Meristem: Meristems are the formative tissues which are made up of group of immature and similar cells, and are in continues state of cell division and forming new cells.
Classification of meristem
They have been classified based on their position, origin and function.
Types of meristems on the basis of position
Based on their position in the plant body there are three types.
1) Apical meristems
These are present at the tips of stem, root and branches. They arise from promeristem and forming growing point at the apices of stems and roots. The meristem present at the stem tip is called shoot apex and that located at the tip of the root is called root apex. Thus, the shoot and root apical meristem constitute the apical meristem. The activities of apical meristem results in longitudinal growth of stem and root.
2) Intercalary meristems
They are the portions of the apical meristems, which got separated from them by the formation of permanent tissues in between. They occupy various position in plants, at the base of leaves in Pinus and various grasses and just below the node in mint etc. The intercalary meristems are short lived and ultimately disappear and they become permanent tissue.
3) Lateral meristems
They occur along the lateral sides of stems and roots, particularly of gymnosperms and dicots. The cells of lateral meristems divide only in longitudinal plane and that results in growth and hence the increase in girth or diameter of the plant. Cambium are the lateral meristems in the plant.
Types of meristems on the basis of origin
On the basis of origin, meristems are classified into three types
1) Promeristems
It is also called primordial meristems or embryonic meristems. It is a group of earliest and youngest meristematic cells occupy a small area and give rise to a new tissues and organs. The cells of promeristems have thin walls, prominent nucleus and dense cytoplasm.
2) Primary meristems
These are the meristems derived from the promeristems and retain the capacity to divide throughout the life of the plant. The apical meristems, intercalary meristem and intrafascicular cambium are represented by primary meristem. The activity of primary meristem results in longitudinal growth called primary growth where as the activities of intrafascicular cambium is responsible for the radial growth called secondary growth.
3) Secondary meristems
These are the meristems, which are derived from primary permanent tissues by a process called dedifferentiation in regions of the plant body. It takes place during secondary growth of stems and roots. The interfascicular cambium and cork cambium are good examples of secondary meristems. They are lateral in position and give rise to secondary permanent tissues that results in radial growth or secondary growth.