What are meristems?
Ø Meristem is a type plant tissue composed of an undifferentiated mass of cells.
Ø They are rapidly dividing cells found in the growing portions of the plant and they give rise various plant organs.
Ø The primary function of meristem is to assist in plant growth.
Ø Living cells other than meristem can also give rise new cells; however, the meristems carry on such activity indefinitely.
Ø Meristematic cells not only add new cells to the plant body but also perpetuate themselves.
Ø Meristematic cells when divide, some portions of division do not differentiate into adult cells but remain meristematic.
Ø The term meristem was first proposed by Karl Wilhelm von Nägeli (1817–1891) in his book Beiträge zur Wissenschaftlichen Botanik (“Contributions to Scientific Botany”).
Ø The word meristem is derived from a Greek word ‘merizein’ meaning ‘to divide’.
Characteristics of Meristematic Cells
Ø The characteristic features of meristematic cells are summarized in the post: Characteristic Features of Meristematic Cells
Classification of meristems
Ø Meristems are classified according to TWO criterions
(1). Classification based on position in the plant body
§ Apical meristem
§ Lateral meristem
§ Intercalary meristem
(2). Classification based on nature of cell giving the meristem
§ Primary meristem
§ Secondary meristem
(I). Classification of meristems based on position in the plant body
(1). Apical Meristem
Ø Apical meristems occur at the growing tips of plant parts such as the tip of stem, roots and leaves.
Ø They are also called as APICAL CELL or apical initial.
Ø Apical meristem may constitute of one or more cells.
Ø The number of apical cells in the apical meristem varies in different plant groups.
Ø Lower plants (Bryophytes and Pteridophytes) usually have a single apical cell.
Ø In Gymnosperms and Angiosperms, the apical meristem composed of a group of cells.
Ø Cells in the apical meristem always maintain their individuality and position.
Ø The activity of apical meristem causes increase in the length of shoot, root and leaves.
Ø Apical meristem produces the primary structure of plants.
Ø Apical meristem is terminal in stem and sub-terminal in roots (due to the presence of root cap in root)
Ø Example of apical meristem: root apex, shoot apex
Ø The tissue zones of shoot apex are:
o Protoderm: give rise to epidermis
o Procambium: give rise to primary vascular tissue (xylem & phloem)
o Ground meristem (fundamental meristem): give rise endodermis, pericycle, cortex, medulla and pith
(2). Lateral Meristem
Ø Lateral meristems are the meristematic tissue present parallel to the organs in which they occur.
Ø They help in increasing diameter of the plant body by adding new cells to the existing tissues.
Ø They divide only in one plane.
o Vascular cambium and Cork cambium (phellogen)
(3). Intercalary meristem
Ø They are not typical meristems since these cells later completely differentiated into permanent tissues.
Ø They are the cells with meristematic activity present between permanent tissue regions in the plant.
Ø They are portions of apical meristem that were separated from the apex during development by layers of differentiated tissues.
Ø Regions with intercalary meristems are the actively growing region behind apical meristem.
Ø Intercalary meristem is commonly found in internodes of vascular plants
Ø They also occur in leaf sheath of some grasses.
Ø In Equisetum (a primitive Pteridophyte) intercalary meristem is present just above the node.
(II). Classification of meristem based on nature of cells giving the meristem:
(1). Primary meristem
Ø Primary meristems are the direct descendants of embryonic cells.
Ø They are continuously involved in cell division and growth of the plant.
Ø Apical meristems are best examples for primary meristem.
Ø Primary meristems give rise the primary plant body.
Ø Primary meristems are usually apical in position.
(2). Secondary meristem
Ø They are the meristems developed from permanent tissues.
Ø Secondary meristem gives rise secondary tissue after primary growth.
Ø Secondary meristems are usually lateral in their position
Ø Example: Cork cambium and Accessory cambia
Ø Vascular cambium is not fall precisely in any of these two categories