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ANATOMY OF A ROSE -- EXPLORING THE SECRET LIFE OF FLOWERS |
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TEN: Living Together THE YOUNG PLANT IS paying close attention. Photosensitive cells in its leaves and stems "see" across the spectrum of visible light, from far-red to ultraviolet. The plant knows it is day and not night. The plant knows that the days are getting longer. The plant is detecting a very hot day, with a lot of short, damaging ultraviolet wavelengths. In the plant, two genes are activated. They produce a colorless pigment, a sunscreen to filter out the harmful light. The young plant is busy sending down roots that taste and test and forage for nutrients. When they reach an area rich in minerals or salts, these roots quickly, greedily, grow lateral rootlets to collect the food. Above ground, the plant also tastes and tests, absorbing chemicals from the air or from the bite of an insect. The plant feels the wind buffet its stalk. It responds with a growth of cells that strengthen those tissues. The plant tingles, sensitive to small electric currents. A thunderstorm is coming. Rain means more growth. There are preparations to make. The big question remains: When to reproduce? At the right hormonal signal, the tip of a leafy shoot will stop adding leaves and start making a flower. Often these hormones are triggered not by light, but by the length of uninterrupted darkness, something being carefully measured by the plant's green leaves. Plants that bloom in the early spring or fall require a cycle of long nights and short days. Plants that bloom in the summer require a cycle of short nights and long days. Some plants need a second cue of temperature. They bud in the fall and bloom in the spring after months of cold weather. Some flowering plants, like tulips and hyacinths, are only cued by temperature. Some plants use rain as their stimulus to flower. Others wait for the dry season. This young plant is waiting for its own burst of hormones, which it will produce when the cycle of darkness is absolutely perfect. In all of this, in all of its short, sweet, individual life, the young plant is influenced by other plants. Most of them are competition. They eat the young plant's food, use its water, absorb its sunlight. The young plant has to respond quickly. If it were to sense now, for example, that it was not getting enough good light, it would begin to grow faster, taller, higher, struggling out of the shade of those nearby growing plants. If it were a sunflower, it would send out a toxic compound to inhibit the growth of that nearby dratted evening primrose. This young plant is also a competitor. In the end, surprisingly, some of these nearby plants may prove beneficial. Even now, the young plant has a relationship with a fungus that helps its roots absorb nutrients. (Fungi are not plants. But they are often closely associated with plants, as friends or enemies.) When this young plant has flowered, other flowers in the area may help attract pollinators or repel pests. Their flowers might be good models to mimic. They might have something to borrow or steal. The young plant lives in a community of plants. We humans have a great attachment to the word community, which we have invested with a sense of nostalgia. We need community. We used to have more community. We grieve for community. We have forgotten, perhaps, that community is not only supportive. Community also stones the adulterer and shames the unconventional. For the individual, community is a mixed blessing, both good and bad. Community is your neighbor. What's he doing now? *** FLOWERS CAN be good neighbors. Red skyrockets next to blue delphiniums bloom sequentially, first one species, then another. This extends the period during which a pollinator can search for food. Some insects need that time in order to reach sexual maturity, reproduce, and start the next generation. Working together, overlapping, different flowers support the pollinators they need for their own reproduction. Flowers that open at different hours of the day also allow pollinators to work all day long. Flowers with different rewards help sustain those pollinators: The bee collecting pollen for the hive might also need a drink of nectar, just to keep going. Flowers communicate with their pollinators through scent or odor molecules released into the air. Plants also talk to nonpollinators, other insects. Often enough, they ask for help. Parasitic wasps sting caterpillars into which the wasps lay eggs. As the wasp larvae grow, they feed on and kill their hosts. Naturally, caterpillars hide from wasps. In a large field of leafy plants, how do wasps find their prey? When plants "taste" certain secretions from a caterpillar, they send out compounds into the air. Wasps recognize and follow these compounds. Quick, the plant says, I've got him. He doesn't suspect a thing. He's right here, under this leaf. He's eating this leaf. Better hurry. In the 1980s, researchers used willows and maples to show that pest damage in one tree could lead to a greater resistance toward pests in neighboring trees. The damaged plants might have been sending out a chemical warning, allowing nearby plants to start their own defense. The "talking trees" studies were met with criticism and ridicule. Today, these scientists are being vindicated. In new and better-controlled experiments, scientists have shown clearly that plants attacked by pests do send out wound signals. Nearby plants, not yet damaged, are then better able to repel these pests or to attract their predators. Perhaps the airborne chemicals are simply being absorbed and used by the undamaged plant. More likely, these chemicals activate genes and begin the plant's own response.
