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ANATOMY OF A ROSE -- EXPLORING THE SECRET LIFE OF FLOWERS |
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FIVE: Sex, Sex, Sex THE JACK-IN-THE-PULPIT IS CONSIDERING a sex change. The violets have a secret. The dandelion is smug. The daffodils are obsessive. The orchid is finally satisfied, having produced over a million seeds. The bellflower is not satisfied and is slowly bending its stigma in order to reach its own pollen. The pansies wait expectantly, their vulviform faces lifted to the sky. The evening primrose is interested in one thing and one thing only. A stroll through the garden is almost embarrassing. *** ABOUT 80 PERCENT OF FLOWERS are hermaphrodites, both male and female. Pollination is the movement of pollen from an anther to a stigma. Fertilization occurs when the sperm from a pollen grain unites with an egg in the ovary. Hermaphrodite flowers could easily pollinate and fertilize themselves. Most don't. Instead, they try to mix and match their pollen and eggs with the pollen and eggs from flowers of the same species: I'll take this. You take that. Here. Yes. Sex, good sex, is all about cross-fertilization. Why? Why have sex at all? In terms of the individual and its offspring, asexual reproduction is so much easier. You don't have to think about males or male parts. You cut your investment in half. You don't have to use up all that energy and time. You just reproduce. In a sexual population, an asexual mutant has many advantages and should quickly spread and take over. In an asexual population, a sexual mutant has many disadvantages and should quickly die out. Scientists are still puzzled by the question: What good is sex? They have some theories. In a cell, when genes divide and replicate themselves, their occasional changes or mistakes are sometimes harmful and even lethal. But when an individual gets a set of genes from two different parents, that dangerous mutation can be neutralized. The normal form of the gene usually takes over, and the mutation is not expressed. In the offspring of asexual reproduction, the harmful genes tend to accumulate. The recombination of genes from two different parents also allows for diversity among offspring. For natural selection to work, the genetic recombination has to create an immediate advantage for those individuals. In a variable world, their variability may mean that more of them survive. Finally, there is the theory of the long run. Natural selection would not favor sex or cross-fertilization because these things are good for the species. Natural selection does not care about the future of the species. But sex and cross-fertilization are good for the species because they prevent the buildup of harmful mutations and because they produce a population that is diverse. When the climate gets colder, when pollinators disappear, when new diseases attack, the population may have individuals that can survive and reproduce. In the long run, species lucky enough to be sexual -- species that, for complex reasons, resist asexuality -- may simply be the ones that last. *** THESE ARE ONLY THEORIES. But you're convinced. You decide to be sexual. And you decide to cross-fertilize. First, you must avoid clogging up your stigma with your own pollen grains. Some flowers, like the delphinium, separate their sexual parts in time. In a version of cross-dressing, they go through a male stage, when their anthers produce pollen. Then, in a matter of hours or days, they go through a female stage, when the stigma is ready to receive pollen. In the passionflower, the stigmas curve down at this point, bending back to fit between their own anthers, closer now to the colored mosaic of petals, closer now to the pollinating bee. A few flowers reverse the process, stigma first, anthers second. Flowers also separate their parts in space. In many flowers, the stigma rises well above the encircling stamens. An insect first plops on the stigma as a good place to land, deposits its pollen, and then goes exploring, rummaging around the petals, and collecting new pollen. In the rockrose, the anthers are sensitive to touch. Once a pollinator has visited the flower, the anthers splay down and away from the central stigma. The position of these organs is never casual. Some plants have two sexes, much like animals. The willow has a male form with flowers that only have stamens and a female form with flowers that only have stigmas. There is a Mr. and Mrs. Mistletoe, a Mr. and Mrs. Stinging Nettle, a Mr. and Mrs. Cottonwood, and a Mr. and Mrs. Holly. This is the most dramatic separation of parts. Some plants have two sexes but on the same inflorescence. Other species mix up their inflorescences with hermaphrodite flowers, male flowers, and female flowers. Plants juggle their sexual parts and move around their sexes as a way of avoiding self-pollination. A few plants also have the ability to choose their sex. An individual bog myrtle will produce only female flowers one year and only male flowers the next. It's not indecision. The myrtle is responding to water or nutrients in the soil, to light, or to temperature. Commonly, female flowers require more resources and more time to produce fruit; in a difficult situation, a plant reasonably decides to be male. A young jack-in-the-pulpit is often