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.
702. Vascular tissue, or tracheary tissue.—This consists of the vessels or ducts, and tracheides, which are so characteristic of the vascular bundle (see [Chapter V]) and forms a conducting tissue for the flow of water. The vascular tissue contains spiral, annular, pitted, and scalariform vessels and tracheides according to the marking on the walls (figs. [58], [59]). These are all without protoplasmic contents when mature. There are also thin-walled living cells intermingled called wood parenchyma. In the conifers (pines, etc.) the tracheary tissue is devoid of true vessels except a few spiral vessels in the young stage, while it is characterized by tracheides with peculiar markings. These marks on the tracheides are due to the “bordered” pits appearing as two concentric rings one within the other. These can be easily seen in a longitudinal section of wood of conifers.
703. Sieve tissue.—This consists of elongated tubular cells connected at the ends, the cross walls being perforated at the ends. These are in the phloem part of the bundle, and serve to conduct downwards the dissolved substances elaborated in the leaves.
704. Fascicular cambium.—This is the living, cell-producing tissue in the vascular bundle, which in the open bundle adds to the phloem on one side and the xylem on the other.
3. THE EPIDERMAL SYSTEM.
705. To the epidermal system belong the epidermis and the various outgrowths of its cells in the form of hairs, or trichomes, as well as the guard cells of the stomates, and probably some of the reproductive organs.