Meristematic Tissues (Meristems)
The term “meristem” is derived from the greek word ‘ Meristos’ which means divisible or having cell division activity. The term meristem was coined by Nageli (1858).
Meristematic tissues are group of immature cells that are capable of undergoing cell division. In plants, meristem is found in zones where growth can take place, for example, apex of stem, root, leaf primordia, vascular cambium, cork cambium, etc.,
a) They are made of living cells
b) Cells are small, oval, polygonal or round in shape
c) They are thin walled with dense cytoplasm, large nuclei and small vacuoles.
d) They undergo mitotic cell division
e) They do not store food materials.
Meristems are classified based on (i) origin and development, (ii) origin of initiating cells (iii) position in plant body (iv) function
1. Types of Meristems
Based on origin and development of initiating cells, meristems can be classified into three types namely i) Promeristem or primordial meristem ii) Primary meristem and iii) Secondary meristem
i) Promeristem or primordial meristem:
· A group of young meristematic cells of a growing organ.
· In plants, they occupy a small area at the tip of shoot and root.
· They further divide to form primary meristem.
ii) Primary meristem:
· They are present below the promeristem at shoot and root apices.
· These cells divide to form permanent tissues.
iii) Secondary meristem:
It is derived from primary permanent tissues which have the capacity of division eg. cork-cambium, cambium of roots and inter fascicular cambium of stem.
On the basis of their position in the plant, meristems are of three types: i) Apical meristem ii) Intercalary meristem and iii) Lateral meristem.
i. Apical meristem:
These are found at the apices or growing points of root, shoot and bring about increase in length. They include both pro-meristem as well as primary meristem.
ii. Intercalary meristem:
It lies between the region of permanent tissues and is part of primary meristem which is detached due to formation of intermittent permanent tissues. It is found either at the base of leaf e.g. Pinus or at the base of internodes e.g. grasses.
iii. Lateral Meristem:
These are arranged parallel to sides of origin and normally divide radially to give secondary permanent tissues. ese increase the thickness of the plant part.
On the basis of their function, meristems have been classified into three types, namely: i) Protoderm meristem, ii) Procambium meristem and iii) Ground Meristem
i. Protoderm meristem:
It is the outermost layer of the young growing region which develops to form epidermal tissues.
ii. Procambium meristem:
It is composed of narrow, elongated, meristematic cells that give rise to the vascular tissues.
iii. Ground Meristem:
It is composed of large, thick-walled cells which develop to form ground tissue system, e g. hypodermis, cortex and pith.
The growth pattern and plane of division of meristematic tissue is important to govern the mode of growth, On this basis tissues can be classified into three types, namely:
i) Mass meristem ii) Rib or file meristem and iii) Plate meristem
i. Mass meristem:
In this type of meristem, cell divisions occur in all planes resulting in an increase in volume. It can be observed in meristems of cortex and pith.
ii. Rib or file meristem:
The cells divide only on one plane e.g. formation of laments in algae.
iii. Plate meristem:
These cells divide in two planes resulting to an increase in the area of an organ e g. Leaf formation.
Meristems are actively dividing tissues of the plant, that are responsible for primary (elongation) and secondary (thickness) growth of the plant.