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FLOWERS: HOW THEY CHANGED THE WORLD Introduction ............................................. 5 Chapter 1, What, exactly, is a flower? .....................15 Chapter 2, What are flowers for? ..........................47 Chapter 3, Flowers and their friends. ......................74 Chapter 4, Flowers and their enemies. .....................108 Chapter 5, How are the flowering plants distinguished? ....142 Chapter 6, What makes flowering plants so special? ........172 Chapter 7, Primates, People and the flowering plants. ....204 Chapter 8. How flowers changed the world. .................238 Notes and bibliography ....................................275 Glossary ..................................................292 William C. Burger Botany Department The Field Museum Chicago, IL 60605 Phone: 312 665 7847 Email: [email protected] Figures. [Full page pen-and-ink plates; to be positioned together in chapter one as a group of four, with fig. 1 to face fig. 2, and fig. 3 to face fig. 4.] Figure 1. A: Geranium maculatum (Wild Geranium): flower from above and two enlarged stamens (upper left), side view of flower with two petals removed and flower bud (center). B: Rosa carolina (Pasture Rose): longitudinal section of flower (right center) and flower bud (lower right). Basal scales apply to leafy stems in this and the other figures. Figure 2. A: Lilium philadelphicum (Wood Lily): two flowering stems (left), flower with two tepals removed (upper left). B: Lilium michiganense (Michigan Lily): enlarged stamen (upper right), enlarged pistil with cross-section of ovary (lower center). Note pistil composed of ovary, style and stigma; stamen with filament and anther. Figure 3. A: Stachys tenuifolia (Hedge Nettle): flowering stems (left), enlarged views of bilabiate flower (top center and right), longitudinal section of flower (upper center). B: Campsis radicans (Trumpet Creeper): cut-open tubular corolla viewed from beneath (right center). Figure 4. A: Rudbeckia hirta (Black-eyed Susan): enlarged head (upper right), cross-section of head (left center), disk florets in female and male phases (right center). B: Taraxacum officinale (Common Dandelion): cross-section of head (center), enlarged ligulate floret (lower left). COLOR [PHOTO] INSERTS: A (1). Bent over, the Michigan lily, with its curled-back tepals, displays six brown anthers and a reddish style. B (2). Beetle visiting a wild geranium flower. Note the five separate petals, ten stamens and central stigmas. C (3). Hoverfly on a meadow rose. Note the five separate petals, many stamens, and central mound of stigmas. D (4). The tubular corolla forms a curved spur in the Impatiens flower, holding nectar for long-tongued insects. E (5). Rapid "buzzing" allows bees to extract pollen from the distal pores of the yellow stamens of Solanum dulcamara. F (6). Spikelet of a grass (Lolium sp.). Only an anther and two filamentous stigmas reveal the florets within. G (7). Only the upper third of a Hydnora flower emerges above the soil. Dark objects within the flower are beetles. H (8). Two pseudo-flowers of skunk cabbage; each spathe encloses a spherical spadix bearing many congested flowers. I (9). Pseudo-flowers of the flowering spurge. Near the ovary base, a tiny fly sips nectar from a greenish nectary. J (10). The head or pseudo-flower of New England aster. The bright yellow disk flowers form a highly visible center. K (11). Inflorescence of Queen Anne's lace. Here, a large umbel of small umbels helps little flowers make a big show. L (12). The many flowers of Banksia are congested on a thick spike which becomes woody, protecting the seeds from fire. M (13). Frontal view of Tabebuia rosea flowers; note the broad landing field, with yellow "honey guides." N (14). Cymbidium orchid flower; note the upper column, and the lower lip with landing field and honey guides. O (15). This Trichoceros orchid mimics the hairy body of a tachnid fly; an example of pollination by pseudo-copulation. P (16). Over time, flowers and their pollinators have created biotas far richer than any that had come before. Introduction. Surely, flowers are among the most endearing aspects of our environment. Be it in a carefully tended garden, a tall-grass prairie, or even in a vacant lot, their bright cheery colors help make the world a more pleasant place to be. Whether borne by a little weed at the edge of your driveway, or covering the surface of a tropical tree, flowers add a significant dash of color to our natural surroundings. However, such floral exuberance is limited to the lush growing season. Prolonged dry seasons or severe winters will cast a pall of lifelessness across the landscape. But start adding warmth in our northern springtime, or bring on the rains after a long tropical dry season, and the natural world becomes transformed. Both the warmer temperatures of springtime, and the renewed growth of a rainy season, begin a flurry of plant activity. Many species flower quickly, while others build their greenery first and display their flowers later. All participate in a seasonal progression that is repeated year after year. Likewise, fruiting and seed production are precisely timed for each species. Unusually stressful seasons, prolonged drought, or local calamity may disrupt these patterns, but plant life soon recovers and is back into synchrony with our planet's yearly journey 'round the sun. Flowering plants follow in cadence with our annual cycle of seasons; that same cadence is also central to our own daily lives --as it has been ever since we were hunters and gatherers, tens of thousands of years ago. The invention of agriculture bound us even more tightly to the seasons. While preparing the ground for planting and caring for domesticated animals may have more severely constrained our lives, these innovations helped multiply our numbers. Thanks to our new "partners"--both plants and animals--we humans now had a more assured food supply. Agriculture became a very special symbiosis between ourselves and a few species of useful