Initially all embryonic cells of an embryo have the capacity to divide and multiply but as the embryo develops into a plant body, this capacity for division is restricted to certain parts of the plant body called meristems which are active throughout the life of the plant body (unlike that of an animal body). When meristematic cells divide, a group of the daughter cells remain meristematic; the other daughter cells called derivatives that differentiate into various tissue elements.
Meristematic cells give rise to all three fundamental mature cell types: parenchyma cells, collenchyma cells and sclerenchyma cells. Their major function is cell division, and so their cell cycle indeed cycles. Before the occurrence of any cell division, usually cells become enlarged accompanied with addition of protoplasmic and cell wall material.
Meristematic cells are isodiametric, compactly arranged with dense cytoplasm, large nucleus, and small vacuoles or without vacuoles and with thin cell wall.The meristems that occur at the apices of stem, root and other branches are called apical meristems, which bring about primary growth of the plants and hence also called as primary meristems. Apical meristems are the completely undifferentiated (indeterminate) meristems in a plant.
The primary meristems in turn produce the secondary meristem types. These secondary meristems are also known as lateral meristems because they are involved in lateral growth. Lateral meristems are arranged parallel to the sides of organs in which they occur.These are referred to as secondary meristems because they produce secondary tissues, and increase the thickness of the plant body. This process is called as secondary growth, seen in dicotyledons and gymnosperms. There are two types of secondary meristems: Vascular cambium and Cork cambium.
- Vascular cambium : The vascular cambium produces more vascular tissue (xylem and phloem), which provide support for the shoot system in addition to transporting water and nutrients. Because the xylem and phloem that come from the vascular cambium replace the original (primary) xylem and phloem, and add to the width of the plant, they are called secondary xylem and secondary phloem.The vascular cambium is only one cell thick and forms a ring around the stem of a plant. On its interior, it adds secondary xylem and on its exterior, it adds secondary phloem. In trees, the layers of secondary xylem form wood. The layers of the secondary phloem form bark.
- Cork cambium: The cork cambium makes cork, which is a tough, insulating layer of cells. These cells have wax in them, which helps them protect the stem from water loss. The cork layer also protects the plant from insects and pathogens such as fungi and bacteria, and can insulate the tree from fire. Cork is also part of the bark, and it falls off over time.
The pith is made up of primary cells (originating from an apical meristem) and cortex forms outer layer which surrounds the vascular tissue.
In many plants in addition to apical meristems, lateral meristems, intercalary meristems are found. This is also a primary growth which is found inserted between permanent tissues, in the bases of internodes and leaf sheaths of grasses. This type of meristem is only found in monocots. Sometimes, as in gynophore of groundnut, xylem may be present in intercalary meristem. Wherever stem is jointed, elongation of internodes is due to intercalary meristem. Example: Bamboos. Even prolonged growth of leaves, flowers and fruits may be regarded as an intercalary growth.