Paper 2000 Question: 1 (a) Al-Beruni
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| V EVOLUTION OF FLOWERS
Flowering plants are thought to have evolved around 135 million years ago from cone-bearing gymnosperms. Scientists had long proposed that the first flower most likely resembled today’s magnolias or water lilies, two types of flowers that lack some of the specialized structures found in most modern flowers. But in the late 1990s scientists compared the genetic material deoxyribonucleic acid (DNA) of different plants to determine their evolutionary relationships. From these studies, scientists identified a small, cream-colored flower from the genus Amborella as the only living relative to the first flowering plant. This rare plant is found only on the South Pacific island of New Caledonia. The evolution of flowers dramatically changed the face of earth. On a planet where algae, ferns, and cycads tinged the earth with a monochromatic green hue, flowers emerged to paint the earth with vivid shades of red, pink, orange, yellow, blue, violet, and white. Flowering plants spread rapidly, in part because their fruits so effectively disperse seeds. Today, flowering plants occupy virtually all areas of the planet, with about 240,000 species known. Many flowers and pollinators coevolved—that is, they influenced each other’s traits during the process of evolution. For example, any population of flowers displays a range of color, fragrance, size, and shape—hereditary traits that can be passed from one generation to the next. Certain traits or combinations of traits appeal more to pollinators, so pollinators are more likely to visit these attractive plants. The appealing plants have a greater chance of being pollinated than others and, thus, are likely to produce more seeds. The seeds develop into plants that display the inherited appealing traits. Similarly, in a population of pollinators, there are variations in hereditary traits, such as wing size and shape, length and shape of tongue, ability to detect fragrance, and so on. For example, pollinators whose bodies are small enough to reach inside certain flowers gather pollen and nectar more efficiently than larger-sized members of their species. These efficient, well-fed pollinators have more energy for reproduction. Their offspring inherit the traits that enable them to forage successfully in flowers, and from generation to generation, these traits are preserved. The pollinator preference seen today for certain flower colors, fragrances, and shapes often represents hundreds of thousands of years of coevolution. Coevolution often results in exquisite adaptations between flower and pollinator. These adaptations can minimize competition for nectar and pollen among pollinators and also can minimize competition among flowers for pollinators. Comet orchids, for example, have narrow flowers almost a foot and a half long. These flowers are pollinated only by a species of hawk moth that has a narrow tongue just the length of the flowers. The flower shape prevents other pollinators from consuming the nectar, guarantees the moths a meal, and ensures the likelihood of pollination and fertilization. Most flowers and pollinators, however, are not as precisely matched to each other, but adaptation still plays a significant role in their interactions. For example, hummingbirds are particularly attracted to the color red. Hummingbird-pollinated flowers typically are red, and they often are narrow, an adaptation that suits the long tongues of hummingbirds. Bats are large pollinators that require relatively more energy than other pollinators. They visit big flowers like those of saguaro cactus, which supply plenty of nectar or pollen. Bats avoid little flowers that do not offer enough reward. Other examples of coevolution are seen in the bromeliads and orchids that grow in dark forests. These plants often have bright red, purple, or white sepals or petals, which make them visible to pollinators. Night-flying pollinators, such as moths and bats, detect white flowers most easily, and flowers that bloom at sunset, such as yucca, datura, and cereus, usually are white. The often delightful and varied fragrances of flowers also reveal the hand of coevolution. In some cases, insects detect fragrance before color. They follow faint aromas to flowers that are too far away to be seen, recognizing petal shape and color only when they are very close to the flower. Some night-blooming flowers emit sweet fragrances that attract night-flying moths. At the other extreme, carrion flowers, flowers pollinated by flies, give off the odor of rotting meat to attract their pollinators. Flowers and their pollinators also coevolved to influence each other’s life cycles. Among species that flower in response to a dark period, some measure the critical night length so accurately that all species of the region flower in the same week or two. This enables related