Meristematic tissues are a group of young cells that are in a continuous state of division.
These tissues are mostly found at the apices of root and shoot.
The main characteristics of cells of meristematic tissues are:
(i) They are living and thin walled
(ii) Vacuoles are few and small in size
(iii) The cells contain a dense protoplasm and conspicuous nuclei
(iv) The cells are spherical, oval or polygonal in shape
(v) They do not store reserve food material and are in an active state of metabolism.
Types of Meristems:
I. Classification based on origin and development:
On the basis of origin and development of initiating cells, meristems can be divided into three types:
(i) Promeristem or priordial meristem:
A group of young meristematic cells of a growing organ. It is the early embryonic meristem from which other advanced meristems are derived. In a plant, it occupies a small area at the tip of stem and root. It further divides to form primary meristem.
(ii) Primary meristem:
These are derived from promeristem. They are present below the promeristem at shoot and root apices. These cells divide and form permanent tissues.
(iii) Secondary meristem:
It is derived from primary permanent tissues which have the capacity of division e.g. Cork-cambium, cambium of roots and inter fascicular cambium of stem.
II. Classification on basis of position:
On the basis of their position in the plant body, meristems are of three types (Fig. 3.1)
(i) Apical meristem:
These are found at the apices or growing points of root and shoot and bring about increase in length. It includes both pro-meristem as well as primary meristem.
Several theories have been put forward to explain the activity of apical meristem:
Apical cell theory:
The theory was first proposed by Hofmeister (1857) and advanced by Nageli (1878). According to this theory, a single apical cell is the structural and functional unit of apical meristem which governs the entire process of apical growth. However, such organization has been found only in cryptogams.
Histogen cell theory:
This theory was given by Hanstein (1868). According to this theory root and shoot apices consists of the central or inner mass called Plerome surrounded by the middle region composed of isodiametric cells called periblem and the outermost uniseriate layer of Dermatogen. Dermatogen gives rise to epidermis, periblem to cortex and endodermis and plerome to vascular bundle and pith. These three layers were called Histogen by Hanstein.
Tunica-corpus theory:
This theory was proposed by Schmidt (1924). According to this theory, mass of dividing cells are of two types: Tunica, the outer consisting of one position of different meristems or more peripheral layers of cells, forming the outer region and Corpus, the central undifferentiated multilayered mass of cell. Epidermis is derived from outer layer of tunica and other tissues from remaining layer of tunica and corpus.
(ii) Intercalary meristem:
It lies between the region of permanent tissues and is considered| as a part of primary meristem which has become detached due to formation of intermediate 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 the sides of origin and normally divide periclinally or radially and give rise to secondary permanent tissues. These increase the thickness of the plant part.
III. Classification on basis of function:
On the basis of their function, meristems have been classified into three types:
(i) Protoderm meristem:
It is the outermost layer of the young growing region which develops to form epidermal tissue system.
(ii) Procambium meristem:
It is composed of narrow, elongated, prosenchymatous, meristematic cells that gives rise to the vascular tissues system.
(iii) Ground Meristen:
It is composed of large, thick-walled cells which develop to for ground tissue system, i.e. hypodermis, cortex and pith.
IV. Classification on basis of plane of divisions:
The growth pattern and plane of division of meristematic tissue is important to govern the mode of growth.
These tissues can be divided into three types:
(i) Mass meristem:
In such 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 filaments 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.