Pollination

Pollination involves the transfer of the male gametophyte in seed plants, or pollen, to the receptive surface of the female organs, the stigma, for fertilization of the ovule. Although many plants are capable of self-fertilization, only some weedy and ephemeral plants routinely do so. Some seed plants disperse pollen by wind, such as many conifers; the grasses, sedges, and rushes (Poaceae, Cyperaceae, and Juncaceae); and temperate species of oaks and beeches (Fagaceae), elms (Ulmaceae), birches (Betulaceae), and some willows (Salicaceae). The unparalleled success of the angiosperms, however, at least partly results from a symbiotic relationship with pollinating insects that began approximately 140 million years ago and that evolved to include some birds and bats 100 million years later.

Pollination of flowers by animals has many advantages. It allows less pollen to be wasted, and (probably most important) it allows reproduction of plants distant from each other. Growing distant from other individuals of the species, as many trees do in tropical forests, allows more efficient exploitation of resources; it avoids intraspe-cific competition, and helps prevent serious outbreaks of plant-feeding insects that specialize on that species. Because reproduction in most of the 220,000 species of angiosperms is entirely dependent on certain kinds of animals, gene flow is controlled by the foraging behavior of the pollinator. The diversity of this group has been attributed to sexual isolation among populations caused by pollinators, leading to the rapid formation of many species.

The main pollinating groups are certain birds, bats, and many groups of insects. Among each group there has evolved particular kinds that are highly specialized for feeding exclu sively on the nectar and sometimes the pollen of flowers. These obligate pollinators almost always can hover in flight, and they have a long, extensible tongue for probing the recesses of flowers. Conversely, flowers have pollination "syndromes," or suites of features that adapt them for pollination by particular animals (Table 1). In the Western Hemisphere the main bird pollinators are the hummingbirds (Family Trochilidae) and honeycreepers (some Thraupidae). Sunbirds (Nectarinidae) are the main pollinators of Africa and Asia; honey-eaters (Meliphagidae) in Australia; and honey-creepers (Drepanidinae) in Hawaii. Bird flowers tend to be large, vivid red, with deep corollas. Among bats the main pollinators are some Pteropinae (fruit bats) of Africa and southeast Asia, though the Macroglossinae (nectar-feeding bats) are more specialized pollinators in southeast Asia and Australia. In the

Table 1

Specialized Pollinators and Characteristics of the Flowers They Visit

Pollinating Qroup

Region

Flower Characteristics

Anthesis

Color

Size

Form

Scent

Nectar

Plant Growth

Birds

Mostly tropical

Day

Vivid red

Medium to large

Tubular, zygomor-phic

Minimal

Copious

Trees, vines

Bats

Tropical

Night

Subtle, pale

Large

Radial, open, brushy

Musty, fruity

Very abundant

Trees, cacti

Long-tongued flies

Tropical to xeric

Day

Blue, violet

Small

Deep tubular, Minimal radial

Abundant

Herbs

Saprophagous flies

Tropical to temperate

Day and night

Green, light yellow

Large

Tubular vessels

Musky, putrid

None

Vines, herbs

Syrphid flies

Tropical to temperate

Day

White, yellow

Small to medium

Exposed, radial

Minimal

Moderate

Herbs

Butterflies

Tropical to temperate

Day

Blue, violet

Medium

Exposed/ tubular

Mildly sweet

Abundant

Trees, vines, herbs

Moths

Tropical to temperate

Night

White

Medium to large

Bell, to deep tubular

Fragrant

Copious

Vines, herbs

Bees

Tropical to xeric

Day

Various

Various

Various

Fragrant

None to abundant

Herbs to trees

Western Hemisphere, the Subfamily Glos-sophaginae in the family of leaf-nosed bats (Phyllostomidae) has specialized pollinators. Bat flowers tend to be large and fleshy, with numerous exposed, "brushy" stamens. Pollinating bats occur largely in the tropics and deserts, such as those that pollinate the large flowers of cacti. Some small monkeys and arboreal species of rats and opossums pollinate a few species of tropical trees. The proportion of angiosperms pollinated by vertebrates probably amounts to less than 1 percent of all species.

