during daylight hours. Their moth counterparts usually fly at night and pollinate a different type of tubular flower, ones that are usually white or very pale in color, making the flowers easier to see in the dark, and flowers that produce a sweet-smelling aroma, which also makes locating them easier. Hawk moths have especially long tongues and can pollinate tropical flowers with the corolla tube up to ten inches long. One such flower, Posoqueria latifolia (Rubiaceae), has a special arrangement of the stamens that causes them to be held together under tension until the anther mass is touched by the pollinator. At this point, it is released with great force and the stamens then throw a mass of pollen into the face of the pollinator. This pollen mass is carried onto the next flower, where the style is now properly positioned to accept the pollen.

Beetles.

Although somewhat rare in temperate areas, this is quite common in the tropics. Beetles often fly at dusk, enter the inflorescence, and stay there until the following evening at dusk. Beetle pollination syndromes often involve thermogenesis, an internal heating of some part of the inflorescence caused by the rapid oxidation of starch. The inflorescence of Philodendron (Araceae) consists of a leaflike spathe that surrounds the spadix where the flowers are aggregated. The flowers of philodendron are unisexual, with the female flowers aggregated near the base and the male flowers occupying the remainder of the spadix. In most cases, it is the spadix that warms up and the temperature is commonly well above ambient temperature (that of the surrounding air). The elevated temperature is associated with the emission of a sweet scent that helps attract the beetles. Once inside the base of the spathe (the tube portion), the beetles feast on the lipid-rich sterile male flowers at the base of the male spadix, and they also often use this space for mating. On the following day, when the beetle is departing, the stamens release their pollen and the beetle departs covered with it. Beetles pollinate many species of palms (Arecaceae), members of the Cyclanthaceae, many Araceae, and even giant tropical water lilies such as Victoria amazonica. The skunk cabbage mentioned earlier under fly pollination is also thermogenic, and it is this feature that enables it to melt its way through the snow in the early spring.

Birds and Mammals.

Although vertebrate pollinators are not as common as insect pollinators, they do exist, and include birds and mammals. Bird pollination syndromes usually involve colorful, scentless flowers that are designed to attract birds, which have excellent vision but a poor sense of smell. In the western hemisphere, hummingbirds are the most common pollinators, and their typically long tongues mean that hummingbird flowers are typically long and tubular. Many hummingbird-pollinated flowers are either red or have red-colored parts, such as bracts, which attract the bird to the inflorescence. Many tropical members of the Gesneriaceae have yellow rather than red flowers, but the leaves associated with the flowers are heavily marked with red or maroon and are clearly visible to the hummingbird pollinators.

Mammal pollination is rare but is becoming increasingly more well known among tropical animals. White-faced monkeys (Cebus capuchinus ) are known to pollinate balsa trees (Ochroma pyramidale in Bombacaceae) as they search deep in the big tubular flowers for insects. Bats are more common as effective pollinators because they are skilled fliers. Because bats fly at night, the bat pollination syndrome involves pale-colored, usually large, often pen-dent broadly open tubular flowers, such as Coutarea hexandra, a tropical member of the coffee family (Rubiaceae). However, bat pollination syndromes may also involve such plants as Inga (Leguminosae), which have many flowers with broad tufts of stamens through which the bat can extend its tongue to forage for pollen or nectar. Some unusual mammal pollinators include giraffes, who are known to pollinate Acacia trees with their facial hairs, and lemurs, who pollinate Strelitzia in Madagascar.

see also Breeding Systems; Flowers; Interactions, Plant-Insect; Interactions, Plant-Vertebrate; Reproduction, Fertilization and; Reproduction, Sexual.

Thomas B. Croat

Bibliography

Faegri, K, and L. van der Pijl. The Principles of Pollination Ecology. New York: Pergamon Press, 1966.

Percival, Mary S. Floral Biology. New York: Pergamon Press, 1965.

Real, Leslie. Pollination Biology. New York: Academic Press, 1983.