In certain plants, the epidermis is imbued with flint or sil´ica; so that even when burnt to an ash the stomata are still quite distinct. Examples of this may be found in the stalk or culm of grasses, as in straw, the shining epidermis of which is siliceous; or the epidermis of canes. Among the lower plants, this peculiarity is especially curious in the species of _Equisétum_, or mares’-tails. The manner in which the veins of leaves are arranged is worthy of special attention, as it forms one of the characters by which the two leading divisions of the Vegetable Kingdom are characterized. Thus in one of these divisions the veins are branched, so as to form a network throughout the leaf; the plants with these netted veins, to which belong our trees, shrubs, and most herbs, are the Dicotylédons, or Ex´ogens; while in the second division, the veins run parallel to each other, being little or not at all branched, and not forming a network. The plants with parallel veins, among which are our grasses, lilies, &c., are the Monocotylédons or En´dogens. _Stems._--In the stems of plants, the tissues are arranged round a centre; otherwise, in the simpler and lower plants, they agree in structure with leaves, the centre being occupied by some element of fibro-vascular tissue, as simple wood-cells, a few vessels or ducts. In the higher or flowering plants, the stem exists in two distinct forms, corresponding to the differences above noticed in the arrangement of the veins of the leaves; these must be considered separately. In the _Dicotyledons_ or _Exogens_ (Pl. I. fig. 36), the centre of the stem, in a transverse section, is seen to be occupied by the pith or _medulla_, which is represented in the figure by the innermost circle. Immediately outside and around this is a narrow ring, indicating the section of a sheath to the pith, and called the _medullary sheath_. Next comes a broad ring of wood of the first year’s growth (fig. 36 _a_), traversed, from the pith to the bark, by wedge-shaped paler rays, termed the medullary rays. Outside the first year’s wood is the newer and paler wood of the second year (_b_); and so on, a new ring of wood being added outside the preceding layer for each year of growth of the stem. On the outer side of the wood is the inner bark or _liber_ (fig. 36 _c_); and outside this is the spongy outer bark (_d_), covered by its epidermis. These structures are of different composition, as may be best seen in longitudinal sections. The pith and the medullary rays consist of cellular tissue, the cells being mostly rounded in the former, and more closely pressed together and squarish in the latter. The medullary sheath consists of vascular tissue; and the wood, of wood-cells traversed longitudinally by bundles of vascular tissue and ducts, the latter being larger and more distinct towards its outer boundary. The liber is composed of woody fibre, and the outer bark of cellular tissue.