Meristematic tissues, or simply meristems, are tissues in which the cells remain forever young and divide actively throughout the life of the plant. When a meristematic cell divides in two, the new cell that remains in the meristem is called an initial, the other the derivative. As new cells are added by repeated mitotic divisions of the initial cells, the derivatives are pushed farther away from the zone of active division. They stretch, enlarge and differentiate into other types of tissues as they mature. Meristematic cells are generally small and cuboidal with large nuclei, small vacuoles, and thin walls.
A plant has four kinds of meristems: the apical meristem and three kinds of lateral—vascular cambium, cork cambium, and intercalary meristem.
These are located at opposite ends of the plant axis in the tips of roots and shoots. Cell divisions and subsequent cellular enlargement in these areas lengthen the above and below ground parts of the plant. The meristems also influence the shapes of the mature plants since the patterns for subsequent growth are laid down in the meristems.
Vascular cambium. Some plants grow in diameter by producing new tissues laterally from a cylinder of tissue called the vascular cambium, which extends throughout the length of the plant from the tips of the shoots to the tips of the roots. It is present in allperennial and in some annual plants. Tissues produced by cell divisions of the vascular cambium are secondary tissues.
Cork cambium. Cork cambia (singular: cambium), also called phellogens, are found in the bark of roots and stems of woody plants where they produce cork cells. The cork cambia originate just under the epidermis of the primary body and in some tree species are long cylinders running parallel to the vascular cambium. In other species, more discrete, disk-like cork cambia in the trunks produce flat plates of bark tissues that break off in large scales as the tree ages.
Intercalary meristem. Grasses have intercalary meristems located along the stems near the nodes. Cell divisions in this tissue push the stem upward. Grasses and other monocots have no lateral meristems so any lateral increase in size is the result of primary tissue cell enlargement, not cell divisions.
Primary (transitional) meristems
The cells produced by divisions in the apical meristem region are soon identifiable as three zones of distinct tissues that differentiate below the apical meristems. These are the primary meristems, called sometimes the transitional meristems: theprotoderm, the procambium, and the ground meristem. They give rise to the tissue systems of the primary plant body.