Growth takes place in two stages in plants: first there is the division of cells of an undifferentiated type (simple, thin-walled parenchyma) adding to the number of cells; then there is the enlargement of some of the cells produced by these divisions.
Dividing cells of the undifferentiated type are not present throughout the plant, but are concentrated in particular places. In addition to these, certain cells in most organs remain relatively undifferentiated and may begin to divide if the appropriate conditions arise and after they have un-dergone a process known as dedifferentiation. Such cells give rise to adven-titious roots and buds, or to the callus tissue which forms during wound healing. They are of great importance to the horticulturalist. The ability of such cells to divide is a basic requirement for the success of many forms of vegetative propagation and grafting.
Cells that divide actively to produce the primary plant body are associat-ed together in meristems. These comprise the apical meristems at the tips of shoot and root and the tips of lateral shoots or roots. Some plants have ac-tive meristems just above and near to most nodes; these are the intercalary meristems.
When secondary growth occurs, that is, growth in thickness, lateral meristems are involved. The vascular cambium occurs in dicotyledons and gymnosperms and is the best known of the lateral meristems. Growth in thickness of stem and root causes the primary covering layer of the plant, the epidermis, to split. A secondary protective barrier between delicate tis-sues and the outer world is developed to replace the epidermis. It consists of layers of cork cells, derived from the specialized cork cambium or phello-gen, also a lateral meristem.
In the dicotyledon leaf, cells continue to divide in various areas of the ex-panding lamina, some until the mature size has almost been attained, when they cease division and the products expand. Leaves in monocotyledons aredifferent in that most have an additional basal zone of meristematic tissue that continues growth for long periods, until the mature leaf size has been reached.
Certain monocotyledons have secondary growth in stem thickness (sec-ondary thickening meristem), although many of the larger ones do not, for example the palms. Dracaena (Ruscaceae) and Cordyline(Laxmanniaceae),Klattia, Pattersonia, Nivenia and Witsenia (Iridaceae) serve as examples inwhich there is a special zone of meristematic cells in the outer part of the cortex (that part of the stem to the outside of the region containing primary vascular bundles). In the cortex entire vascular bundles are formed, with new secondary ground tissue between them.
Clearly, the growing plant is exceedingly complex, containing areas that are juvenile and have actively dividing cells close to other tissues that are fully formed and mature.