Before commencing an intensive study of any aspect of botanical science or of any particular problem which deals with plants, it will be well for us to make a brief survey of the plant kingdom as a whole, and of some of the more important structures and functions of plants in general.
About 250,000 differents kinds or species of plants have been discovered and described, and every year botanical exploration and careful study bring more of them to our knowledge. We have seen that the problem of systematic botany is to name this host of plants and to arrange and classify its members to a logical system. Over many of the details of such a classification difference of opinion still exists, but there is now rather general agreement as to the main groups into which the plant kingdom should be divided. Four such divisions are commonly recognized.
These are lowly plants, various in their structure, activities, and methods of reproduction, but agreeing in the possession of a simple body without roots or leaves and in multiplying by single-celled spores. The majority of Thallophytes inhabit water or moist places and are small and soft-bodied plants.
There are two main series of Thallophytes: The Algae which possess the green pigment chlorophyll and are thereby able to manufacture their own food, and which include all the seaweeds and their fresh-water allies; and the Fungi, which lack chlorophyll and consequently are obliged to obtain their food from living animals and plants or from dead organic material. Here belongs the vast array of bacteria, molds, blights, rusts, toadstools, mushrooms, and similar plants, many o fwhich live as parasites and are often the cause of serious disease of man and the lower organisms.
These plants are distinguished from the Thallophytes chiefly by their more highly developed sexual structures and their more complicated methods of reproduction. The plant body of the Bryophytes has no roots, and in many cases consists of only a flat, strap-like mass of green tissue, but the higher members of the group possess very simple stems and leaves. The plants are small and inconspicuous, and generally thrive best in moist situations. Bryophytes are subdivided into the simple and lowly Liverworts (Hepaticae) and the commoner and more highly specialized Mosses (Musci).
These possess true roots, stems, and leaves, essentially similar in structure to those of the Seed Plants, but they still reproduce by spores rather than by seeds. Compared with Bryophytes, the plant body is large and vigorous, and it is well adapted to life on land. The three important subdivisions of the Pteridophytes are: The Ferns (Filicales), possessing large and feathery leaves on the backs of which the spores are produced; The Club Mosses or Ground Pines (Lycopodiales), which have spore-bearing cones, solid stems and scale-like, spirally arranged leaves; and the Horsetails (Equisetales) also possessing cones but with jointed, hollow stems and minute, whorled leaves.
The dominant and and familiar portion of the earths vegetation today consists of these plants, which are well adapted for life on land and often attain great size. Their distinctive feature is the production of a complex, many-celled reproductive body, the seed, in which is contained an embryo plant and a supply of stored food.
Seed Plants are very numerous and exceedingly varied in form and structure, ranging from small and delicate herbs to huge trees over three hundred feet tall. They are the most conspicious and best known of all the divisions of the plant kingdom, and provide the great bulk of the foods, timbers, fibers, and other vegetable products which form the basis of our civilization.
Two major subdivisions of the Seed Plants are recognized: The Gymnosperms, which have primitive, often cone-like flowers and bear their seeds openly exposed on scales as in our common coniferous trees; and the Angiosperms, in which there is usually a typical flower with its various floral parts, including an ovary in which the seeds are enclosed during their development. There are only about 450 species of Gymnosperms living today, but the Angiosperms are an enormous group of more than 130,000 species and are our most familiar plants. They are divided again into two main groups, the Dicotyledons and the Monocotyledons, which differ from each other in the structure of the seed, leaf, stem, and flower.
Underlying the differences by which these various groups are distinguished from one another, there are many fundamental similarities in structure and function which are common to all plants; but the marked changes which appear as we pass from the lowest to the highest types make very difficult a concise description of the characteristics of the plant kingdom as a whole. It will therefore, be profitable for us to confine our attention, at first, mainly to those plants which are most familiar to everyone and of most obvious and immediate interest to man - the Seed Plants.
The great variety of plant types and the diversity of conditions under which they live renders it difficult to make general statements about plants which are universally true, for exceptions to any such statement may usually be found. Indeed, variability is one of the most notable characteristics of all life. The student should therefore bear in mind that many of the facts and principles set forth briefly and simply in an elementary text are to be taken as true for typical cases and under ordinary conditions, and they should be careful not to accept them as necessarily and universally true for all plants and under all conditions. Living things are too complicated to be described completely in simple formulas.