MODERN BOTANY - P. RAVEN - 1990

CHAPTER 1. INTRODUCTION TO BOTANY

Evolution of Communities

The colonization of land by plants profoundly transformed the face of the continents. Looking down from an airplane upon the vast expanses of deserts or mountain ranges below, one can easily imagine what the world looked like before this historic milestone. Yet even in such seemingly barren landscapes, the terrestrial traveler will discover a remarkable diversity of plant life scattered among rocks and sand. In regions with a more temperate climate and more frequent rainfall, plant communities dominate the land, defining its very character. In fact, they largely create the land itself. The tropical rain forest, meadow, steppe, tundra—these names conjure up distinct landscapes (Fig. 1-9) whose primary features are plants, whether rising in dark-green canopies overhead, carpeting the ground with wildflowers, or rolling as far as the eye can see in golden waves of steppe grasses. Only when describing specific biomes—that is, large-scale natural communities characterized by distinct, climate-determined combinations of plants and animals—are other features considered alongside trees, shrubs, and herbs, such as the presence of deer, antelopes, rabbits, or wolves.

Class="center">Fig. 1-9. These photographs illustrate the striking diversity of Earth's biological communities. A — A tropical rain forest (in this case, in Trinidad), the richest and most complex biome on Earth. At least two thirds of all organismal species inhabit the tropics, where a single hectare may support as many tree species as the entire territory of the United States and Canada combined—approximately 700. B — Deciduous forests of the temperate zone, covering much of the eastern United States and southeastern Canada, are dominated by trees that shed their leaves with the approach of the cold winter. In early spring, these forests burst into a vibrant, fresh green once more. C — Savannas, tropical biomes characterized by a sharply defined dry season. In Africa (the famous Ngorongoro Crater in Tanzania is shown here), they are inhabited by massive herds of herbivorous mammals, such as these wildebeests. The tree in the photograph is an euphorbia. D — Regions with a Mediterranean-type climate are rare on Earth. They are characterized by a cool, moist winter—the season of plant growth—and a hot, dry summer, during which plants enter a state of dormancy. Shown here is the western fringe of the Mojave Desert in California, near Lancaster, blanketed with California poppies and other herbaceous species that thrive under these conditions.

How did vast plant communities, which often occupy significant continental areas, come to be? To some extent, we can retrace the Evolution of the plant species that compose them, as well as the animals that inhabit them. However, even when synthesizing all accumulated knowledge, we are only beginning to grasp the incomparably more complex development of entire systems of organisms that make up various communities. Together with their abiotic environment, these systems are known as ecosystems. Ecosystems are discussed in more detail below; for now, it is sufficient to define them as stable communities of successive generations of individuals. Some of their components, such as large trees, live for up to several thousand years, whereas others, notably microorganisms, persist for only a few hours or even minutes; yet ecosystems as a whole exhibit a remarkable stability (rather than static permanence) and, once equilibrium is reached, remain unchanged for centuries. Our great-grandchildren will someday walk through a forest that remembers our great-grandparents. Where they saw pine, mulberry, blueberry, and vole, one will—even generations later (provided, of course, the forest still survives)—encounter essentially the same species of plants and animals, and in very similar proportions.

An ecosystem Functions as a cohesive whole despite the fact that many of its constituent organisms compete for resources. In fact, every living creature, down to the tiniest bacterial Cell or fungal spore, serves as food for other organisms. As a result, solar energy captured by green plants is transferred through finely tuned trophic chains involving diverse groups of organisms before it is eventually dissipated. Moreover, the interactions of these organisms with one another and with their abiotic environment drive an orderly cycling of elements, such as nitrogen and phosphorus. Energy must be continuously supplied to the ecosystem, whereas the elements that make up Living organisms are returned to the soil with their remains and, following decomposition by resident Fungi and Bacteria, re-enter the cycle. Energy transfer and nutrient cycling involve a complex sequence of events in which each group of organisms plays a strictly defined role; consequently, it is impossible to alter any single component of an ecosystem without risking the disruption of the equilibrium upon which its stability depends.

The productivity of virtually all ecosystems is underpinned by plants, Algae, and photosynthetic bacteria—the only living forms capable of absorbing solar energy and synthesizing the organic molecules essential for life, both for themselves and for all other organisms. There are approximately half a million autotrophic species and at least 8 to 10 times as many entirely dependent heterotrophic ones. Animals, including humans, can obtain many vital substances, such as Essential Amino Acids, Vitamins, and mineral components, solely from plants or other photosynthetic

forms. Furthermore, the oxygen released into the atmosphere by these organisms makes the very existence of life on land and in the upper layers of the ocean possible. It is required by the vast majority of creatures, including the autotrophs themselves, for metabolic Respiration.

The Emergence of Humans

Humans appeared relatively recently in the course of organic evolution (Fig. 1-10). If the entire past history of Earth is compressed into a single 24-hour day (beginning at midnight), Cells arose in warm seas just before dawn, the first Multicellular Organisms appeared much later after sunset, and the first human emerged merely half a minute ago (approximately 2 million years ago). Yet humans have managed to alter the face of the planet more profoundly than any other animal—and almost as much as land plants—reshaping the biosphere to suit their own needs, ambitions, or whims.

Fig. 1-10. The clock of biological evolution. Life arose quite early in Earth's history (by 6 a.m. on the 24-hour scale). Multicellular organisms were absent until the twilight of this "day," and Homo sapiens appeared less than a minute before midnight.

