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by Michael J. Pelczar, Jr., Rita M. Pelczar, William Campbell Steere
Theophrastus, a Greek philosopher who studied first with Plato and then became a disciple of Aristotle, is credited with founding botany. Only two of an estimated 200 botanical treatises written by him are known to science: originally written in Greek about 300 bc, they have survived in the form of Latin manuscripts, De causis plantarum and De historia plantarum. His basic concepts of morphology, classification, and the natural history of plants, accepted without question for many centuries, are now of interest primarily because of Theophrastus’ independent and philosophical viewpoint.
Pedanius Dioscorides, a Greek botanist of the 1st century ad, was the most important botanical writer after Theophrastus. In his major work, an herbal in Greek, he described some 600 kinds of plants, with comments on their habit of growth and form as well as on their medicinal properties. Unlike Theophrastus, who classified plants as trees, shrubs, and herbs, Dioscorides grouped his plants under three headings: as aromatic, culinary, and medicinal. His herbal, unique in that it was the first treatment of medicinal plants to be illustrated, remained for about 15 centuries the last word on medical botany in Europe.
From the 2nd century bc to the 1st century ad, a succession of Roman writers—Cato, Varro, Virgil, and Columella—prepared Latin manuscripts on farming, gardening, and fruit growing but showed little evidence of the spirit of scientific inquiry for its own sake that was so characteristic of Theophrastus. In the 1st century ad, Pliny the Elder, though no more original than his Roman predecessors, seemed more industrious as a compiler. His Historia naturalis—an encyclopaedia of 37 volumes, compiled from some 2,000 works representing 146 Roman and 327 Greek authors—has 16 volumes devoted to plants. Although uncritical and containing much misinformation, this work contains much information otherwise unavailable, since most of the volumes to which he referred have been destroyed.
The printing press revolutionized the availability of all types of literature, including that of plants. In the 15th and 16th centuries, many herbals were published with the purpose of describing plants useful in medicine. Written by physicians and medically oriented botanists, the earliest herbals were based largely on the work of Dioscorides and to a lesser extent on Theophrastus, but gradually they became the product of original observation. The increasing objectivity and originality of herbals through the decades is clearly reflected in the improved quality of the woodcuts prepared to illustrate these books.
In 1552 an illustrated manuscript on Mexican plants, written in Aztec, was translated into Latin by Badianus; other similar manuscripts known to have existed seem to have disappeared. Whereas herbals in China date back much further than those in Europe, they have become known only recently and so have contributed little to the progress of Western botany.
The invention of the optical lens during the 16th century and the development of the compound microscope about 1590 opened an era of rich discovery about plants; prior to that time, all observations by necessity had been made with the unaided eye. The botanists of the 17th century turned away from the earlier emphasis on medical botany and began to describe all plants, including the many new ones that were being introduced in large numbers from Asia, Africa, and America. Among the most prominent botanists of this era was Gaspard Bauhin, who for the first time developed, in a tentative way, many botanical concepts still held as valid. In 1665 Robert Hooke published, under the title Micrographia, the results of his microscopic observations on several plant tissues. He is remembered as the coiner of the word cell, referring to the cavities he observed in thin slices of cork; his observation that living cells contain sap and other materials too often has been forgotten. In the following decade, Nehemiah Grew and Marcello Malpighi founded plant anatomy; in 1671 they communicated the results of microscopic studies simultaneously to the Royal Society of London, and both later published major treatises.
Experimental plant physiology began with the brilliant work of Stephen Hales, who published his observations on the movements of water in plants under the title Vegetable Staticks (1727). His conclusions on the mechanics of water transpiration in plants are still valid, as is his discovery—at the time a startling one—that air contributes something to the materials produced by plants. In 1774, Joseph Priestley showed that plants exposed to sunlight give off oxygen, and Jan Ingenhousz demonstrated, in 1779, that plants in the dark give off carbon dioxide. In 1804 Nicolas de Saussure demonstrated convincingly that plants in sunlight absorb water and carbon dioxide and increase in weight, as had been reported by Hales nearly a century earlier.
The widespread use of the microscope by plant morphologists provided a turning point in the 18th century—botany became largely a laboratory science. Until the invention of simple lenses and the compound microscope, the recognition and classification of plants were, for the most part, based on such large morphological aspects of the plant as size, shape, and external structure of leaves, roots, and stems. Such information was also supplemented by observations on more subjective qualities of plants, such as edibility and medicinal uses.
