Meristematic Tissue: Structure and Classification (Key Points)

different types of meristematic tissues

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

apical meristematic tissue

(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.

Ø  Example:

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

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