All plants are by this system first divided into the Vasculares and the Cellulares; and to explain the difference between these two great divisions, it will be necessary to say a few words on the construction of plants, though this subject belongs properly to vegetable physiology. All plants are composed of two kinds of matter: viz. Cellular Tissue, which may be compared to the flesh of animals; and Vascular Tissue, which consists of spiral vessels and ducts, which may be compared to the nerves and veins. If any one will take the leaf of a hyacinth and break it by doubling it first on one side and then on the other, and will then draw the parts gently a little way asunder, the spiral vessels will be seen distinctly with the naked eye; as though very fine, they are sufficiently strong to sustain the weight of the lower part of the leaf for a short time. Now these vessels are much more conspicuous in some plants than in others; and in some of the inferior plants, such as lichens and fungi, they are wanting altogether. Their presence or absence has therefore been chosen to mark the two great divisions of the Natural System; the Vasculares being those plants which have both vascular and cellular tissue, and the Cellulares being those which have cellular tissue only.

The Vasculares are much more numerous than the Cellulares; and they are re-divided into sub-classes, which it also requires the aid of vegetable physiology to explain. All plants, when in a growing state, require the moisture taken up by their roots to be elaborated, and mixed with air in the leaves before it becomes sap; that is, before it is fit to contribute to their nourishment. Now, when a seed begins to germinate, its root descends into the ground and its plumule, or ascending shoot, rises upwards; but the leaves folded up in the latter are too weak and tender to elaborate the sap; and besides, they cannot act till they are fully expanded, and they want nourishment to give them strength to unfold; the roots are also not sufficiently developed to absorb moisture. To supply the wants of the young plant while it remains in this feeble and imperfect state, a quantity of albumen is laid up in the seed; and it is evident that this substance must be extremely nourishing, as it forms, when ground, what we call flour. The young plant is thus provided with food, till its roots are sufficiently developed to obtain it from the soil; and to enable it to elaborate this food and to turn it into sap, it is, in most cases furnished with one or more seed-leaves, Fig.102.—Cotyledons, Leaves, and Wood of a Dicotyledonous Plant. or cotyledons, (see c in fig. 102,) which drop off as soon as the true leaves are sufficiently advanced to be able to perform their proper functions. The cotyledons differ in number, form, and substance in the different genera; but in all plants, they are very different from the true leaves, being admirably contrived for answering the end for which they are designed; and it having been found that the plants having two or more cotyledons differ widely in many other respects from those having only one coty ledon, the number of the cotyledons has been chosen as the distinguishing mark of the second great division of plants according to the Natural System.

The Vascular plants are therefore again divided into the Dicotyledonous plants, which have two or more cotyledons; and the Monocotyledonous plants, which have only one cotyledon: and to these modern botanists add the Acotyledonous plants, which have no cotyledon, as some of them have spiral vessels, or at least ducts, though most of the Acotyledonous plants belong to the Cellulares. These divisions are not only marked by their cotyledons, but they are so distinct in other respects, that it is sufficient for a botanist to see a leaf, or even a bit of wood without leaves, of any plant, to know at first sight to which of these three divisions the plant belongs.

If the leaf of a Dicotyledonous plant be examined, it will be found to have a strong vein up the centre, from which other veins proceed on each side (as shown at a in fig. 102); and if it be held up to the light, the rest of the leaf will be found to be intersected by numerous smaller veins, so as to appear like network, and hence these leaves are said to be reticulated. The trunk and branches of trees belonging to this division consist, when young, of pith, wood, and bark. At first the substance within the bark is little more than pith, but as the returning sap deposits every year a layer of wood just within the bark, which presses against the previous layers so as to contract them, they press in turn against the pith, which becomes smaller and smaller every year, till at last, in old trees, it is scarcely perceptible. The layers of wood are always perfectly distinct from each other, and they are called concentric rings (b); while faint lines, with which they are intersected, and which proceed like rays from the remains of the pith in the centre, are called medullary rays. A layer of wood being deposited every year, the age of the tree may be discovered by counting the concentric rings; also if the tree has grown rapidly, the layers will be thicker than if it has grown slowly; and if it has had one side more exposed to the sun than the other, the remains of the pith will be on one side instead of in the centre, and the layers will be thinner on that side than on the other. The newest layer of wood, which is called the alburnum or sap-wood, is of a paler colour and more porous texture than the rest of the tree, and it is of less value as timber. It is from the manner in which the successive layers of wood are deposited that Dicotyledonous trees are called exogens, which signifies, to increase from the outside.

If the leaf of a Monocotyledonous plant be held up to the light, the principal veins will be found to form parallel lines of nearly equal thickness, the central one being very little, if any, Fig.103.—Monocotyledonous Plant. larger than the others (see a in fig. 103). The trees belonging to this division are all natives of the tropics, and their softest and newest wood is in the centre, where fresh deposits are made every year inside the old wood; and hence, these trees are called endogens, which signifies, to increase from within. The wood of these trees has neither medullary rays nor concentric rings; and a section of it appears pierced with numerous holes (b), as may be seen by cutting off a slice of bamboo. The germination of a Monocotyledonous plant, with the cotyledon remaining in the ground, is shown at c.

The Dicotyledones and the Monocotyledones have all visible flowers, and are hence called PhanerogamÆ; but the Acotyledones have no visible flowers, and they are hence called Cryp togamÆ, which signifies that their flowers are hidden. The most remarkable of the Cryptogamous plants are the ferns, some of which become lofty trees; the wood of which is in curious wavy lines, as it appears to be formed by the footstalks of the decayed leaves growing together and becoming woody. The veins in the leaves or fronds of the ferns are forked.

Besides the great divisions already mentioned, the Dicotyledonous plants have been divided into the DichlamydeÆ, or those having both calyx and a corolla; and the MonochlamydeÆ, or those having only a calyx; but there are so many exceptions, as to render this division of little value. The MonochlamydeÆ are not subdivided, but the DichlamydeÆ are again divided into the ThalamiflorÆ, in which the petals and the stamens grow separately out of the thalamus or flat part of the receptacle, and generally from below the pistil; the CalyciflorÆ, in which the stamens and petals are either attached to the calyx, or to a lining of it formed by the dilated receptacle; and the CorolliflorÆ, in which the petals grow together, so as to form a cup for the pistil, and which have the stamens attached to the corolla, but quite distinct from the calyx. The Monocotyledones have also been re-divided into the Petaloid, or those with regular flowers, like the bulbous plants, and OrchidaceÆ; and the Glumaceous plants, or those with scales or glumes instead of petals, as in the sedges and the grasses. The Acotyledones are divided into those with leaves, as the ferns; and those without leaves, as the mosses, lichens, and fungi.

I have only to add that each sub-class is divided into numerous orders, which are differently arranged by different botanists; the object being to place those nearest together which are most alike. As no one of these arrangements appears to be decidedly better than the others, I have adopted that given in Mr. Loudon’s Hortus Britannicus; marking, where, they occur, the new orders which have been formed, and the alterations in the old ones that have been made since that work was written.