Delphinium Most of this research has been done on crops. Normally, we wouldn't cheer on a wasp laying an egg in a paralyzed caterpillar -- except when that caterpillar is infesting our corn. We are relieved to know that tomato plants can defend themselves aggressively. We are interested in cabbage and cabbage aphids, in lima beans and spider mites, in sugar beets and armyworm larvae. As we learn more about plants, it is not hard to suspect similar defenses in a meadow of wildflowers: deep blue monkshood, purple-blue delphinium, sky blue flax, yellow columbine, golden sunflower, pale lousewort, elephant's head, shooting star, red skyrocket, Indian paintbrush. Perhaps these plants, too, are talking to each other. *** A MEADOW, OF COURSE, is not really a suburban neighborhood. It's more like a shopping mall. Everyone is here to do business. Reasonably, in this mall, there can only be so many shoe stores, so many restaurants, so many boutiques. When two flower species are too similar, when they compete too directly, one of them, reasonably, should go out of business. That doesn't seem to happen. More often, flowers find a way to adapt. Some shift to self-fertilization. Some start offering a different reward, perhaps an entirely new product, like resin or oil or perfume. Some change their flowering time. The red skyrocket grows near a certain penstemon. In different parts of the United States, either species might flower earlier or later, depending on what and how well the other one is doing. Between specific plants, however, and between species, competition can be intense. In the Southwest, the creosote bush and a small shrub called burro-weed share the desert's resources. The plants have become territorial, spaced well apart. A burro-weed root will stop growing if it enters into the root zone of a creosote bush. The creosote is emitting a growth inhibitor. Even the root of another creosote bush will be stopped, barred by the same chemicals in the soil. Burro-weeds are less effective against invading creosote bushes. But when the root from one burro-weed touches the root of another, there is also a decline in growth. When roots from the same plant meet, nothing happens. The plant recognizes both a self and a nonself. Allelopathy is a plant's bad magic, the release of substances that harm nearby plants. As early as 1 A.D., the Greek scientist Pliny saw that not much grew under the black walnut, its shade being "heavy" and "poisonous." Weeds like lamb's-quarter, thistle, nutgrass, and chickweed probably do not just compete for resources; they also affect the healthy growth of nearby vegetation. Many mustards and sunflowers are likely allelopathic, as are goldenrods and asters. In nature, pure stands of one kind of tree or grass might well suggest an enforced segregation: No trespassing. Get out. This means you. Some plants actively prey on each other. The seeds of witchweed only germinate in the presence of cereals like sorghum, maize, and barley, as well as crops of tobacco and cowpeas. As these plants start their growth, the weed also grows rapidly underground, reaching with witchy fingers toward its victim and developing a specialized rootlike organ. This organ allows the parasite to suck out nutrients and water from the roots of the host plant. Eventually the witchweed appears above ground and produces a pretty, red flower. By then, the farmer has probably lost her field of sorghum. In parts of Africa and Asia, witchweed can affect as much as 40 percent of arable land. These are places where a failed crop is a family disaster, where children die from hunger, where witchweed is as deadly as war. Other parasitic plants, like cancer-root or mistletoe, attack hardwood trees. The sandalwood tree gets its food from nearby grasses. Indian pipe has a ghostly, white, tubular stem with a single white blossom. The plant lacks chlorophyll and feeds off the fungi that have become part of a nearby tree's root system. In a meadow of wildflowers, among the delphinium and columbine, the strangest parasitism may be practiced by another kind of fungus, the rust fungus Puccinia, which infects mustard plants and reprograms them to grow in unnatural ways. An infected mustard plant looks very different from an uninfected one. The alien may have twice as many leaves and leaf rosettes and be half as tall. Elongated stems are crowned not by mustard flowers but by clusters of bright yellow petallike leaves that exude a sticky, sugary substance. Bees, butterflies, and flies visit this pseudoflower. In the same way that they spread pollen, they spread the reproductive cells, or spores, of the fungus, which now need to find and combine with other fungal spores. Importantly, the odor of the pseudoflower is different from the odor of the host plant's flower and vegetation. The scent is also different from concurrently blooming flowers such as buttercups and phlox. The pseudoflower produces a unique smell, which may encourage constancy, helping pollinators carry the fungal reproductive cells to another fungal fake, not to another flower. From a distance, a botanist might mistake this counterfeit flower for a real one. Up close, the rust fungus Puccinia can fool you and me. *** IN FLORAL COMPETITION, in that struggle to win, a different approach resembles aikido: Use your