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.