plants to interbreed, and provides pollinators with enough pollen and nectar to live on so that they too can reproduce. The process of coevolution also has resulted in synchronization of floral and insect life cycles. Sometimes flowering occurs the week that insect pollinators hatch or emerge from dormancy, or bird pollinators return from winter migration, so that they feed on and pollinate the flowers. Flowering also is timed so that fruits and seeds are produced when animals are present to feed on the fruits and disperse the seeds. VI FLOWERS AND EXTINCTION Like the amphibians, reptiles, insects, birds, and mammals that are experiencing alarming extinction rates, a number of wildflower species also are endangered. The greatest threat lies in the furious pace at which land is cleared for new houses, industries, and shopping malls to accommodate rapid population growth. Such clearings are making the meadow, forest, and wetland homes of wildflowers ever more scarce. Among the flowers so endangered is the rosy periwinkle of Madagascar, a plant whose compounds have greatly reduced the death rates from childhood leukemia and Hodgkin’s disease. Flowering plants, many with other medicinal properties, also are threatened by global warming from increased combustion of fossil fuels; increased ultraviolet light from ozone layer breakdown; and acid rain from industrial emissions. Flowering plants native to a certain region also may be threatened by introduced species. Yellow toadflax, for example, a garden plant brought to the United States and Canada from Europe, has become a notorious weed, spreading to many habitats and preventing the growth of native species. In some cases, unusual wildflowers such as orchids are placed at risk when they are collected extensively to be sold. Many of the threats that endanger flowering plants also place their pollinators at risk. When a species of flower or pollinator is threatened, the coevolution of pollinators and flowers may prove to be disadvantageous. If a flower species dies out, its pollinators will lack food and may also die out, and the predators that depend on the pollinators also become threatened. In cases where pollinators are adapted to only one or a few types of flowers, the loss of those plants can disrupt an entire ecosystem. Likewise, if pollinators are damaged by ecological changes, plants that depend on them will not be pollinated, seeds will not be formed, and new generations of plants cannot grow. The fruits that these flowers produce may become scarce, affecting the food supply of humans and other animals that depend on them. Worldwide, more than 300 species of flowering plants are endangered, or at immediate risk of extinction. Another two dozen or so are considered threatened, or likely to become extinct in the near future. Of these species, fewer than 50 were the focus of preservation plans in the late 1990s. Various regional, national, and international organizations have marshaled their resources in response to the critical need for protecting flowering plants and their habitats. In the United States, native plant societies work to conserve regional plants in every state. The United States Fish and Wildlife Endangered Species Program protects habitats for threatened and endangered species throughout the United States, as do the Canadian Wildlife Service in Canada, the Ministry for Social Development in Mexico, and similar agencies in other countries. At the international level, the International Plant Conservation Programme at Cambridge, England, collects information and provides education worldwide on plant species at risk, and the United Nations Environmental Programme supports a variety of efforts that address the worldwide crisis of endangered species. Pollination I INTRODUCTION Pollination, transfer of pollen grains from the male structure of a plant to the female structure of a plant. The pollen grains contain cells that will develop into male sex cells, or sperm. The female structure of a plant contains the female sex cells, or eggs. Pollination prepares the plant for fertilization, the union of the male and female sex cells. Virtually all grains, fruits, vegetables, wildflowers, and trees must be pollinated and fertilized to produce seed or fruit, and pollination is vital for the production of critically important agricultural crops, including corn, wheat, rice, apples, oranges, tomatoes, and squash. Pollen grains are microscopic in size, ranging in diameter from less than 0.01mm (about 0.0000004 in) to a little over 0.5 mm (about 0.00002 in). Millions of pollen grains waft along in the clouds of pollen seen in the spring, often causing the sneezing and watery eyes associated with pollen allergies. The outer covering of pollen grains, called the pollen wall, may be intricately sculpted with designs that in some instances can be used to distinguish between plant species. A chemical in the wall called sporopollenin