Insects, in contrast, pollinate perhaps 90 percent of all angiosperms. Flower visitors have been found in most orders, and, indeed, most species of insects have no particular adaptations for pollinating, even though they may be significant pollinators, particularly in the Coleoptera, nematocerous Diptera, and symphytan and lower apocritan Hymeno-ptera. For example, important generalized pollinators include some thrips (Thysano-ptera) on large Asian dipterocarp trees; cer-atopogonid midges on the chocolate tree, Theobromia cacao; as well as muscids and other kinds of calyptrate flies on umbellifers in northern latitudes. Various tiny Diptera become temporarily trapped within vessels of Dutchman's pipe vine (Aristolochia), Cerope-gia milkweeds (Asclepiadaceae), and aroids (Araceae). The flowers of these plants occur deep in the vessels. Particular species of flies are lured to the vessels by musky or putrid odors and cannot escape because of slippery surfaces and false windows. When they are released by changes in the shape of the vessel, generally a day later, they are dusted with pollen. The most effective and pervasive pollinators, though, are the long-tongued flies (Bombyliidae, Acroceridae, Apioceridae, and Nemestrinidae), the flower flies (Syr-phidae), Lepidoptera, the pollen or hover wasps (Vespidae: Masarinae), and, by far the most important, the bees (Apoidea). These groups alone comprise approximately 150,000 species.

There are about 20,000 species of bees, essentially all of which provision their nests with larval food made of nectar and pollen. Many forage on a particular family or even genus of plants (oligolectic), though the highly social honeybees (bumblebees, stingless bees, and especially Apis) visit diverse flowers (polylectic). Bees are intelligent insects, and individuals quickly learn which flowers are most rewarding, or how to force themselves into a flower, or even to steal nectar by chewing a hole in the bottom of the corolla from the outside. Although many zygomorphic flowers are pollinated only by bees, bees visit a wide range of flower types, so there is no typical "bee flower" syndrome.

Orchids (Family Orchidaceae) have the most impressive array of specialized adaptations for pollination. They are also among the most diverse families of plants (with approximately 25,000 species), and they evolved probably only within the past 40 million years. Such rapid diversification may be related to the exotic modes of reproduction. The flowers of some species mimic the shape of female wasps; males transfer pollen when they attempt to mate with the dummies. Other, South and Central American orchids secrete scents specific to one of nearly 200 species of orchid bees (Euglossinae). Male bees don't feed from the flowers, but collect scents that they use for displaying to females. Some orchids mimic the appearance and odor of mushrooms, and are pollinated by flies that normally congregate on mushrooms.

In the mid-Cretaceous, approximately 100 million years ago, angiosperms radiated explosively. There are only two fossil records from the Cretaceous of insects specialized for vis iting flowers. One is a nemestrinid fly approximately 120 to 130 million years old, the other a bee, Cretotrigona prisca, perhaps 65 to 70 million years old. The fly fossil possesses a long proboscis and wing venation typical of a hovering fly, and it may have been the earliest specialized pollinator. Cretotrigona is a member of the recently evolved group of stingless bees, found throughout the world's tropics. Bees evolved from generalized sphe-cid wasps somewhere in the mid-Cretaceous. The fossil record otherwise indicates that pollinators of Cretaceous flowering plants were small, unspecialized flies, wasps, moths, and beetles. Insects were on the scene when angiosperms first appeared in the Cretaceous, and both quickly adapted to specialized pollination.

—David Grimaldi

See also: Angiosperms; Arthropods, Terrestrial Bibliography

Barth, Friedrich G. 1985. Insects and Flowers, the Biology of a Partnership. Princeton, NJ: Princeton University Press; Buchmann, Stephen L., and Gary Paul Nabhan. 1996. The Forgotten Pollinators. Washington, DC: Island Press; Grimaldi, David. 1999. "The Coradiations of Insects and Angiosperms in the Cretaceous." Annals of the Missouri Botanical Garden 86: 373-406; Proctor, Michael, Peter Yeo, and Andrew Lack. 1996. The Natural History of Pollination. Portland, OR: Timber Press.

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