Cambium (Lat. cambio, 'I change'). The growing point of any stem is composed of one or more actively dividing cells. In the growing point of a fern a single apical cell can still, though with difficulty, be distinguished amid the mass of embryonic cells from which the future tissues are forming, but in higher plants all the cells of this 'meristem,' as it is termed, are equally endowed with the power of division. As the plant grows, the new cells constantly formed behind the apex are modified at first into the embryonic layers from which epidermis, fibro-vascular bundles, and parenchyma are developed, but these are no longer capable of continued growth, and the bundles are said to be definite. Hence monocotyledons and herbaceous dicotyledons do not increase in thickness; but in coniferous and perennial dicotyledonous stems a layer of embryonic cells remains undifferentiated between the wood and bast of the fibro-vascular bundles, and continues in a state of division as the cambium. The bundles thus grow in thickness by the differentiation from the cambium layer of new layers of wood and bast which press apart those already formed. (For cut and fuller details, see BARK.) The wood and bark thus undergo an increase in thickness, which is most marked in spring and early summer, but proceeds more slowly towards autumn, and stops entirely in winter. The bundles are thus said to be indefinite, and their seasonal rhythm of growth is recorded in the familiar rings of wood and layers of bark (see BARK and WOOD). These are always easily separable, since the delicate intervening cambium-cylinder gives way much more easily than the formed tissues, leaving, when torn, its thin-walled protoplasmic cells as a moist and viscid layer covering the harder surfaces of these. In thin transverse sections of stems the cambium cells can readily be made out, with their closely parallel walls indicating their plane of division longitudinal and parallel to the outer surface of the stem.
In exceptional cases, a cylindrical layer of meristem may persist outside the fibro-vascular bundles altogether, and immediately under the epidermis and cellular envelope, which become thickened as a false bark or rind. From this may be continuously deposited entirely new bundles of definite type, the stem thus increasing in thickness, as in the dragon-tree (Dracena) and other arborescent monocotyledons. In the dicotyledonous Bougainvillea also the stem is thickened in essentially the same way. See DICOTYLEDONS.
The layer of embryonic cells from which cork is formed is called cork-cambium by many writers. See CORK.