Figure 23.13 Flower markings on coneflowers (Rud-beckia). A. In ordinary light. B. In ultraviolet light.
Many bee-pollinated flowers are delicately sweet and fragrant. In contrast, flowers pollinated by beetles tend to have stronger, yeasty, spicy, or fruity odors. Beetles don't have keen visual senses, and flowers pollinated by them are usually white or dull in color. Some beetle-pollinated flowers don't secrete nectar but either furnish the insects with pollen or have food available on the petals in special storage cells, which the beetles consume.
Some flowers, including the stapelias of Africa (Fig. 23.14), smell like rotten meat. Short-tongued flies pollinate such flowers, which tend to be dull red or brown. These plants are related to our milkweeds, although superficially they don't resemble milkweeds at all. They are often called carrion flowers because of their foul odor and appearance. Flies with longer tongues may also pollinate bee-pollinated flowers.
Moth- and butterfly-pollinated flowers, like bee-pollinated flowers, often have sweet fragrances. Night-flying moths visit flowers that tend to be white or yellow—colors that stand out against dark backgrounds in starlight or moonlight.
Red flowers are sometimes pollinated by butterflies, some of which can detect red colors, but these insects are more often found visiting bright blue, yellow, or orange
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flowers. The nectaries of these flowers are at the bases of corolla tubes or spurs, where only moths and butterflies with longer tongues can reach. However, an enterprising bumblebee will occasionally bypass convention and chew through the base of a spur to get at the nectar.
Birds—particularly the hummingbirds of the Americas and the sunbirds of Africa—and the flowers they pollinate are also adapted to one another (Fig. 23.15). The birds do not have a keen sense of smell, but they have excellent vision. The flowers they visit are often bright red or yellow and usually have little if any odor. Bird-pollinated flowers also are typically large, are part of a large, sturdy inflorescence (flower cluster), or, in some cases, are produced on trunks of trees.
Birds are highly active pollinators and tend to burn energy rapidly. They must feed frequently to sustain themselves. Many of the flowers birds prefer produce copious amounts of nectar, thereby assuring repeated visits. The nectar is often produced in long floral tubes that keep most insects out. Some native California fuchsias and their relatives have long threads extending from each pollen grain (Fig. 23.16). When a hummingbird inserts its long bill into such a flower, the pollen grain threads catch on short, stiff hairs located toward the base of its bill, and in this way, the bird unknowingly transfers pollen from one flower to another.
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Bat-pollinated flowers, found primarily in the tropics, tend to open only at night when the bats are foraging (Fig. 23.17). These flowers are dull in color, and like flowers pollinated by birds, they either are large enough for the animal to insert part of its head or consist of ball-like inflorescences containing large numbers of small flowers that dust the visitor with pollen.
The very large Orchid Family, with approximately 35,000 species, has pollinators among all the types mentioned. Some of the adaptations between orchid flowers and their pollinators are extraordinary (Fig. 23.18). The pollen grains of most orchids are produced in little sacs called pollinia (singular: pollinium) that typically have sticky pads at the bases. When a bee visits such a flower, the pollinia are usually deposited on its head. The "glue" of the sticky pads dries almost instantly, causing the pollinia to
Figure 23.17 A bat visiting a flower of an organ-pipe cactus. The bat's head is covered with pollen. (Photo by Donna J. Howell)
adhere tightly. In some orchids, the pollinia are forcibly slapped on the pollinator through a trigger mechanism within the flower.
Members of Ophrys, a genus of orchids found in Europe and in North Africa, have a modified petal that resembles a female bumblebee or wasp. Male bees or wasps emerge from their pupal stage a week or two before the females and apparently mistake the flowers for potential mates. They try to copulate with the flowers, and while they are doing so, pollinia are deposited on their heads. When they visit other flowers, the pollinia catch in sticky stigma cavities. During a single visit by the pollinators, the pollinia removed from one flower are replaced with fresh pollinia from the next flower visited.
The pollinia of orchids pollinated by moths and butterflies are attached to their long tongues with sticky clamps instead of pads. The pollinia of some bog orchids become attached to the eyes of their pollinators, which happen to be female mosquitoes. After a few visits, the mosquitoes are blinded and unable to continue their normal activities.
Among the most bizarre of the orchid pollination mechanisms are those in which the pollinator is dunked in a pool of watery fluid secreted by the orchid itself; the pollinator escapes under water through a trapdoor. The route of the insect ensures contact with 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 disappears. The temporarily stupefied insect then recovers and flies away.
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