A meristem is a localized region of active cell division in plants from which permanent tissue is derived. New cells formed by activity of a meristem become variously modified to form characteristic tissues of the adult (epidermis, cortex, vascular tissue, etc). A meristem may have its origin in a single cell, e.g., in ferns, or in a group of cells as in angiosperms (flowering plants). The principal meristems in the latter group occur at the tips of stems and roots (apical meristems or growing points), between xylem and phloem of vascular bundles (cambium) in the cortex (cork cambium), in young leaves, and, for example, in many grasses, at the base of internodes (intercalary meristems). Meristems may also arise in response to wounding.