Growth in plants is restricted to specific regions of active cell division called meristems. Plants have different kinds of meristems. The meristems that occur at the tips of roots and shoots produce primary tissues are called apical meristems (refer figure). Root apical meristem occupies the tip of a root where as the shoot apical meristem occupies the region of the stem axis. During the formation of leaves and elongation of stem the cells that are ‘left behind’ from shoot apical meristem, constitute the axillary bud. Such buds are present in the axils of leaves and are capable of forming a branch or a flower. The meristem which occurs between mature tissues is known as intercalary meristem. They occur in grasses and regenerate parts removed by the grazing herbivores. Both apical meristems and intercalary meristems are primary meristems. They appear early in life of a plant and contribute to the formation of the primary plant body.
The meristem that occurs in the mature regions of roots and shoots of many plants, and appear later than primary meristem is called the secondary or lateral meristem. They are cylindrical meristems. Fascicular vascular cambium, interfascicular cambium and cork-cambium are examples of lateral meristems. They produce the secondary tissues. Divisions of cells in both primary and secondary meristems, form new cells that become structurally and functionally specialised and lose the ability to divide. Such cells are termed permanent or mature cells and constitute the permanent tissues. During the formation of the primary plant body, specific regions of the apical meristem produce dermal tissues, ground tissues and vascular tissues.