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ANATOMY OF A ROSE -- EXPLORING THE SECRET LIFE OF FLOWERS |
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NINE: Travelin'Man POLLEN HAS ITCHY FEET. Pollen has a job to do, going down that long, lonesome highway, bound to leave, bound for glory. You can't hold him back. Hit the road, Jack. Pollen is a travelin' man. *** THE HAZEL TREE SIGHS, a puff of pollen. In the first mild weather of spring, the male catkins of the hazel tree hang down like lambs' tails, their tiny flowers ribboned, two by two. When the wind blows, the catkins bobble. A cloud of yellow colors the air. A fine, masculine dust. On separate catkins, the female flowers of the hazel tree barely reveal their crimson tips. Well-bred, aloof, they will reject any pollen from the parent tree. The yellow cloud drifts, casting its pollen shadow. In an otherwise prosaic research paper, I find this sentence, which I am tempted to write as a poem, with the appropriate line breaks:
A paternal plant A single catkin on a hazel tree can contain four million grains of pollen. The tree may produce several thousand catkins. The light, dry grains are designed for flight, to be cast into the breeze and lifted as high as nineteen thousand feet or borne as far as three thousand miles. It's a wild ride and often pointless. Most of the pollen will fall back to the earth to dry out in the sun or to drown in a pond or to dust the wrong plant. A few will swell and rupture in the mucous membrane of a human nose, triggering the defense of our immune system: Help, help! A foreign body, more fluid! More fluid over here! The result is an eruption of tears and sneezing. A pollen shadow is cast over the land. One by one, each grain is accounted for. This is where the hazel tree uses math. Four million grains of pollen multiplied by several thousand catkins increases the chances that some of these will land on a compatible stigma. The majority of plant species in the world depend on the discriminating animal pollinator. But in terms of biomass -- the plants that cover the earth's land -- most plants cast their sperm into the air. For the dominant trees of a forest canopy, for conifers and pines, for grasses and sedges and rushes, wind pollination is the most effective choice. Even swarms of insects could not handle this job. In areas where insects and birds are scarce, such as salt marshes and some deserts, flowers also rely on the wind. Flowers judge, as best they can, how and when to release their pollen. To avoid storms, wind-pollinated plants tend to bloom in the milder days of early spring and autumn. Similarly, grass flowers may open early in the morning or late in the afternoon, when the turbulence caused by heat won't take their pollen into the next state. In a dead calm, grass flowers prevent their pollen's release by holding their grains in the spoonshaped lower end of the anthers. On the very nicest of days, when the air is slightly fresh, when the sun is pleasantly warm, when troublesome insects have not yet appeared or are already gone, we live and breathe in an effluvia of male sex cells. *** POLLEN GRAINS VARY in their tininess. The forget-me-not's is three microns (three thousandths of a millimeter). Pollen from a pumpkin can be eighty times larger than that, visible to the human eye. Most species of plants have pollen grains about thirty microns long. Each pollen grain is enclosed by a sturdy outer shell that may be sculpted variously with spines, warts, or rounded or angular ridges. These patterns are unique to groups of plants and sometimes to a single species. Wind-pollinated grains tend to be relatively smooth and aerodynamically efficient. The most elaborate surfaces, with scary protrusions and medieval spikes, are found in flowers pollinated by insects, the better to catch and latch onto a thorax. In animal-pollinated flowers, the pollen we see is usually a clump or sac of smaller grains held together by a gluey cement that adheres easily to the beak of a bird or the shell of a beetle. The adhesive oils produced by the grain contain pigments that cause pollen to appear yellow, orange, green, blue, black, or brown. The colors may attract pollinators. The oils may give off scent, serve as a water-repellent, or protect the grain from ultraviolet light. Pollen leaves an anther in various ways. Often the anthers naturally dry and split along preformed seams. The drying process can be a quiet one or so melodramatic that the stamen jerks and curls. At the slightest touch, the anthers in some orchids machine-gun-spray their pollen: bang, bang, bang. Flowers often protect their pollen. Some anthers have the ability to close again if the environment becomes too wet or cold. The anther cones of certain flowers contain pollen grains that can only be released through pores at the anther's tip. This also keeps the pollen safe and dry, until the right suitor comes along. A bee lands on the anther and vibrates its thoracic muscles in the frequency needed to free the pollen. The wrong vibration fails to produce pollen or produces fairly little. Honeybees do not seem to vibrate very well. Instead, they do strange and useless things, like trying to stick their tongues into the anther pore. Bumblebees have better buzz. About 8 percent of the world's flowers, including tomatoes, potatoes, blueberries, and cranberries, require a bumblebee at the window, serenading the pollen out.
