Botany as a Science
Botany is the branch of biology concerned with the scientific study of plants. Traditionally, botanists studied all organisms that were not generally regarded as animal. However, advances in our knowledge about the myriad forms of life, especially microbes (viruses and bacteria), have led to spinning off from Botany the specialized field called Microbiology. Still, the microbes are usually covered in introductory Botany courses, although their status as neither animal nor plant is firmly established.
Plants are living entities, and material presented within Biology will have relevance here, most particularly at the cellular and subcellular levels of organization (Chapter 2). Both plants and animals deal with the same problems of maintaining life on planet Earth — their approaches seem quite different, but the end result is the same: continued existence in an organized state, as part of a universe whose tendency is towards greater disorganization. Back on Earth, however, it is a fact that microbes, plants, and animals comprise a very interdependent system. We divide them apart, because our minds work best that way. We categorize and learn common features or properties of the categories. This approach is neither right nor wrong, but is clearly efficient for our minds. Nonetheless, it is desirable to regularly step back and realize that the boundaries between categories are often just constructs, and exceptions to our categories usually abound.
It was alluded to in the opening definition that Botany is a science. Just what makes Botany, or anything else a science? It is important to acquire a grasp of the fundamentals of science itself to fully appreciate both how botanical knowledge was gained as well as how it can be used. It usually becomes uninteresting to acquire facts simply for the sake of knowing. Humans do not just appreciate mountains because they are there, they climb them because they are there!
Biology is defined as the study of life, and Botany is that discipline within Biology concerned with the study of living organisms called plants and with certain other living things that are not plants (but are not animals either).
Like many words in common usage that apply to biological entities or concepts, the term plant is more difficult to define than might be at first obvious. Although botanists describe a Kingdom Plantae, the boundaries defining members of Plantae are more inclusive than our common concept of a "plant". We are tempted to regard plant as meaning a multicellular, eukaryotic organism that generally does not have sensory organs or voluntary motion and has, when complete, a root, stem, and leaves. However, botanically only vascular plants have a root, stem, and leaves, and even some vascular plants, such as certain carnivorous plants and duckweed, fall afoul of that definition. But to be fair, the vascular plants are the plants we tend to encounter every day and that most people would readily regard as "plants".
A more significant point of departure between Plantae and plants occurs among the seaweeds. Technically, only a relatively minor group of seaweeds (the chlorophytes or green algae) are members of the Kingdom Plantae. The majority of seaweeds, like the kelps (very large brown algae from the Order Laminariales), despite a superficial appearance of such, lack true stems, leaves, roots, and any kind of vascular systems as found in higher plants. Thus, the kelps are not Plantae; but are they plants? Certainly if we regard the green algae as plants, it is difficult to exclude the more prominent red and brown algae of our coastal waters.
Another, much broader definition for plant is that it refers to any organism that is photoautotrophic—produces its own food from raw inorganic materials and sunlight. This is not an unreasonable definition, and is one that focuses on the role plants typically play in an ecosystem. However, there are photoautotrophs among the Prokaryotes, specifically photoautotrophic bacteria and cyanophytes. The latter are sometimes called (for good reasons) blue-green algae. Then there arises the problem that many people would consider that a mushroom is a plant; a mushroom is the fruiting body of a fungus (Kingdom Fungi) and not photoautotrophic at all, but saprophytic. However, there are more than a few species of flowering plants, fungi, and bacteria that are not autotrophic, but parasitic.
We cannot hope to offer a firm answer. The list of characteristics that separate the Plantae from the other biological kingdoms provides at least a technical definition, but realize it is only a technical definition. The problem this lack of precision or agreement in the definition of "plant" presents is one of understanding statements, often encountered in Wikipedia (and other) articles, of the sort: ...xylem is one of the two transport tissues of plants. In general it cannot be assumed this means all plants, algae through flowering plants. It very probably does not include fungi or bacteria. Indeed, it is usually safest to assume the discussion is about vascular plants (essentially the ferns, conifers, flowering plants, and a few others; see discussion below on "General Terminology") unless stated differently (e.g., ...in vascular and non-vascular plants this is such and such).
Plants as Organisms
The distinction between life and non-life is not as easily made as you might think. There exist intracellular "parasites" that are progressively less alive in terms of being metabolically active.
Plants and their Uses
There can be no disputing the fundamental significance of plants to the ecology of our planet. Photosynthetic plants utilize energy arriving from the sun to create complex organic molecules from inorganic substances, and by this process contribute oxygen to the atmosphere. Advanced animal life is very much dependent upon this source of oxygen, as well as the organic molecules that form the basis of nearly every food web on the planet. However, humans utilize plants in many ways, especially as sources of pleasure, food, and material for shelter, clothing, and more. Consider here the role plants play in our everyday lives and in our economy.
Introduction to Plant Classification
At the beginning of this chapter it was suggested that each of us categorizes information we encounter on a daily basis. Our minds seem to want to find relationships between facts and observations, to erect mental bins in which to place new items with previous "facts". This natural human process is the basis for prejudice, in as much as "facts" categorized together can become strongly associated. But these are personal constructs. In order for scientists of many races, speaking many languages, and coming from all manner of backgrounds and experiences to work productively together to solve common problems, the objects with which they work must be classified within a universally accepted framework.
The classification of living things is called systematics, or taxonomy, and ideally should reflect the evolutionary history (phylogeny) of the different organisms.
Taxonomy arranges organisms in groups called taxa, while systematics seeks clues to their relationships. The dominant system of Scientific Classification is called Linnaean taxonomy, and includes classification ranks as well as an organism naming convention called binomial nomenclature.
Traditionally, all living things were divided into five kingdoms:
However, this five-kingdom system has been replaced by Carl Woese's three-domain system, which focuses on phylogenic roots and comparison of DNA structures. The older approach utilized visual observation as the basis of classification. The three domains reflect whether cells have nuclei (eukaryotic) or not (prokaryotic), as well as differences in cell membranes and cell walls.
Recall (and review as necessary) how these groupings relate to the sequence of events in the evolutionary history of life as summarized in Timeline of Evolution. You will return to the subject of Scientific Classification to consider in much more detail the groups of organisms studied in Botany, beginning with Chapter 7. First, however, we shall turn our attention to the structure and function of cells and eventually to gain an understanding of plant structure (plant anatomy) and function (plant physiology).
In Section II of this text we will delve much deeper into "plant" systematics. But you should be aware of some general terms related to classificatory schemes that are used regularly in discussing plants. You have probably encountered these terms many times, although may not be aware of their exact definitions. For example, much of the material in Section I of this textbook is biased towards flowering plants. That is, much of the descriptive material here as well as at Wikipedia refers specifically to these. Flowering plants are angiosperms; plants that have flowers and produce seeds, and comprise the majority of the plants we would normally encounter in say a nursery if not on the street, field, or empty lot. Seed-bearing plants include both the angiosperms and the gymnosperms, the latter now treated as a modern group called conifers. The conifers are also common plants, especially in higher latitudes, but bear cones instead of flowers. Both conifers and flowering plants develop vascular tissues internally that conduct fluids (especially water) throughout the plant. Included in the vascular plants are ferns. Ferns have vascular tissue, but reproduce by spores. They do not produce seeds and do not bear flowers.
- Discussion of this chapter