In 1753 Linnaeus published his master work, Species Plantarum, which contains careful descriptions of 6,000 species of plants from all of the parts of the world known at the time. In this work, which is still the basic reference work for modern plant taxonomy, Linnaeus established the practice of binomial nomenclature—that is, the denomination of each kind of plant by two words, the genus name and the specific name, as Rosa canina, the dog rose. Binomial nomenclature had been introduced much earlier by some of the herbalists, but it was not generally accepted; most botanists continued to use cumbersome formal descriptions, consisting of many words, to name a plant. Linnaeus for the first time put the contemporary knowledge of plants into an orderly system, with full acknowledgment to past authors, and produced a nomenclatural methodology so useful that it has not been greatly improved upon. Linnaeus also introduced a “sexual system” of plants, by which the numbers of flower parts—especially stamens, which produce male sex cells, and styles, which are prolongations of plant ovaries that receive pollen grains—became useful tools for easy identification of plants. This simple system, though effective, had many imperfections. Other classification systems, in which as many characters as possible were considered in order to determine the degree of relationship, were developed by other botanists; indeed, some appeared before the time of Linnaeus. The application of the concepts of Charles Darwin (on evolution) and Gregor Mendel (on genetics) to plant taxonomy has provided insights into the process of evolution and the production of new species.
Systematic botany now uses information and techniques from all the subdisciplines of botany, incorporating them into one body of knowledge. Phytogeography (the biogeography of plants), plant ecology, population genetics, and various techniques applicable to cells—cytotaxonomy and cytogenetics—have contributed greatly to the current status of systematic botany and have to some degree become part of it. More recently, phytochemistry, computerized statistics, and fine-structure morphology have been added to the activities of systematic botany.
The 20th century has seen an enormous increase in the rate of growth of research in botany and the results derived therefrom. The combination of more botanists, better facilities, and new technologies, all with the benefit of experience from the past, has resulted in a series of new discoveries, new concepts, and new fields of botanical endeavour. Some important examples are mentioned below.
New and more precise information is being accumulated concerning the process of photosynthesis, especially with reference to energy-transfer mechanisms.
The discovery of the pigment phytochrome, which constitutes a previously unknown light-detecting system in plants, has greatly increased knowledge of the influence of both internal and external environment on the germination of seeds and the time of flowering.
Several types of plant hormones (internal regulatory substances) have been discovered—among them auxin, gibberellin, and kinetin—whose interactions provide a new concept of the way in which the plant functions as a unit.
The discovery that plants need certain trace elements usually found in the soil has made it possible to cultivate areas lacking some essential element by adding it to the deficient soil.
The development of genetical methods for the control of plant heredity has made possible the generation of improved and enormously productive crop plants.
The development of radioactive-carbon dating of plant materials as old as 50,000 years is useful to the paleobotanist, the ecologist, the archaeologist, and especially to the climatologist, who now has a better basis on which to predict climates of future centuries.
The discovery of alga-like and bacteria-like fossils in Precambrian rocks has pushed the estimated origin of plants on Earth to 3,500,000,000 years ago.
The isolation of antibiotic substances from fungi and bacteria-like organisms has provided control over many bacterial diseases and has contributed biochemical information of basic scientific importance as well.
Areas of study
For convenience, but not on any mutually exclusive basis, several major areas or approaches are recognized commonly as disciplines of botany; these are morphology, physiology, ecology, and systematics.
Morphology deals with the structure and form of plants and includes such subdivisions as: cytology, the study of the cell; histology, the study of tissues; anatomy, the study of the organization of tissues into the organs of the plant; reproductive morphology, the study of life cycles; and experimental morphology, or morphogenesis, the study of development.
Physiology deals with the functions of plants. Its development as a subdiscipline has been closely interwoven with the development of other aspects of botany, especially morphology. In fact, structure and function are sometimes so closely related that it is impossible to consider one independently of the other. The study of function is indispensable for the interpretation of the incredibly diverse nature of plant structures. In other words, around the functions of the plant, structure and form have evolved. Physiology also blends imperceptibly into the fields of biochemistry and biophysics, as the research methods of these fields are used to solve problems in plant physiology.
Ecology deals with the mutual relationships and interactions between organisms and their physical environment. The physical factors of the atmosphere, the climate, and the soil affect the physiological functions of the plant in all its manifestations, so that, to a large degree, plant ecology is a phase of plant physiology under natural and uncontrolled conditions; in fact, it has been called “outdoor physiology.” Plants are intensely sensitive to the forces of the environment, and both their association into communities and their geographical distribution are determined largely by the character of climate and soil. Moreover, the pressures of the environment and of organisms upon each other are potent forces, which lead to new species and the continuing evolution of larger groups.
Systematics deals with the identification and ranking of all plants; it includes classification and nomenclature (naming) and enables the botanist to comprehend the broad range of plant diversity and evolution.
