Elementary botany . er wall of marginal cell; g, i\ hypoderma of elongated scle-renchyma; p, chlorophyll-bearing paren-chyma; pr, contracted protoplasmic con-tents. X800. (After Sachs.) 699. Cork.—In many cases there is a development of cork tissueunderneath the epidermis. Cork tissue is developed by repeated divisionof parenchyma cells in such a way that rows of parallel cells are formedtoward the outside. These are in distinct layers, soon lose their proto-plasm and die; there are no intercellular spaces and the cells are usuallyof regular shape and fit close to each other. In some plants th
Elementary botany . er wall of marginal cell; g, i\ hypoderma of elongated scle-renchyma; p, chlorophyll-bearing paren-chyma; pr, contracted protoplasmic con-tents. X800. (After Sachs.) 699. Cork.—In many cases there is a development of cork tissueunderneath the epidermis. Cork tissue is developed by repeated divisionof parenchyma cells in such a way that rows of parallel cells are formedtoward the outside. These are in distinct layers, soon lose their proto-plasm and die; there are no intercellular spaces and the cells are usuallyof regular shape and fit close to each other. In some plants the cell wallsare thin (cork oak), while inothers they are thickened(beech). The tissue givingrise to cork is called corkcambium, or phellogen, andmay occur in other parts ofthe plant. For example,where plants are wounded theliving exposed parenchymacells often change to corkcambium and develop a pro-tective layer of cork. Thewalls of coik cells contain asubstance termed suberin,which renders them nearly waterproof-. Fig. 416-Section through a lenticel of Betula alba show-ing stoma at top, phellogen below producing rowsof flattened cells, the cork. (After De Bary.) 35§ RELATION TO ENVIRONMENT. 700. Lenticels.—These are developed quite abundantly underneathstomates on the twigs of birch, cherry, beech, elder, etc. The phellogenunderneath the stoma develops a cushion of cork which presses outwardin 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*—This consists of thick-walled cells, usually with-out living contents which are elongated and taper at the ends so that thecells, or fibers, overlap. It is common as one of the elements of the vas-cular bundles, as wood fibers and bast fibers. 702. Vascular tissue, or tracheary tissue.—This consists of the vessels orducts, and tracheides, which are so characteristic of the vascular bundle(see Chapter V) and forms a conducting
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