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. [Illustration: FIG. 102.—COTYLEDONS, LEAVES, AND WOOD OF A DICOTYLEDONOUS PLANT.] 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, 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