The Development of agriculture (discussed in detail in Chapter 30), which began at least 11,000 years ago, eventually enabled large numbers of people to live in cities. This allowed human culture to specialize and diversify. One of the distinguishing traits of every culture is its perspective on itself and The Nature of other living creatures, including plants. Ultimately, the science of biology emerged within human communities whose existence was made possible by the domestication of plants. The branch of biology dedicated to The Study of plants—and, traditionally, bacteria, fungi, and photosynthetic protists (algae)—is called botany.

Botany as a Science

The study of plants has spanned millennia; however, like all branches of science, it has only branched into specialized disciplines over the past 300 years. Little more than a century ago, botany was still considered a branch of medicine, pursued primarily by physicians as a hobby or a core professional pursuit. Today, however, it is a vital scientific discipline with numerous sub-disciplines. Plant physiology examines how plants function—that is, how they absorb and transform energy, grow, and develop. Plant Morphology investigates plant form, plant anatomy explores internal Structure, and plant systematics, or Taxonomy, deals with plant Nomenclature and Classification. Among many other specialized fields are Cytology (the Study of Cells), genetics (the science of heredity), and ecology, which investigates the relationships between living organisms and their environment.

In the past, all organisms were divided into plants and animals; with the discovery of microscopic forms, this dichotomy was extended to them as well. Fungi were classified as plants, likely because most of them are immobile and their growth habit resembles ordinary green plants more than animals. The differences between bacteria and all other creatures are greater than those separating any other groups, as will be shown in the next chapter and in greater detail in Chapter 11. Viruses (Chapter 12) are not actually living organisms at all; rather, they are mere fragments of the genetic material of other creatures that reproduce by hijacking the cellular metabolic machinery. Some of them originated from bacteria, while others apparently evolved from eukaryotes.

Eukaryotes encompass many highly diverse groups of unicellular organisms. Heterotrophic eukaryotes, traditionally called Protozoans, are classified as animals, whereas autotrophic ones are grouped with plants (algae). However, anyone who studies these autotrophs and heterotrophs in detail will recognize that close relationships exist between them, rendering METABOLISM/2.html">THE CONCEPT OF two distinct evolutionary lineages untenable. Consequently, all Unicellular Eukaryotes are now united into The Kingdom Protista, discussed in detail in Chapters 14 and 15. Several evolutionary lines of algae (green, brown, and red) independently acquired multicellularity. All true plants are also multicellular and are directly linked only to green algae, from which they are believed to have evolved during the colonization of land. This unique combination of traits—multicellularity, terrestrial habit, immobility, and the capacity for Photosynthesis—justifies placing plants in a separate kingdom, one defined much more narrowly today than in the past.

Whereas plants obtain nourishment through photosynthesis (with the exception of a few species that have lost this ability but clearly evolved from photosynthetic ancestors), animals ingest their food, and fungi (as shown in Chapter 13) absorb it after extracellular Digestion via secreted Enzymes. Each of these three multicellular evolutionary lineages is considered a distinct eukaryotic kingdom; the remaining eukaryotes (a highly heterogeneous assemblage) are assigned to the kingdom Protista.

This book examines the organisms traditionally studied by botanists—namely, plants, bacteria, viruses, fungi, and autotrophic protists (algae)—essentially all living things except the animals studied by zoologists. Although we do not include algae, fungi, bacteria, and viruses within the plant kingdom, nor refer to them as plants in this book, they are discussed here by tradition and because they are typically included in botanical curricula, much as botany itself was once considered a branch of medicine. It should be noted, however, that virology, bacteriology, phycology (the study of algae), and mycology (the study of fungi) are fully independent, well-established fields of knowledge.

Botany and the Future

In Chapter 1, we touched upon a broad range of topics, from THE ORIGIN OF life on Earth to the evolution of plants and ecosystems, as well as the development of agriculture and civilization. These profound questions are of interest to people across many diverse professions. Botanists and agronomists face the pressing challenge of feeding the planet's rapidly growing population, as discussed in Chapter 30. Modern plants represent the most promising renewable energy source for human enterprise, while fossilized plants have formed the vast reserves of gas, oil, and coal upon which modern industrial civilization depends. The Role of plants, along with algae and photosynthetic bacteria, deserves our attention for another, even more vital reason. As the primary producers of the global ecosystem, these organisms supply all other creatures, including humans, with energy, oxygen, and numerous other life-sustaining substances. By studying botany, one can better appreciate the critical environmental challenges of our time and, through understanding them, contribute to creating a healthier world.

Exciting new possibilities for human utilization of plants, unlocked in recent years, are discussed throughout this book. Today, we can stimulate plant growth, combat pests and weeds, and produce hybrids far more effectively than ever before. The Significance of these novel techniques grows continuously alongside the expansion of scientific discoveries and their Practical Applications. For instance, Introduction/32.html">Genetic Engineering (discussed in Chapter 30) makes it theoretically possible to transfer natural or synthetic genes from certain species of plants and animals to others in order to confer specific traits. This approach, first successfully applied in 1973, has already spurred billions of dollars in investments and raised hopes for solving many of the formidable problems facing humanity.

Future discoveries will undoubtedly surpass our wildest dreams, vastly expanding the boundaries of current knowledge.

Moving on to Chapter 2, where our focus shifts to The Cell—invisible to the naked eye—It is important to keep these broader issues in mind. A foundational understanding of plant biology is valuable in its own right and essential for many fields of endeavor. At the same time, botany is becoming increasingly intertwined with critical societal challenges and the difficult decisions that will be required to address them. Our own future, the future of the planet, and the future of all plants—whether individual species or the ecosystem components upon which all life, including human, depends—rely on our knowledge. Thus, this book is intended not only for future botanists, teachers, or researchers, but for all educated people, scientists and laypersons alike, who will be tasked with finding these solutions.