All plants may be said to have had a natural habitat originally determined for them at the different geological epochs of time at which vegetation in some form or other may be supposed to have commenced, but the secondary causes which now determine the habitat of plants may be briefly stated to be: The chemical nature of the soil in which they are found - some plants delight in siliceous soil; others in calcareous, or limestone, or gypseous soils; other plants again prefer a soil impregnated with sea-salt; others gravel or clay soil; others prefer to grow in the cracks of granite rocks; and other plants appear more accommodating than any of these in their requirements, and seem to follow man to minister to his comfort or his luxury, and are indiscriminately found in any of these soils, especially if they have been in any way decomposed or intermixed: The elevation of land above the level of the sea, and, within certain limits, the latitude of the countries in which they are found, where the daily average annual amount of sun-heat during the season of growth of plants is above the degree of heat necessary first to start them into growth, and sufficient to enable them to complete their full growth, and which in the case of annual plants must necessarily be above freezing-point, or they could not germinate: The relative situation of islands to continents or mainland: The presence of vast tracts of land covered with fresh or salt water, and the currents in the latter: The state of the atmosphere, its dryness or moisture, its currents of wind, its chemical composition - some plants delight in an atmosphere charged with chloride of sodium or sea-salt which would be destroyed by an application of such salt to their roots: The amount of sunlight, often as important as sun-heat, and, in fact, as we approach the poles, replacing its want: The power of the chemical or actinic rays of the sun: The inherent vital power in the plant itself to resist destructive agencies.

Upon some or other of the above or similar secondary causes, with here and there anomalous exceptions, the habitat of plants may be said to depend.

We must now proceed to the main part of our subject - the plant - and try to elucidate some of its characteristics.

Each plant is an aggregation of cells; each cell consists of a little transparent spherical sac or globe; the outer membrane enclosing it is made of cellulose, which, by the by, is insoluble in water, containing in it liquid or viscid granular matter, sometimes called the primordial utricle, or protoplasm, made of carbon, oxygen, hydrogen, and nitrogen, and some sulphur and phosphorus; and within the protoplasm is a nucleus. These cells grow either by internal or external growth, or by division of the cell into two cells (by plant growth is meant either formation of new cells, or increase of cells already made, or thickening of cell walls). Each cell nourishes another cell; and where there is no pressure the spherical or globe shape will remain; but where there is pressure this appearance is soon lost sight of, and it then assumes various forms and shapes, and is called cellular tissue. The outer covering of all plants is called the epidermis, and is in reality cellular tissue. As a consequence of cell growth, each plant has the power of reparation of injuries done to it.

For convenience of classification, each plant will be found to belong to some collection of plants called a species, supposed to have had, in monoecious plants, a common ancestor, or in dioecious plants, a pair of ancestors; and each species will belong to a group of one or more species called a genus (or genera, in plural), and each genus not less permanent or distinct than the different species included in it.

Each genus, in like manner, will be found to have been grouped with other genera, and then placed in a family, as a natural order is called; and each family in its turn will again have to be arranged into one or other of the following four great classes: First, Dicotyledonous plants - that is, plants whose seed is furnished with two cotyledons, or seed-lobes: by far the larger number of species of flowering-plants belong to families of this class, and from the structure of their stems they are also called Exogenous. Second, Mono-cotyledonous plants - that is, plants whose seed is furnished with only one cotyledon, or at least only one is apparent, and from the structure of their stems they are called Endogenous. Third, Polycotyle-donous plants, with three or more cotyledons, of which the family of Coniferae or Eir tribe and Palms are good examples; and, Fourth, Acotyledonous plants - that is, plants whose seeds or spores have no cotyledons, of which Eerns, Mosses, and Fungi are good examples, and from the structure of their stems they are called Acrogenous. Our remarks are applicable for the most part only to plants belonging to the first and second of these great classes.

Each plant is named with its generic and specific name, generally in Latin or Greek. Many of the specific names have been chosen according to the characters of parts of the plant, such as the leaf, as serrata, dentata, salicifolia; or according to their local situations or habitat, such as, arvense, pratense, nemorosum, sylvaticum, aquatica, rupestre, or nivalis; or according to the uses or attributes of the plant, such as somniferum; or according to its time of flowering, as vernum, cestivum, or autumnalis.

The origin of species has recently been the subject of much controversy, and much time and labour expended to prove that the characteristics of species are not of such permanent nature as to be entitled to be regarded as the landmarks in plant classification they have hitherto been. However this may be, it will be well to bear in mind that whatever may be the final result of these inquiries, and which really do no harm, it will be always necessary to make use of specific descriptions of the characteristics of plants in order to render them intelligible, and to arrive at accurate views of Geographic Botany.

Each plant has, for the purpose of holding it in its place, and for obtaining nourishment and for storing it up sometimes, as in biennial plants, a root or descending axis, or earth end, or alkali extremity, formed at the end of the embryo, but in connection or continuation with the stem, and from which numerous root-fibres, covered with fine hair-like roots - in reality elongations of cells in the epidermis or outer covering of the root-fibre. Each plant, in like manner, has a stem or ascending axis in connection or continuation with the root, the plumule (of which more hereafter), elongated, with leaves and buds on it, sometimes called the air end, or stem fabric of the plant. Where the root ends and the stem begins in the full-grown plant, or in the seed after germination where the radicle ends and the plumule begins, there is situate what is called the neck (collum, in Latin), by gardeners "the collar." It is a guide in planting trees by which the danger may be avoided either of planting too deeply beneath the surface of the soil, and which would therefore be burying part of the stem, or of leaving it too much exposed above the surface, so that the cold of winter might injure and destroy the plant; it is a very sensitive part, and easily injured by the spade or by the foot in the operation of treading.

Each plant, not a monocotyledonous one, at its earliest growth, will have a simple root, called a tap-root, from which other rootlets, called root-fibres, branch off; the length of this tap-root depends on the physical nature of the soil and its resistance to downward growth. In seedling fruit-trees in their early life it should be cut off to encourage root-fibres, so that they may be more easily transplanted, and be encouraged to produce their roots nearer the surface of the ground, where they will be more accessible to the sun's rays and the gardener's spade than they would be if buried deeply beneath the surface of the soil.

The following are some of the different forms of roots: the tap-root, as in Carrot; the fusiform root, as in Radish; the napiform root, as in Turnip; the fibrous root, as in grasses, or in plants of simple annual growth; the tubercular root, or tubercles, as in Orchis; the annulated or ringed root; the nodulose root (Dropwort), or the fasciculated tuber, as in Dahlia; and branched roots which become woody, such as in roots of trees and shrubs.