| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature Night Bloomers And Their Matchmakers
The lights of Las Vegas start to flicker on at sunset, tempting passersby with the promise of riches. In the nearby Mojave Desert, the yucca plant sits quietly under the stars, and, to the human eye, its apparent stillness seems in complete contrast with the neon advertisements in the neighboring valley. But this unassuming plant sends out signals that moths can read as easily as flashing lights. When darkness falls, night-blooming plants here and around the world begin to call pollinators to their flowers. During the day, when most interactions between insects and plants occur, flowers advertise their presence with bright colors to attract insects, which use vision to find an appropriate flower. Flowers that bloom at night take a different approach. They cater their resources to appeal to the senses of nighttime pollinators, which excel at reading chemical signals. The practice of blooming in darkness arose independently in plant families ranging from water lilies to cacti, suggesting it must be highly beneficial. This arrangement can also benefit daytime pollinators. For example, certain bumblebees in the Himalayas find warm shelter for the night inside furry composite Sussurea flowers. In exchange, the bees’ actions pollinate the hospitable plants. Other times, daytime visitors looking for a flower’s nectar are forced into staying overnight. The giant water lily, Victoria amazonica, which produces leaves capable of supporting a weight of 150 pounds, blooms for only two nights. It attracts beetles during the day, then traps them inside when it closes for the night. Well-doused with pollen, the unwitting prisoners are released on the second day, before the flower sinks. While most nocturnal pollinators are either bats or moths, some bees can also extend their activity into the night. The carpenter bee, Xylocopa tenuiscapa, is one of the few nocturnal bee species whose activity is relatively unaffected by low nighttime temperatures, and who use moonlight for navigation. In India, these bees are the principal pollinators of the night-blooming tree, Heterophagma quadriloculare. In the Amazon rainforest, the small red flowers of the mimosoid tree (Parkia velutina) attract Megalopta bees, which populate the canopy after dark and feed on pollen. Pollination by nocturnal insects is considered to be an intermediate stage as this plant genus evolves from insect pollination by day to bat pollination by night. The yucca has become completely dependent on the yucca moth, Tegeticula yuccasella, during the course of its evolution. The insect tricks the plant into nurturing its young. The female moth collects pollen from one yucca flower and forms it into a ball. She then lays her eggs into the ovary of another plant’s flower and places the pollen ball on its stigma. Fertilizing the flower in this way ensures that the ovary (and the eggs inside it) will not wither. The larvae that hatch from these eggs will eat only a fraction of the seeds, leaving the rest to complete their normal development. Until recently, T. yuccasella was believed to pollinate most yuccas. However, recent studies have revealed that 13 closely related moth species pollinate the eight species of yucca found in the United States. Such diversity arises when plant and insect populations evolve together. The hawkmoths, which in flight and size can be easily confused with hummingbirds, are among the most important nocturnal pollinators. In Madagascar, the extraordinary number and diversity of long-spurred orchids likely evolved in unison with the diverse group of long-tongued archaic hawkmoths found on this isolated island. Darwin suspected a relationship between hawkmoths and orchids existed decades before it was observed. After studying the long nectar reservoirs of the night-blooming orchid Angraecum sesquipedale, Darwin predicted that it would be pollinated by a nocturnal hawkmoth with a tongue ten to eleven inches long. The Xanthopan morgani predicta hawkmoth, with its twelve-inch proboscis, was described 40 years later. In this case, the adaptation extends beyond the custom fit of the moth’s tongue to the orchid’s flower structure. The orchid’s pollen adheres to the moth’s head by means of a sticky pad, to which pollen is connected with elastic cord. By the time the hawkmoth visits another plant, the cord has dried up, and the pollen is successfully transferred. After a pollinator absconds with a plant’s pollen, it normally has only one chance to pollinate the right flower. So plants must make sure that insect messengers deliver their precious cargo to the correct address. To do this, plants have evolved all kinds of fragrances they release into the night. Flowers that bloom at night produce some of the most sublime fragrances. The night-blooming ylang-ylang tree of the Philippines produces an extract added to Chanel No 5. And many jasmines that bloom at night are used extensively in the perfume industry. As early as 600 B.C., Greek poet Sappho wrote of the influence that night fragrances might have on humans’ romantic side:
Its beauties charm the gods above;
Orchid fragrances become a true “breath of love” to insects when they mimic moth pheromones. As some day-blooming orchids fool male bees by resembling a much-desired female in appearance, night-blooming orchids produce trails of pheromone-like chemicals that moths follow through the darkness until the landing platform of a flower becomes visible. This chemical deception insures that only one species of pollinator will visit the flower, improving the odds that the pollen will reach a fertile destination. Another notable adaptation found in night-blooming flowers is nectar secretion by night. Experiments have shown that if nectar is artificially removed from the flower of Caesalpinia gilliesii after 9 p.m., the flower reacts by increasing its nectar secretion. The reverse is observed earlier in the day. In this way, the flowers encourage visits by animals active only at night. Another example of this strategy is the Mexican columnar cactus, Stenocereus stellatus, in which nectar production peaks between 2 and 4 a.m. The three species of glossophagine bats that pollinate this cactus forage during that time. Plants have been offering their pollination jobs to bats for millions of years. The practice arose independently in 27 families and at least 500 species of plant. Not surprisingly, bats have evolved in parallel. For example, the front teeth of glossophagine bats have disappeared, as they are no longer required for chewing fruit. Instead, these nectar-feeding bats have developed elongated skulls and extended tongues covered with specialized papillae for collecting nectar. Their facial hair traps pollen during feeding and helps cross-fertilize these plants. The flower of the Malaysian Oroxylum iridicum tree opens two-and-a-half hours after sunset for only a couple of hours, which coincides precisely with nectar-foraging activity of the dawn bat (Eonycteris spelaea). The cup-shaped flowers are just wide enough to admit this bat’s shoulders, effectively excluding larger bats. Because bats can move pollen over considerable distances, plants such as the columnar cactus can maintain wide ranges. This cactus relies entirely on bats for pollination services. When its flowers were covered at night during experiments, they produced no fruit. In fact, some plants have become so dependent on bats for pollination that they cater to these winged mammals’ bizarre tastes. Some have begun to produce strong organic odors instead of the usual fruity fragrances. The Key tree-cactus (Cereus robinii) found in the Florida Keys and Cuba, whose flowers open only for a single night, refutes the old vampire myth by attracting bats with a garlic-like odor. Bat-pollinated bromeliads Guzmania mucronata and Vriesea gladioliflora smell like cooked cabbage, while G. confusa has an aroma reminiscent of garlic and burned rubber. The interaction between night bloomers and their pollinators relies on a number of signals that escape human perception. Had humans not lost the ability to pick up on these nocturnal signals during the course of their evolution, our brains would probably have suffered an information overload. Instead, our information gathering became almost entirely visual and auditory. As a species we took an alternate route, developing sophisticated machines such as volatile collectors, gas chromatographs, and mass spectrometers to intercept and analyze the chemical communications that surround us. Our almost monthly advancements in the fields of volatile and pheromone chemistry are only beginning to shed light on what plants and animals whisper to one another in the dark. Andrei Sourakov is a research scientist who has worked at the California Academy of Sciences and in Florida. |
