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ramifying in every direction, by which the sap is brought into contact with the air. These vessels are arranged in two sets; the upper one, in exogenous plants, communicating with the vessels of the wood, and the lower with those of the bark. The nutriment taken up by the root passes through the upper series to the upper surface of the leaves, and after undergoing the necessary chemical changes, returns by the lower through the bark to the root, depositing various secretions by the way. The spaces between the vessels are filled up with cellular matter, in which numerous minute pores establish communication between the fluid in the vessels and the air; they are chiefly on the under surface in most plants.
In exogenous plants the principal fibres or veins of the leaf branch off from a central vein or midrib, distributing themselves in numerous net-like ramifications over the whole leaf, as in Fig. 3. In plants with endogenous stems, on the other hand, they are usually arranged in a parallel series, diverging from the stalk and uniting again at the apex of the leaf, being connected during their course only by minute cross-veins at right angles to the others, as in Fig. 4. The leaves are generally supported on a stalk called a petiole, but are sometimes stalkless, or, to speak technically, sessile. At or near the base of the petiole two or more leaf-like appendages are often found, to which the name of stipules is given. The angle between the leaf and the stem is called the axil, and is the place where the bud of the ensuing growth is developed. In some few plants the buds drop off, and falling on the earth become bulbs, and thus propagate the plant, as in the Tiger Lily; a bulb is, in fact, only an underground bud.
Leaves are either simple, or composed of a number of leaflets arranged upon the same stalk, when they are styled compound. Simple leaves are either entire, or variously cut and divided; but their forms are far too numerous to admit of mention here, and for the explanation of the technical terms by which botanists have been compelled to distinguish the diversified figures of these im-