makes the wall resistant to decay. Although the single cell inside the wall is viable, or living, for only a few weeks, the distinctive patterns of the pollen wall can remain intact for thousands or millions of years, enabling scientists to identify the plant species that produced the pollen. Scientists track long-term climate changes by studying layers of pollen deposited in lake beds. In a dry climate, for example, desert species such as tanglehead grass and vine mesquite grass thrive, and their pollen drifts over lakes, settling in a layer at the bottom. If a climate change brings increased moisture, desert species are gradually replaced by forest species such as pines and spruce, whose pollen forms a layer on top of the grass pollen. Scientists take samples of mud from the lake bottom and analyze the pollen in the mud to identify plant species. Comparing the identified species with their known climate requirements, scientists can trace climate shifts over the millennia. II HOW POLLINATION WORKS Most plants have specialized reproductive structures—cones or flowers—where the gametes, or sex cells, are produced. Cones are the reproductive structures of spruce, pine, fir, cycads, and certain other gymnosperms and are of two types: male and female. On conifers such as fir, spruce, and pine trees, the male cones are produced in the spring. The cones form in clusters of 10 to 50 on the tips of the lower branches. Each cone typically measures 1 to 4 cm (0.4 to 1.5 in) and consists of numerous soft, green, spirally attached scales shaped like a bud. Thousands of pollen grains are produced on the lower surface of each scale, and are released to the wind when they mature in late spring. The male cones dry out and shrivel up after their pollen is shed. The female cones typically develop on the upper branches of the same tree that produces the male cones. They form as individual cones or in groups of two or three. A female cone is two to five times longer than the male cone, and starts out with green, spirally attached scales. The scales open the first spring to take in the drifting pollen. After pollination, the scales close for one to two years to protect the developing seed. During this time the scales gradually become brown and stiff, the cones typically associated with conifers. When the seeds are mature, the scales of certain species separate and the mature seeds are dispersed by the wind. In other species, small animals such as gray jays, chipmunks, or squirrels break the scales apart before swallowing some of the enclosed seeds. They cache, or hide, other seeds in a variety of locations, which results in effective seed dispersal-and eventually germination-since the animals do not always return for the stored seeds. Pollination occurs in cone-bearing plants when the wind blows pollen from the male to the female cone. Some pollen grains are trapped by the pollen drop, a sticky substance produced by the ovule, the egg-containing structure that becomes the seed. As the pollen drop dries, it draws a pollen grain through a tiny hole into the ovule, and the events leading to fertilization begin. The pollen grain germinates and produces a short tube, a pollen tube, which grows through the tissues of the ovule and contacts the egg. A sperm cell moves through the tube to the egg where it unites with it in fertilization. The fertilized egg develops into an embryonic plant, and at the same time, tissues in the ovule undergo complex changes. The inner tissues become food for the embryo, and the outer wall of the ovule hardens into a seedcoat. The ovule thus becomes a seed—a tough structure containing an embryonic plant and its food supply. The seed remains tucked in the closed cone scale until it matures and the cone scales open. Each scale of a cone bears two seeds on its upper surface. In plants with flowers, such as roses, maple trees, and corn, pollen is produced within the male parts of the plant, called the stamens, and the female sex cells, or eggs, are produced within the female part of the plant, the pistil. With the help of wind, water, insects, birds, or small mammals, pollen is transferred from the stamens to the stigma, a sticky surface on the pistil. Pollination may be followed by fertilization. The pollen on the stigma germinates to produce a long pollen tube, which grows down through the style, or neck of the pistil, and into the ovary, located at the base of the pistil. Depending on the species, one, several, or many ovules are embedded deep within the ovary. Each ovule contains one egg. Fertilization occurs when a sperm cell carried by the pollen tube unites with the egg. As the fertilized egg begins to develop into an embryonic plant, it produces a variety of hormones to stimulate the outer wall of the ovule to harden into a seedcoat, and tissues of the ovary enlarge into a fruit. The fruit may be a fleshy fruit, such as an apple, orange, tomato, or squash, or a dry