opponent's strength. In Batesian mimicry, a flower tries to look like another species. The orchid resembles a lily that provides nectar. The orchid does not provide nectar. This kind of deceit must be restrained. Too many mimics defeat themselves; their pollinators wise up and switch to another food source or don't wise up and die from lack of food. Batesian mimics depend on the abundance of their model. The more successful the model, the more successful the mimic. Most Batesian mimics pretend to be a flower that offers a reward like nectar or pollen. Some flowers push the envelope. One orchid, bobbing in the breeze, tries to imitate the general movement of an insect so that a certain territorial bee will swoop in and buffet/pollinate the flower in an effort to drive away the offending insect/orchid. The bees do not always cooperate. The orchid also self-fertilizes. Batesian mimicry is most often cited in animals, where the point is not to attract pollinators but avoid predators. The harmless kingsnake looks like the poisonous coral snake. An ugly, nontoxic caterpillar looks like an ugly, toxic caterpillar. In both instances, the resemblance to something else benefits the mimic and no one else. Another kind of mimicry, called Mullerian, is rather different. Here, the resemblance is mutually beneficial. A number of plant families include species with inflorescences of small, white florets. These umbellifers all have similar shapes and are visited by a variety of insects. They may be showing Mullerian mimicry, much like the convergence of so many yellow-centered white "daisies" or yellow-headed dandelions, hawkweeds, and their relatives. Sharing the same advertisements, borrowing from each other, favors the group as a whole by attracting more pollinators. In the U.S. West, as many as nine species from seven different families have flowers that are red and tubular and bloom at the same time. Migratory hummingbirds like red, tubular flowers. Each flower species places its pollen on different parts of the bird's body, which allows each flower to find and fertilize a similar flower. Eight of these species are combining their resources of nectar to support a larger number of customers. The ninth species practices Batesian mimicry. It is red and tubular and nectar-less. At some point, reading botany, the nonbotanist raises her head to ask, "Who was Bates anyway? Who was Muller?" Flashback to the Amazon River, 1848. Henry Walter Bates is traveling tippily in a canoe down a tributary, marveling at the monkeys, fish, and butterflies. Bates is twenty-three years old; his companion, Alfred Russel Wallace, twenty-five. They are English boys trying to make a living as naturalists and collectors. Bates explored the Amazon basin for another ten years. In the end, his collection of insects included eight thousand new species. One day, watching a swarm of South American butterflies, Bates recognized two different species, with the second species closely resembling the first. The first species is distasteful to predators. The second species tastes fine but uses its coloring to deceive predators. On his return to England, Bates read a paper about these butterflies before the Linnaean Society, the premier scientific group of his age. Some years later, the Brazilian zoologist Fritz Muller described another kind of mimicry. Two bad-tasting species might begin to look like each other in order to maximize their protection from predators. Viceroy butterflies were once thought to be Batesian mimics of monarch butterflies. Actually, to birds, both insects taste bad. The butterflies have combined their strengths. The predator learns, doubly quick, to spit them out. Bates's companion, Alfred Russel Wallace, also left the Amazon basin and continued collecting in Malaysia. What Wallace saw on those islands was similar to what Charles Darwin had seen in the Galapagos Islands, twenty years earlier. Excited, Wallace sent Darwin a letter. Darwin wrote back. Finally, concerned that Wallace might publish his theories first, Darwin finished his long-overdue work on natural selection. In 1858, the two papers, written separately by the two men, were read before the Linnaean Society. Darwin's Origin of Species was published within a year. Soon after, Bates presented his work on butterflies. It seemed a perfect example of how natural selection worked. Individuals that resembled the toxic models were favored, not eaten by predators. More of them passed on their genes. More individuals looked more like the model. The species, as a whole, became a mimic. Darwin sent Bates an enthusiastic letter. They began talking to each other. *** SCIENCE AND FLOWERS have a few things in common. It's all about community: cooperating, competing, stealing, borrowing, exploiting, combining. A field of wildflowers bobbles and glows: deep blue monkshood, purple-blue delphinium, sky blue flax, yellow columbine, golden sunflower, pale lousewort, elephant's head, shooting star, red skyrocket, Indian paintbrush. The young plant is out there somewhere, having already flowered, waving slightly in the breeze, exuding its sweet scent. Things are looking pretty good. Pollinators are interested. Pests aren't bad. The soil is excellent. And the neighbors seem friendly.
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