plants and animals. Based on this new relationship, and in the right locations, humans were able to build grand civilizations. Even today, in our complex technological world, it is the flowering plants that provide us with nearly all the vegetable energy that sustains us. Flowering plants also provide the feed and pasture for most of the animals that help nourish us. Clearly, flowers and the plants that bear them have played a critical role in the human saga. With a love for flowering plants that only avid gardeners can truly appreciate, my mother imparted some of her enthusiasms to me. When I was little, I was quite convinced that plants produced flowers because they were happy. I had noticed that the woods had very few flowers, and it seemed obvious that those woodland plants weren't nearly as happy as those in my mother's carefully tended garden. Better yet, that happiness was shared with busy bees and lovely butterflies who regularly visited our garden. Each year of my childhood, I became further captivated by the colorful opulence of the blooming season. The early flowers of springtime were accompanied by the joyful songs of birds. As the weeks progressed, flowering expanded across the landscape, even as the birds were quieting down. By August flowering was reaching its peak, now accompanied by a chirping chorus of little six-legged creatures. Cooler temperatures in September witnessed a sharp decline in flowering but a bonanza of fruits and vegetables. By mid-October cold temperatures had ended flowering, but now the leaves of hardwoods exploded into a final burst of brilliant color. Then, in November, the last leaves would flutter to the ground and the woods took on the barren austerity of a long winter. Insect serenades had ended with the first hard freeze; both field and forest now fell silent--except for cold winds whistling across bare branches. Long cold winter, mostly gray and unpleasant, could, on occasion, produce moments of splendor when moist snow covered every twig and branch in ermine. Finally, windy March brought longer days, warmer temperatures, and weird choruses arising from ponds and swamps, as frogs and toads announced the coming of spring. By mid-April our first wild flowers were blooming again on the forest floor, and by May the flowering spectacle was beginning in fields and prairies. The blooming cycle had resumed. As a child I had no conception of how important flowers were in the life of the landscape, or that there might have been times, eons ago, when the world's green landscapes were devoid of flowers. Nor did I realize that grasses and sedges also had flowers, albeit little ones. And I had no idea of what flowers might actually be doing--though I had figured out that plants with different kinds of flowers produced very different kinds of fruits, ranging from blueberries and tomatoes to apples and pumpkins. It would only be much later that I came to understand how all these devices functioned in the central purpose for every form of life: reproduction. All complex living things, in many generations over time, suffer countless tribulations and eventual death. Only those species and populations that reproduce, not just successfully but abundantly, survive such onslaughts. Bad weather, virulent diseases, and hungry predators incessantly erode the living world. Only successful reproduction can counter these destructive forces. Flowers exist because they are part of the reproductive exuberance one finds throughout the living world. Flowers are functional; they are a critical way in which one group of plants has "learned," not only to survive, but to prosper. More significantly, flowering plants are "photosynthesizing autotrophs" --they energize themselves by capturing the energy of sunlight. We humans and other members of the animal world are "heterotrophs," using other organisms or organic matter to energize ourselves. Thus, even though they don't seem to be too lively, green plants are the fundamental energy resource for most of the biosphere. Because flowering plants are so dominant amidst the greenery that adorns our planet's land surfaces, they provide most ecosystems with their enabling energy. The flowering plants--botanists call them the angiosperms--number about 260,000 species. With all land plants estimated to number about 300,000 species, it is clear that the angiosperms are the biggest game in town. However, flowering plants are by no means the most species-rich lineage of living things. When you add together all the species of butterflies, moths, beetles, bees, wasps, grasshoppers and flies, the number of just these insect species exceeds a million. Beetles alone number over 460,000 species, with more species being discovered every day. But all those beetles don't add up to a lot of biomass. Pack a thousand of most any beetle species in a gallon jug and you'll have space left over. Here's another reason why flowering plants are so important: they are big. While a few, like pond scum (Lemna and its allies), are smaller than a penny, most flowering plants are substantial in size--at least when compared to beetles. So, if we consider size and energy-capturing photosynthesis together, flowering plants turn out to be the most significant organisms of the rain forest, the tall grass prairie, the savanna woodland, and the agricultural landscapes of our planet. Though commonplace, a field of flowers with all its buzzing bees and chattering birds can seem to be so complex, yet so coherent in its overall functioning, as to border on the miraculous. Whether an exquisite flower or an elaborate butterfly, these manifestations of nature have inspired the age-old question: How did such wonderful creatures come to be? For many people the answer has been to imagine an intelligent Deity, and each little marvel the product of His design (later becoming known as "Intellligent Design"). That the ordinary operations of nature--birth and death, success and failure--might have fashioned so many natural wonders is difficult to imagine. Charles Darwin challenged this kind of