fruit, such as an almond, walnut, wheat grain, or rice grain. Unlike conifer seeds, which lie exposed on the cone scales, the seeds of flowering plants are contained within a ripened ovary, a fleshy or dry fruit. III POLLINATION METHODS In order for pollination to be successful, pollen must be transferred between plants of the same species—for example, a rose flower must always receive rose pollen and a pine tree must always receive pine pollen. Plants typically rely on one of two methods of pollination: cross-pollination or self-pollination, but some species are capable of both. Most plants are designed for cross-pollination, in which pollen is transferred between different plants of the same species. Cross-pollination ensures that beneficial genes are transmitted relatively rapidly to succeeding generations. If a beneficial gene occurs in just one plant, that plant’s pollen or eggs can produce seeds that develop into numerous offspring carrying the beneficial gene. The offspring, through cross-pollination, transmit the gene to even more plants in the next generation. Cross-pollination introduces genetic diversity into the population at a rate that enables the species to cope with a changing environment. New genes ensure that at least some individuals can endure new diseases, climate changes, or new predators, enabling the species as a whole to survive and reproduce. Plant species that use cross-pollination have special features that enhance this method. For instance, some plants have pollen grains that are lightweight and dry so that they are easily swept up by the wind and carried for long distances to other plants. Other plants have pollen and eggs that mature at different times, preventing the possibility of self-pollination. In self-pollination, pollen is transferred from the stamens to the pistil within one flower. The resulting seeds and the plants they produce inherit the genetic information of only one parent, and the new plants are genetically identical to the parent. The advantage of self-pollination is the assurance of seed production when no pollinators, such as bees or birds, are present. It also sets the stage for rapid propagation—weeds typically self-pollinate, and they can produce an entire population from a single plant. The primary disadvantage of self-pollination is that it results in genetic uniformity of the population, which makes the population vulnerable to extinction by, for example, a single devastating disease to which a l the genetically identical plants are equally susceptible. Another disadvantage is that beneficial genes do not spread as rapidly as in cross-pollination, because one plant with a beneficial gene can transmit it only to its own offspring and not to other plants. Self-pollination evolved later than cross-pollination, and may have developed as a survival mechanism in harsh environments where pollinators were scarce. IV POLLEN TRANSFER Unlike animals, plants are literally rooted to the spot, and so cannot move to combine sex cells from different plants; for this reason, species have evolved effective strategies for accomplishing cross-pollination. Some plants simply allow their pollen to be carried on the wind, as is the case with wheat, rice, corn, and other grasses, and pines, firs, cedars, and other conifers. This method works well if the individual plants are growing close together. To ensure success, huge amounts of pollen must be produced, most of which never reaches another plant. Most plants, however, do not rely on the wind. These plants employ pollinators—bees, butterflies, and other insects, as well as birds, bats, and mice—to transport pollen between sometimes widely scattered plants. While this strategy enables plants to expend less energy making large amounts of pollen, they must still use energy to produce incentives for their pollinators. For instance, birds and insects may be attracted to a plant by its tasty food in the form of nectar, a sugary, energy-rich fluid that bees eat and also use for making honey. Bees and other pollinators may be attracted by a plant’s pollen, a nutritious food that is high in protein and provides almost every known vitamin, about 25 trace minerals, and 22 amino acids. As a pollinator enters a flower or probes it for nectar, typically located deep in the flower, or grazes on the pollen itself, the sticky pollen attaches to parts of its body. When the pollinator visits the next flower in search of more nectar or pollen, it brushes against the stigma and pollen grains rub off onto the stigma. In this way, pollinators inadvertently transfer pollen from flower to flower. Some flowers supply wax that bees use for construction material in their hives. In the Amazonian rain forest, the males of certain bee species travel long distances to visit orchid flowers, from which they collect oil used to make a powerful chemical, called a pheromone, used to attract female bees for mating. The bees