=698. Sclerenchyma, or stone-tissue.=—This is also a strengthening tissue and consists of cells which do not taper at the ends and the walls are evenly thickened, sometimes so thick that the inside (lumen) of the cell has nearly disappeared. Usually such cells contain no living contents at maturity. Sclerenchyma is very common in the hard parts of nuts, and underneath the epidermis of stems and leaves of many plants, as in the underground stems of the bracken fern, the leaves of pines (fig. 415), etc. [Illustration: Fig. 414. Transverse section of portion of tomato stem. _ep_, epidermis; _ch_ chlorophyll-bearing cells; _co_, collenchyma; _cp_, parenchyma.] [Illustration: Fig. 415. Margin of leaf of Pinus pinaster, transverse section, _c_, cuticularized layer of outer wall of epidermis; _i_, inner non-cuticularized layer; _c´_, thickened outer wall of marginal cell; _g_, _i´_, hypoderma of elongated sclerenchyma; _p_, chlorophyll-bearing parenchyma; _pr_, contracted protoplasmic contents. ×800. (After Sachs.)] [Illustration: Fig. 416. Section through a lenticel of Betula alba showing stoma at top, phellogen below producing rows of flattened cells, the cork. (After De Bary.)] =699. Cork.=—In many cases there is a development of “cork” tissue underneath the epidermis. Cork tissue is developed by repeated division of parenchyma cells in such a way that rows of parallel cells are formed toward the outside. These are in distinct layers, soon lose their protoplasm and die; there are no intercellular spaces and the cells are usually of regular shape and fit close to each other. In some plants the cell walls are thin (cork oak), while in others they are thickened (beech). The tissue giving rise to cork is called “cork cambium,” or phellogen, and may occur in other parts of the plant. For example, where plants are wounded the living exposed parenchyma cells often change to cork cambium and develop a protective layer of cork. The walls of cork cells contain a substance termed _suberin_, which renders them nearly waterproof. =700. Lenticels.=—These are developed quite abundantly underneath stomates on the twigs of birch, cherry, beech, elder, etc. The phellogen underneath the stoma develops a cushion of cork which presses outward in the form of an elevation at the summit of which is the stoma (fig. 416). The lenticels can easily be seen. 2. THE FIBROVASCULAR SYSTEM. =701. Fibrous tissue.=[39]—This consists of thick-walled cells, usually without living contents which are elongated and taper at the ends so that the cells, or fibers, overlap. It is common as one of the elements of the vascular bundles, as wood fibers and bast fibers.