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ANATOMY OF A ROSE -- EXPLORING THE SECRET LIFE OF FLOWERS |
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FOUR: The Shape of Things to Come THE PASSIONFLOWER IN MY NEIGHBOR'S YARD looks constructed, designed by an engineer who has heard about flowers but never actually seen one, designed by a woman in love with helicopters. The passionflower is layered. Five green sepals and five green petals form the base. A fringe of spiky modified petals swirls above like a sea anemone in concentric color: an outer ring of lavender, a ring of white, a wide ring of purple, a ring of green, a thin ring of purple, a ring of light green, a center of dark purple. From this center rises a stalk almost an inch high. Five pads resembling the brake pads of a bicycle hang down. Their bottoms shine with yellow pollen meant to dust any bee or fly drawn to the mosaic of rings and the nectar at its base. Above the brake pads, three stigma lobes splay out like the rotors of a funny hat, a helicopter hat, a beanie hat. The whole thing looks preposterous. When explorers from Europe first saw a passionflower in the New World, they immediately sent one to the pope, claiming that the flower reminded them of the thorny wreath of Christ and his passion on the cross. What on earth were they thinking? Perhaps, like me, they simply had to have some response, a metaphor to hold on to in the face of the passionflower's incomparable and willful existence. It's a sea anemone. It's a helicopter. It's the passion of Christ. *** THE PASSIONFLOWER IS a circle that can be divided into equal parts. Circle flowers are accessible to a variety of insects that simply land anywhere and walk to the flower's center. Circle flowers are easy to manipulate. They're democratic. Botanists call the passionflower a perfect flower because it has both male and female organs. At the center of a perfect flower is the female organ, or carpel, which holds the ovules, or unfertilized eggs. The base of the carpel is the ovary. From the ovary rises a neck, or style, topped by one or more stigmas. The stigma receives the pollen grain. Sperm in the pollen grain will travel down the style to fertilize the eggs.
Passionflower Surrounding the female carpel is usually a ring of male organs, or stamens. Each stamen is made up of a stalk ending in an anther, which produces pollen. The petals, collectively called the corolla, surround the stamens. The leaflike sepals that first enclosed the bud, collectively called the calyx, surround and grow under the petals. If you look at flowers a lot, every day, like eating a good breakfast and getting regular exercise, you will remember these terms. Otherwise, you will not. You will remember the metaphor, a word like helicopter. Some flowers do not have all these parts. They may have only the male or the female organ. They may have only one carpel or many carpels. A carpel may have one ovule or, like some orchids, half a million.
Parts of a Flower Some circle flowers, such as daisies, are really inflorescences, or arrangements of flowers. At the center of these is a community of individuals. Each may have its own tiny carpel, stigma, stamens, corolla, and calyx. Many flowers, of course, are not circles. Cut in half, a bilaterally symmetrical flower will have two mirror images. The lower part may look very different from the upper. In bilateral flowers, the petals have often fused to become a funnel, a bell, a trumpet, a pipe, a hollow slipper, a long spur, a mushroomy gill, or something that resembles a wasp, a bee, or something else. Stamens may be fused to the inside of the corolla or to other parts of the flower, like the ovary or style. In all flowers, sepals can start acting like petals. When botanists cannot tell the difference, they call the sepal/petal a tepal or a petaloid sepal. (A tepal may be what really forms the base of the passionflower.) In grouping plants, the assumption is that each species in a plant family evolved from the same ancestor. But within a plant family, flowers can include a startling range of flower shapes, from circles to gullets to spurs. Indeed, as flowers evolve through time, they seem to be in a state of constant transformation: melding, moving, merging, flowing. This liquidity is entirely practical. Flowers change their shape in response to pollinators, predators, or the environment. A flower may "want" to attract a certain bee, defend itself against ants, or conserve water. Bilaterally symmetrical flowers, for example, tend to control more precisely how a pollinator gets pollen and leaves it behind. Many orchids have a beautifully decorated lower lip, which provides insects with a good place to land. From here, the animal stands and pushes its head or entire body into the flower's upper gullet. As the pollinator backs out, the pollen clings to the thorax, abdomen, or some other place likely to contact the stigma of the next orchid. In a pea blossom, a single large banner petal signals the bee. Two smaller petals, or wings, surround a keel petal. When the bee lands on the keel, its weight causes the petal to dip. The enclosed stamens pop out and dust the bee. In a delphinium, a long spur is attached to a miniature petal with wings. An outer ring of five tepals attracts the bumblebee and provides support as the insect inserts its head between the wings and flicks its long tongue into the spur, searching for nectar. Stamens at the top of the spur cover the insect's head with pollen. When hummingbirds pollinate the delphinium, they hover and make no use of the flower's support. Instead, the pollen rubs off on their beaks. Form follows function. It's nicely alliterative. But flowers are a little more complex than that. Flowering plants are called angiosperms, sperma for "seed" and angeion for "inside a vessel," because their closed, fleshy carpels protect the developing seed from predators and a hostile environment. Before angiosperms, gymnosperms (gymno for "naked") like conifers were the dominant vegetation. In the history of plant evolution, carpels proved to be a tremendous maternal leap. The angels in heaven cried out at this one. Hosannas resounded. Obviously it is important how well a closed carpel protects its seed. In a few flowers, the ovary is high above the other organs and thus more vulnerable to attack. In roses and cinquefoils, the ovary is still high but is surrounded by other organs and petals that give it more protection. In orchids -- and in the small disk flowers of a daisy -- the