carry pollen between flowers as they collect the oils from the orchids. Flowers are designed to attract pollinators, and the unique shape, color, and even scent of a flower appeals to specific pollinators. Birds see the color red particularly well and are prone to pollinating red flowers. The long red floral tubes of certain flowers are designed to attract hummingbirds but discourage small insects that might take the nectar without transferring pollen. Flowers that are pollinated by bats are usually large, light in color, heavily scented, and open at night, when bats are most active. Many of the brighter pink, orange, and yellow flowers are marked by patterns on the petals that can be seen only with ultraviolet light. These patterns act as maps to the nectar glands typically located at the base of the flower. Bees are able to see ultraviolet light and use the colored patterns to find nectar efficiently. These interactions between plants and animals are mutualistic, since both species benefit from the interaction. Undoubtedly plants have evolved flower structures that successfully attract specific pollinators. And in some cases the pollinators may have adapted their behaviors to take advantage of the resources offered by specific kinds of flowers. V CURRENT TOPICS Scientists control pollination by transferring pollen by hand from stamens to stigmas. Using these artificial pollination techniques, scientists study how traits are inherited in plants, and they also breed plants with selected traits—roses with larger blooms, for example, or apple trees that bear more fruit. Scientists also use artificial pollination to investigate temperature and moisture requirements for pollination in different species, the biochemistry of pollen germination, and other details of the pollination process. Some farmers are concerned about the decline in numbers of pollinating insects, especially honey bees. In recent years many fruit growers have found their trees have little or no fruit, thought to be the result of too few honey bee pollinators. Wild populations of honey bees are nearly extinct in some areas of the northern United States and southern Canada. Domestic honey bees—those kept in hives by beekeepers—have declined by as much as 80 percent since the late 1980s. The decline of wild and domestic honey bees is due largely to mite infestations in their hives—the mites eat the young, developing bees. Bees and other insect pollinators are also seriously harmed by chemical toxins in their environment. These toxins, such as the insecticides Diazinon and Malathion, either kill the pollinator directly or harm them by damaging the environment in which they live. Fertilization I INTRODUCTION Fertilization, the process in which gametes—a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually—that is, when a male and a female are needed to produce an offspring (see Reproduction). This article focuses on animal fertilization. For information on plant fertilization see the articles on Seed, Pollination, and Plant Propagation. Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus. Fertilization is not instantaneous—it may take 30 minutes in sea urchins and up to several hours in mammals. After nuclear fusion, the fertilized egg is called a zygote. When the zygote divides to a two-cell stage, it is called an embryo. Fertilization is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed—a sperm cell or an egg cell. Each gamete contains only half the genetic material of the original cell. During sperm and egg fusion in fertilization, the full amount of genetic material is restored: half contributed by the male parent and half contributed by the female. In humans, for example, there are 46 chromosomes (carriers of genetic material) in each human body cell—except in the sperm and egg, which each have 23 chromosomes. As soon as fertilization is complete, the zygote that is formed has a complete set of 46 chromosomes containing genetic information from both parents. The fertilization process also activates cell division. Without activation from the sperm, an egg typically remains dormant and soon dies. In general, it is fertilization that sets the egg on an irreversible pathway of cell division and embryo development. Directory: 2014 2014 -> For want of a nail 2014 -> The Global Warming that Wasn’t 2014 -> Assembly, No. 3901 state of new jersey 216th legislature 2014 -> Stop Militarizing Law Enforcement Act of 2014 2014 -> Ihbb asian Championships 2014 middle school bowl Round 4 First Quarter 2014 -> Unit: Automotive and Auto Body 2014 -> Ihbb european Championships 2014 Bowl Round 4 First Quarter 2014 -> International History Bee and Bowl 2013-14 Asian Division Question Set Bowl Round Six 2014 -> United states patent and trademark office 2014 -> Walnut creek, ca branch executive committee meeting minutes Download 0.98 Mb. Share with your friends:
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