Pollen Grains Many insects have bodies adapted to get, eat, and transport pollen. More than most, bees have reshaped themselves into pollen porters. The hind leg of a honeybee worker is a kind of Swiss Army knife. It includes a pollen basket, or concave region with hairs that anchor pollen; a pollen rake, or row of stiff bristles; a pollen press, or flattened area; and more pollen combs, or rows of stiff hairs. These parts work in coordination as pollen is passed from the forelegs to the middle legs to the hind legs, where it is packed into a pellet. Pollinators claw, scratch, pry, grab, press, and pack. On its part, pollen is hardly passive. In some flowers, pollen can actually jump across the gap that separates it from an insect. The force of desire is static electricity. Plants have their own electrostatic fields, which are strongest on clear, warm, sunlit days, with the "terminal" ends of the plant, the flowers, having the greatest charge. Specifically, the dry parts of a flower usually have a negative charge. Bees just leaving a hive also tend to have a slight negative charge. But as they fly through the air, and as friction strips away electrons, the insects become positively charged. When the foraging bee nears a negatively charged dry anther, pollen grains can leap out and attach themselves to the insect's body. Later, as a passenger on the bee, the pollen grain also becomes positively charged. It can now jump again, this time to a negatively charged stigma. A sailor reaching land. A pilot touching ground. A traveler wearying of traveling. Cold, lost, the wayfarer knocks at the cottage door. There is a light in the window. There is the smell of home. If the pollen grain is lucky, this will be a compatible flower of the same species, not on the same inflorescence, and not too closely related, not a daughter or granddaughter of the pollen's parent plant. The surface of every pollen shell, whether smooth or sculpted, has openings that allow the grain to release moisture and lose weight when it leaves the anther. These same holes can now absorb moisture, which will rehydrate the grain when it lands on a stigma. Some stigmas, as in certain daisies, are dry. The outer cells of these stigmas first "read" and approve the identity and source of the pollen and then secrete the necessary liquids. In many flowers, the receptive stigma is already wet. Here the grain sticks easily and absorbs sugar water from the stigma's surface. Soon the grain cracks, swells, and sprouts a tube. In some flowers, the pollen tube grows by "drilling" its way through the style's tissue. In other cases, the style already has a hollow channel or jelly-like areas easily penetrated. Pollen from compatible flowers is welcome. Pollen from incompatible flowers usually goes off in the wrong direction, explodes, or stops growing. In the meadow saffron, a tube will reach the ovule about twelve hours after pollination. In some flowers, the tube whistles down in six hours. At the ovule is an entrance pore into which the tube delivers two sperm cells, one for the endosperm, one for the egg cell. This is the good news. In flowers pollinated by animals, this is what happens to "good" pollen grains. *** THE BAD ONES, of course, get eaten. Most pollen is sacrificial. For many insects, pollen is breakfast, lunch, dinner, and snacks. To a bee going shopping, the nutrition label is promising: 16-30 percent protein, 1-10 percent fat, 1-7 percent starch, no sugar, various vitamins, various minerals. Flowers can be extraordinarily generous. The corn poppy produces over 2-1/2 million pollen grains per flower, overloading its visitors in the expectation that just a few of these grains will not get consumed, that a few, out of millions, will get taken to another corn poppy. To entice customers, other flowers produce fake, sterile pollen, less costly to make, but almost as nutritious. Smaller hidden stamens, with fertile pollen, are positioned so that the insect can contact them easily. For the honeybee, pollen is usually plentiful, easy to find, and labor-intensive. This is not convenience food. The bee shakes, rakes, presses, packs, and flies back to the bee colony. Here the pollen must be treated chemically so that it won't germinate, processed for storage, and made into "bee bread" to be eaten by larvae and adults. (Nectar, which becomes honey, is stored differently.) The hard, spiky walls of the grain are a problem. It can take three hours for a honeybee to digest a pollen mass. *** ONCE ON A STIGMA, pollen promptly responds to moisture and other chemical signals. Without those signals, the outer rind of a pollen grain is so stable that it has been called the most resistant organic material known, a natural polymer as tough as industrial plastic. Although the inner potency of pollen is short-lived, the surface fiercely resists rot, pressure, and temperature extremes. Scientists have found pollen grains, eaten but not digested, whose walls have survived for thirty thousand years in the stomach of a frozen mammoth. Fossilized pollen can be much older. Archaeologists, naturally, love pollen. So do paleontologists, climatologists, geologists, and forensic scientists. The Neanderthals, for example, buried their dead with whole flowers some fifty thousand years ago. We know this because of the pollen grains left behind: ancient versions of blue hyacinth, yellow groundsel, knapweed, and yarrow. The idea that these people's love of flowers somehow makes them more real to us. Suddenly, we see them weeping. We see a belief in the afterlife. We see culture. In 1994, a mass grave with the skeletons of thirty-two young men was uncovered in Magdeburg, Germany. The victims were either Germans killed by the Gestapo in the early spring of 1945, or they were Soviet soldiers killed by Soviet secret police for refusing to break up a German revolt in June 1953. The nasal cavities of seven skulls produced plantain, lime tree, and rye pollen, all heavily emitted in June. The murderers were Russian. Traces of pollen also have been found on the Shroud of Turin, a linen cloth that bears the image of a wounded man, which some believe to be the burial shroud of Jesus Christ. Since 1538, the cloth has been enshrined in a cathedral in Italy. Pollen from species of bean caper and tumbleweed confirm that the material originally came from Israel. Pollen keeps traveling. Traditional Navajos believe that the "pollen path" is the way between the gods and humanity. It is the harmony that should exist between us.
In the house of life I wander We are all walking the pollen path. We all breathe in that fine, masculine dust (and suffer, some of us, from our body's defensive, surprised reaction). A paternal plant spreads a thin and particulate sheet of itself over the habitat. Flowers everywhere catch the falling clouds. The crimson tips of the hazel tree hide, wait, and receive. The elaborate stigmas of grass comb the air. The corn poppy prepares a feast. The pollen grain leaps ecstatically from the bee. One-half of something meets the other half. Bound to leave, bound for glory.
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