ovary is covered by layers of fused tissues, buried deep and well defended. Sometimes that defense is an example of form following function. But sometimes it is more accidental, a result of the fusion of flower parts for other reasons. As Peter Bernhardt points out in his book The Rose's Kiss, "An orchid flower needs a lot of fusion to hold a pollinator between its lip petal and column." Thus flower organs have united. In so doing, they covered the ovary. This is good. But it may have also resulted in other changes, other problems to solve, other advantages to exploit. The flower keeps fiddling. Form follows function. It's mostly true. *** I AM CUTTING INTO the tiny spur of a delphinium, pretending to search for that source of nectar that feeds the hummingbird. I use a little scalpel, a little pair of tweezers, and a magnifying glass. My fingers seem gigantic. I peer. I squint. I insert my scalpel. I will never see what I really want to see, what I fancy dwells at the bottom of the delphinium's spur. What does evolution look like? It is not so crazy to squint for it in a small place. You could well say that evolution is minuscule since it begins with changes in a gene or cell. As cells divide and duplicate themselves, the genes carried on their chromosomes must also divide and duplicate. In this process, the gene occasionally mutates, or changes. For the organism, the change may be good or bad or neutral. In any case, the duplicated gene is slightly different. In cross-breeding, one set of genes from a parent combines with a set of genes from another parent to create a new individual. This results in even more change and more genetic variation within the group. The process of natural selection takes over. A beneficial genetic change may help an individual survive and reproduce in a particular environment. The change may be passed on to the individual's offspring, who will have an advantage over other individuals. These changes accumulate, individual by individual, generation by generation, until the population itself has taken a new shape. The idea bears repeating. Genetic changes that give the individual a closer fit to its environment and that therefore result in an individual's higher rate of survival and reproduction will gradually become the status quo. A species can be broadly defined as a group of organisms able to breed with each other to produce a new, fertile generation. A species can change over time into another species. In this case, called phyletic evolution, only one species remains. Speciation, however, is when a species splits into two species. Speciation is why we have roses and orchids and philodendrons. Natural selection is only a part of this process. Something else has to happen first. Often, two or more populations become isolated from each other for a variety of reasons and in a variety of ways. A continent drifts north. An island rises from the sea. An asteroid crashes into earth. External forces separate populations, as might internal ones. The separated groups evolve along different paths. At some point, they become separate species. In a study of monkeyflowers, researchers found that a change in one gene alone may have been enough to increase nectar flow and double hummingbird visits. Another small gene change altered the flower's pigment and reduced bee pollination by 80 percent. Relatively few genetic changes may be necessary for reproductive isolation -- and for speciation. Evolution can be "fast" or "slow," measured in hundreds or millions of years. It can begin with a gene or a volcanic eruption. The process has no direction, no purpose. Evolution requires the randomness of genetic change, followed by the decided nonrandomness of natural selection, complicated by the extreme randomness of external events. Evolution is not hard to see, although it may be impossible to fully understand. What does evolution look like? Look around. That tree. That bush. That insect. I can find evolution in the spur of a delphinium, just as I can find it everywhere in all the living world. I will not find it in a dismembered flower. And suddenly I feel like a serial killer, surrounded by body parts. I will see evolution only in the process of life, immanent, like some version of God. It is not the thing itself. It is not the tree. It is the shaper of trees. It is the shaper of flowers. Flowers pollinated by hummingbirds often have curved corolla tubes. This curve makes the bird's beak push against the tube and touch the anthers. In response, some hummingbirds evolved a matching curved beak, which made their feeding more efficient. In response, some flowers evolved even curvier corollas, which made the curved beak push again against the top of the corolla tube. Some of us respond to this information with a kind of awe, as though we were hearing organ music, as though the light were filtering through colored window panes. The flowers of the delphinium grow in a spiral on a tall spike. The flowers at the bottom of the spike are older, larger, and in a female stage. The anthers have shed their pollen and the mature stigmas are now receiving pollen. The upper flowers are younger, smaller, and in a male stage. The anthers are producing pollen and the stigmas are not yet mature. The older, larger, lower flowers tend to have more nectar, and the bumblebee's strategy is to start at the bottom, forage up, and then fly down to the next delphinium. This works well for the bee in terms of flying costs and nectar reward; its energy use is most efficient. This also works well for the delphinium, for the bee is now carrying pollen to the female flower of another plant. The delphinium has evolved to maximize its pollination, based on the bee's behavior. The bee has evolved to maximize its foraging, based on the plant's architecture. The match of delphinium and bee is not perfect, nor is it simple or unchanging. The delphinium also wants to attract other pollinators, and the bee wants to visit other food sources. Concerning his own theory of evolution, Charles Darwin wrote: "There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved." Charles Darwin did not have too much trouble putting a creator into the heart of evolution. The pope, I presume, had no trouble seeing the passion of Christ in a passionflower. I have trouble with these things every day. I am a product of the last half of the twentieth century. Hoping to find God, I cut up the delphinium. The shape of things now is not the shape of things to come.
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