age-old notion when he suggested that the manifest perfection of nature might have been forged by the selection of those that are better adapted, over eons of time. What Darwin's idea did was to interpret biological diversity as the product of a long evolutionary history. And by inferring only natural processes, he opened the history of life to scientific inquiry. Darwin himself, for example, conjectured that humans originated in Africa, because that's where chimps and gorillas live, the animals most similar to us. Other scientists had postulated that Asia was mankind's original home, and orangutans more closely related to us. After more than a century of intensive investigations, fossil discoveries in Africa have sustained Darwin's original suggestion. Today, using DNA sequences, we know that, of all the animals, the genetic code of chimps is, indeed, closest to our own. And we have also discovered that the peoples of Africa have more genetic diversity among themselves than is found in all the rest of humanity around the world, further supporting our African origins. By focusing on data from nature itself, and natural processes, modern science has confirmed the claims of evolution as a powerful explanatory framework: a framework which has allowed us to understand both the workings of nature and its long history. Plants and people have genes and chromosomes which function in much the same way. There is a profound unity in the diversity of life on planet Earth. Nevertheless, many people still feel that the grandeur of nature is too vast and too intricate to be explained in this way; such sentiments have lead to the promotion of "intelligent design" as an explanatory mechanism. While this idea has been actively discussed in the press over the last ten years, those who promote this view have an interesting problem. Not a single scientific paper utilizing intelligent design theory has been published over this same time period. And the reason is simple. There is absolutely no way to test this general idea, or to seek substantiating evidence in the world around us. One might say that the Intelligent Designer has left no fingerprints. I believe that trying to bring "intelligent design" into modern science is akin to bringing a basketball to the golf course. People don't bring basketballs to the golf course because there's nothing they can do with them once they get there. Similarly, because "intelligent design" has no way of being tested or evaluated in the world of nature, it cannot be a part of the scientific enterprise. And that, quite simply, is why "intelligent design" has no place in courses that claim to teach biological science. Similarly, this is why, in examining and discussing flowers in the following pages, we'll adhere to standard scientific practice and discourse. A powerful methodology, modern science has given us both deep insights and a coherent picture of the natural world. Whether it's geologists studying Earth's long history, or cardiologists trying to learn more about heart disease, science has become humankind's single most intellectually successful activity. Studying plant science has been a vital part of this grand enterprise, which you shall see in the pages that follow. ****** In a highly acclaimed collection, The Immense Journey, Loren Eiseley included an essay titled: "How Flowers Changed the World." What startled me about this piece was not only its deep insights and eloquence, but that it had been written by an anthropologist who was also a poet. No botanist or biologist whom I'm aware of has so clearly captured the true significance of flowering plants in a similar way. Perhaps, I thought, biological scientists, eagerly studying each and every tree, had failed to note the grandeur of the forest. It took a son of the open Nebraska prairie to think deeply about the flowering plants, and to realize the transformation they had wrought upon our world. Eiseley's essay rings as true today as it did when it was first published, almost fifty years ago. A massive accumulation of new scientific knowledge over recent decades has only strengthened his insights. Though the fossil record of plants is quite meager (when compared to that of animals), it does indicate that today's land surfaces are embellished with a diversity of life far greater than in the distant past. Yes indeed, the flowering plants have changed the world. The poet ended his essay as follows: "WIthout the gift of flowers and the diversity of their fruits, man and bird, if they had continued to exist at all, would be today unrecognizable. Archaeopteryx, the lizard-bird, might still be snapping at beetles on a sequoia limb; man might still be a nocturnal insectivore gnawing a roach in the dark. The weight of a petal has changed the face of the world and made it ours."1 Is it possible that a single lineage of green plants, not particularly animated, could fashion major changes on the land surfaces of our planet? Changes that encouraged hordes of animals to follow along in explosive variety? And is it really possible that our human ascendancy was powered, if not by petals, then at least by the plants that bear them? In this book we will do more than simply discuss flowers and remind ourselves of their importance. Our final purpose will be to show how the flowering plants have, over more than 100 million years, transformed much of the living world. But before we do that we need to get up close and personal with flowers and the plants that bear them. After that, we'll discuss what flowers do, why they do it, what helps them, and what hinders them. Then we'll consider what distinguishes the flowering plants, and what makes them so special. Finally, we'll examine how they have transformed terrestrial ecosystems, supported the origin of primates, and helped us humans become the masters of our planet. So let's begin our journey by taking a close look at a few of our common, yet elegant, wildflowers.
Library of Congress Subject Headings for this publication:
Botany -- Miscellanes.
Plants -- Miscellanes.
Flowers -- Miscellanes.