male in its first season. When it is bigger and stronger, when it has stored up a supply of starch, it will consider the more ambitious female lifestyle. *** IN POLLINATION, a pollen grain lands on a sticky stigma. The grain absorbs moisture. The grain swells, cracks, and sprouts a pollen tube, which pierces the stigma and grows down the style. The tube contains two sperm cells that are delivered to the ovary. In fertilization, a sperm cell fuses with an egg to become the embryo of the seed. The second sperm cell fuses with two other cells to become the endosperm, which feeds the embryo. This "double fertilization" means that the seed will have plenty of food and can mature quickly. Double fertilization has given flowers a tremendous advantage over nonflowering plants. It has given humanity an array of fruit and edible seeds: in short, agriculture. A flower cannot always prevent self-pollination. Accidents happen. The wind blows the wrong way. A bee misbehaves. The deed is done. At this point, some flowers can still prevent self-fertilization. In many grasses, when the stigma recognizes a pollen grain as too familiar, it blocks the growth of the tube. Evening primroses block the tube just below the stigma. Lilies and poppies cause the tube to burst further down the style. In the red skyrocket, the tube grows down the style, enters the ovule, and even fertilizes an egg -- which is then aborted. Flowers like these are self-incompatible. Some flowers are staunchly self-incompatible. Others waffle. In some species, pollen from another flower is simply given an advantage. Pollen tubes that result from cross-fertilization, for example, may grow more rapidly down the style. The race is not fixed, but the handicaps are heavy. All self-incompatible systems leak to some degree. A number of flowers simply change their minds. Breeding with yourself is better than not breeding at all. At the last possible minute, unpollinated stigmas may bend down or around to contact their own anthers or to pick up pollen left on the style. A few species have two different flowers, ones that cross-fertilize and ones that self-fertilize. In early spring, violets cover the woodland floor. Later, if a violet fails to get pollinated, the plant produces a second green bud that never opens and barely rises above the ground. Unseen, unnoticed, the bud fertilizes itself. Most flowers self-fertilize as a backup plan. Then there are the habitual "selfers." These tend to be flowers that must bloom and die quickly in unpredictable or harsh environments. The flowers are often small with little color and scent. They may look juvenile or undeveloped. Plants that habitually self-fertilize survive where other plants cannot. Since they reproduce quickly without pollinators, they often colonize new areas. Eventually they become genetically uniform, with fixed genes or traits. Populations of the same species that have followed different lines of self-fertilization are sometimes mistaken for different species. In the nineteenth century, one botanist saw two hundred species of grass in the forms of one tiny self-fertilizing plant. Flowers can take self-fertilization a step further. In a dandelion, the ovaries set seed without the presence or benefit of male sperm. Those seeds are genetic copies of the mother only. Agamospermy (what botanist Peter Bernhardt calls "virgin birth") is found in many plant families. Oddly, some of these flowers still require pollination, which stimulates the ovary even though the pollen has no other role. Dandelions do produce some pollen and seem to attract insects. Dandelions are not stupid. They also have a backup plan. About I percent of seeds in a dandelion head are from cross-pollination. The flower has not lost its ability to produce variable offspring in a changing world. Backup, backup, backup. Various plants reproduce vegetatively, sending out runners or roots that will grow into clones of the parent. The oldest known living plant is a clonal creosote bush in the Mojave Desert. The bush started as a seed some twelve thousand years ago. As you might suspect, this ancient shrub has a backup plan. In the rainy season, it blossoms with small, yellow flowers. *** A SINGLE FLOWER genus can show the range of sexual strategy. The large, showy lady's-smock (Cardamine pratensis) is cross-pollinated by many insects and is largely self-incompatible. The small bittercress (C. amara) is pollinated by flies and is easily capable of self-fertilization. The smaller hairy bittercress (C. hirsuta) is a habitual selfer. Flowers are flexible. Flowers are determined. The flower of a European orchid resembles the female of a certain bee. In parts of the Mediterranean, the flowers of a related species are grabbed by a lusty male bee and pollinated. But when that bee died out in western Europe, the orchid evolved into a habitual selfer. Now, a few days after the flower has opened, its pollinia (masses or sacs of pollen attached to a stem) lazily fall out of the anther, hang in front of the stigma, and wait for a breeze. Give that orchid a pollinator and it would return to cross-breeding. If that pollinator looked like a helicopter instead of a bee, the flower would consider the situation. We humans do as strange -- or stranger -- things for sex.
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