In addition to the major subdisciplines, several specialized branches of botany have developed as a matter of custom or convenience. Among them are bacteriology, the study of bacteria; mycology, the study of fungi; algology or phycology, the study of algae; bryology, the study of mosses and liverworts; pteridology, the study of ferns and their relatives; and paleobotany, the study of fossil plants. Palynology is the study of modern and fossil pollen and spores, with particular reference to their identification; plant pathology deals with the diseases of plants; economic botany deals with plants of practical use to man; and ethnobotany covers the use of plants by aboriginal peoples, now and in the distant past.
Botany also relates to other scientific disciplines in many ways, especially to zoology, medicine, microbiology, agriculture, chemistry, forestry, and horticulture, and specialized areas of botanical information may relate closely to such humanistic fields as art, literature, history, religion, archaeology, sociology, and psychology.
Fundamentally, botany remains a pure science, including any research into the life of plants and limited only by man’s technical means of satisfying his curiosity. It has often been considered an important part of a liberal education, not only because it is necessary for an understanding of agriculture, horticulture, forestry, pharmacology, and other applied arts and sciences, but also because an understanding of plant life is related to life in general.
Because man has always been dependent upon plants and surrounded by them, he has woven them into his designs, into the ornamentation of his life, even into his religious symbolism. A Persian carpet and a bedspread from a New England loom both employ conventional designs derived from the forms of flowers. Medieval painters and great masters of the Renaissance represented various revered figures surrounded by roses, lilies, violets, and other flowers, which symbolized chastity, martyrdom, humility, and other Christian attributes.
Methods in botany
The invention of the compound microscope provided a valuable and durable instrument for the investigation of the inner structure of plants. Early plant morphologists, especially those studying cell structure, were handicapped as much by the lack of adequate knowledge of how to prepare specimens as they were by the imperfect microscopes of the time. A revolution in the effectiveness of microscopy occurred in the second half of the 19th century with the introduction of techniques for fixing cells and for staining their component parts. Before the development of these techniques, the cell, viewed with the microscope, appeared as a minute container with a dense portion called the nucleus. The discovery that parts of the cell respond to certain stains made observation easier. The development of techniques for preparing tissues of plants for microscopic examination was continued in the 1870s and 1880s and resulted in the gradual refinement of the field of nuclear cytology, or karyology. Chromosomes were recognized as constant structures in the life cycle of cells, and the nature and meaning of meiosis, a type of cell division in which the daughter cells have half the number of chromosomes of the parent, was discovered; without this discovery, the significance of Mendel’s laws of heredity might have gone unrecognized. Vital stains, dyes that can be used on living material, were first used in 1886 and have been greatly refined since then.
Improvement of the methodology of morphology has not been particularly rapid, even though satisfactory techniques for histology, anatomy, and cytology have been developed. The embedding of material in paraffin wax, the development of the rotary microtome for slicing very thin sections of tissue for microscope viewing, and the development of stain techniques are refinements of previously known methods. The invention of the phase microscope made possible the study of unfixed and unstained living material—hopefully nearer its natural state. The development of the electron microscope, however, has provided the plant morphologist with a new dimension of magnification of the structure of plant cells and tissues. The fine structure of the cell and of its components, such as mitochondria and the Golgi apparatus, have come under intensive study. Knowledge of the fine structure of plant cells has enabled investigators to determine the sites of important biochemical activities, especially those involved in the transfer of energy during photosynthesis and respiration. The scanning electron microscope, a relatively recent development, provides a three-dimensional image of surface structures at very great magnifications.
For experimental research on the morphogenesis of plants, isolated organs in their embryonic stage, clumps of cells, or even individual cells are grown. One of the most interesting techniques developed thus far permits the growing of plant tissue of higher plants as single cells; aeration and continuous agitation keep the cells suspended in the liquid culture medium.
Plant physiology and plant biochemistry are the most technical areas of botany; most major advances in physiology also reflect the development of either a new technique or the dramatic refinement of an earlier one to give a new degree of precision. Fortunately, the methodology of measurement has been vastly improved in recent decades, largely through the development of various electronic devices. The phytotron at the California Institute of Technology represents the first serious attempt to control the environment of living plants on a relatively large scale; much important information has been gained concerning the effects on plants of day length and night length and the effects on growth, flowering, and fruiting of varying night temperatures. Critical measurements of other plant functions have also been obtained.
Certain complex biochemical processes, such as photosynthesis and respiration, have been studied stepwise by immobilizing the process through the use of extreme cold or biochemical inhibitors and by analyzing the enzymatic activity of specific cell contents after spinning cells at very high speeds in a centrifuge. The pathways of energy transfer from molecule to molecule during photosynthesis and respiration have been determined by biophysical methods, especially those utilizing radioactive isotopes.
An investigation of the natural metabolic products of plants requires, in general, certain standard biochemical techniques—e.g., gas and paper chromatography, electrophoresis, and various kinds of spectroscopy, including infrared, ultraviolet, and nuclear magnetic resonance. Useful information on the structure of the extremely large cellulose molecule has been provided by X-ray crystallography.
When plant ecology first emerged as a subscience of botany, it was largely descriptive; today, however, it has become a common meeting ground for all the plant sciences, as well as for other sciences. In addition, it has become much more quantitative. As a result, the tools and methods of plant ecologists are those available for measuring the intensity of the environmental factors that impinge on the plant and the reaction of the plant to these factors. The extent of the variability of many physical factors must be measured. The integration and reporting of such measurements, which cannot be regarded as constant, may therefore conceal some of the most dynamic and significant aspects of the environment and the responses of the plant to them. Because the physical environment is a complex of biological and physical components, it is measured by biophysical tools. The development of electronic measuring and recording devices has been crucial for a better understanding of the dynamics of the environment. Such devices, however, produce so much information that computer techniques must be used to reduce the data to meaningful results.
The ecologist, concerned primarily with measuring the effect of the external environment on a plant, adapts the methodology of the plant physiologist to field conditions.
The plant sociologist, on the other hand, is concerned with both the relation of different kinds of plants to each other and the nature and constitution of their association into natural communities. One widely used technique in this respect is to count the various kinds of plants within a standard area in order to determine such factors as the percentage of ground cover, dominance of species, aggressiveness, and other characteristics of the community. In general, the plant sociologist has relatively few quantitative factors to measure and must therefore take a subjective and intuitive approach, which, nevertheless, gives extremely useful results and some degree of predictability.
Some ecologists are most concerned with the inner environment of the plant and the way in which it reacts to the external environment. This approach, which is essentially physiological and biochemical, is useful for determining energy flow in ecosystems. The physiological ecologist is also concerned with evaluating the adaptations that certain plants have made toward survival in a hostile environment.
In summary, the techniques and methodology of plant ecology are as diverse and as varied as the large number of sciences that are drawn upon by ecologists. Completely new techniques, although few, are important; among them are techniques for measuring the amount of radioactive carbon-14 in plant deposits up to 50,000 years old. The most important new method in plant ecology is the rapidly growing use of computer techniques for handling vast amounts of data. Furthermore, modern digital computers can be used to simulate simple ecosystems and to analyze real ones.
Experimental research under controlled conditions, made possible by botanical gardens and their ranges of greenhouses and controlled environmental chambers, has become an integral part of the methodology of modern plant taxonomy.
A second major tool of the taxonomist is the herbarium, a reference collection consisting of carefully selected and dried plants attached to paper sheets of a standard size and filed in a systematic way so that they may be easily retrieved for examination. Each specimen is a reference point representing the features of one plant of a certain species; it lasts indefinitely if properly cared for, and, if the species becomes extinct in nature—as hundreds have—it remains the only record of the plant’s former existence. The library is also an essential reference resource for descriptions and illustrations of plants that may not be represented in a particular herbarium.
One of the earliest methods of the taxonomist, the study of living plants in the field, has benefitted greatly by fast and easy methods of transportation; botanists may carry on fieldwork in any part of the world and make detailed studies of the exact environmental conditions under which each species grows.
During the present century, many new approaches have been applied to the elucidation of problems in systematic botany. The transmission electron microscope and the scanning electron microscope have added to the knowledge of plant morphology, upon which classical taxonomy so much depends.
Refined methods for cytological and genetical studies of plants have given the taxonomist new insights into the origin of the great diversity among plants, especially the mechanisms by which new species arise and by which they then maintain their individuality in nature. From such studies have arisen further methods and also the subdisciplines of cytotaxonomy, cytogenetics, and population genetics.
Phytochemistry, or the chemistry of plants, one of the early subdivisions of organic chemistry, has been of great importance in the identification of plant substances of medicinal importance. With the development of new phytochemical methods, new information has become available for use in conjunction with plant taxonomy; thus has arisen the modern field of chemotaxonomy, or biochemical systematics. Each species tends to differ to some degree from every other species, even in the same genus, in the biochemistry of its natural metabolic products. Sometimes the difference is subtle and difficult to determine; sometimes it is obvious and easily perceptible. With new analytical techniques, a large number of individual compounds from one plant can be identified quickly and with certainty. Such information is extremely useful in adding confirmatory or supplemental evidence of an objective and quantitative nature. An interesting by-product of chemical plant taxonomy has resulted in understanding better the restriction of certain insects to specific plants.
Computer techniques have recently been applied to plant taxonomy to develop a new field, numerical taxonomy, or taximetrics, by which relationships between plant species or those within groups of species are determined quantitatively and depicted graphically. Another method measures the degree of molecular similarity of deoxyribonucleic acid (DNA) molecules in different plants. By this procedure it should be possible to determine the natural taxonomic relationships among different plants and plant groups by determining the extent of the relationship of their DNA’s: closely related plants will have more similarities in their DNA’s than will unrelated ones.