Types & Characteristics of Meristematic Tissue | Biology for ACT PDF Download

Tissue

• A group of cells which is similar or dissimilar in shape, having a common origin and usually perform a common function is called tissue.

• The term tissue was coined by Nehemiah Grew.

• The tissues are divided into two groups by Karl Nageli :-

Characteristics of Meristematic Tissue

•  It is an undifferentiated tissue.

•  Cell cycle of meristem is in continuous state of division. Thus, meristematic tissue is composed of immature cells.

•  Meristematic cells have only primary cell wall which is thin and flexible ( elastic) and made up of cellulose. Secondary cell wall is absent.

•  Cells of meristem are small and iso-diametric.

•  They have dense cytoplasm

•  They have prominent and large nucleus.

•  Normally vacuoles are absent in meristematic cells if present then they are small in size and Meristematic cells are metabolically highly active so lack of reserve food in these cells.

•  Plastids are absent in meristems. If they are present, then only in the proplastid stage.

•  They do not have intercellular spaces. Cells are closely fitted ( packed) together. so it is a compact tissue

•  Ergastic substances are absent.

Classification of Meristematic Tissues

[A] Based on Origin and Development 

On the basis of origin and development meristems can be divided into following three types :-

(i) Promeristem/Embryonic/Primordial/Urmeristem 

•  This meristem develops in beginning during embryonic stage. 

• They develop primary meristem.

(ii) Primary Meristem 

• Meristematic cells developed from promeristem are known as primary meristem.

•  These cells are always in division phase and form primary permanent tissue.

(iii) Secondary Meristem 

•  These are the meristems developed from primary permanent tissues.  

• Some of the cells of primary permanent tissues become meristematic and constitute secondary meristem.

•  By the activity of secondary meristems, secondary growth takes place.

Note : Formation of meristem from any permanent tissue is called dedifferentiation

On the basis of position, meristematic tissues are divided into three types :-

(i) Apical Meristem
The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems. They are responsible for increase in the length of plant organs. Example :- Root apex, shoot apex. They are responsible for primary growth.

Note  : In embryonic stage, apex has two regions
(i) Promeristem (ii)  Eumeristem

 Eumeristem is derived from promeristem.

(ii) Intercalary Meristem

•  The meristem which occurs between mature tissues.

•  This is the separated region of apical meristem.

•  By the activity of this meristem length of the plant organs increases.

•  They are present in some plants stem.

•  They may be present either at the base of internode  e.g. grasses, bamboo and Equisetum etc, or at the base of node e.g. Mint. They are also present at the base of leaves e.g. Pinus. By the activity of  this meristem, length of leaves increases.

Note: They are short lived and convert into permanent tissue.

 (iii) Lateral Meristem

•  Lateral meristem occurs in lateral side of plant organ or parallel to the longitudinal axis/tangential plane of plant organ.

•  Activity of lateral meristem increases the girth of plant organ.

•  Lateral meristems are both primary and secondary in origin (mostly secondary in  origin).

•  Primary lateral meristem :
  1. Marginal meristem
  2. Intra fascicular cambium

 1. Marginal Meristem: It occurs at the margin of leaf. Its activity increases the width of leaf.

2. Intra Fascicular Cambium or Fascicular Cambium: This cambium occurs inside the vascular bundle. Except intra fascicular cambium all cambia are secondary in origin.

Secondary Lateral Meristems :– Cork cambium, vascular cambium and inter fascicular cambium.

Note: Lateral meristems are cylindrical.

Old Syllabus NCERT Syllabus

PLANT ANATOMY

•  N.Grew known as father of plant anatomy.

• K.A. Chaudhary known as father of Indian plant anatomy.

Note : The anatomy of seed plants by Katherine Esau was published in 1960.

Other basis on which Meristematic Tissues has been divided:

[C] Based on Plane of Division

(i) RIB-MERISTEM / FILE MERISTEM :

Meristem in which anticlinal division occurs in one plane. For example, tunica is a type of rib-meristem.

Note : Formation of some cells of cortex and pith takes place by this meristem.

(ii) PLATE-MERISTEM :

The meristem which divides anticlinally into two planes at right angle to each other. By this division a plate like structure is formed. Formation of leaf blade takes place by the activity of this meristem.

(iii) MASS-MERISTEM :

Meristem which divides in all possible planes resulting it the increase in the volume of plant body (organ).

Example : The formation of embryo and endosperm takes place by the activity of this meristem.

[D] CLASSIFICATION BASED ON RATE OF DIVISION

Cytohistological zonation theory :- According to Foster, meristem is classified into two region on the basis of rate of division :-

1. Summit

2. Flank Vegetative shoot apex :-

(I) SUMMIT :

The rate of division is slow in this region. This region is located at the apex.

(II) FLANK :

The rate of division is very fast in this region. This region lies behind the summit and leaf primordia are formed by this region.

• Shape of vegetative shoot apex →   Conical or dome shaped

•  Shape of Reproductive shoot apex →  Broad & flat  

• Shoot apex isterminalin position  

• Growth of leaf primordium →  First apical then marginal.  

• Function of leaf primordium is to provide protection to shoot apex.

REPRODUCTIVE SHOOT APEX :
During reproductive phase i.e., at the time of flowering vegetative shoot apex transforms into reproductive shoot apex.This change of shoot apex is induced by florigen & light.

•  In reproductive shoot apex, Summit zone is more active and it forms stamen & Carpel and flank zone is less active and it forms sepal and petal.

COMPOSITION OF APICAL MERISTEM IN DIFFERENT PLANTS

• Apical meristem is absent in lower Algae and Fungi. All the cells of these plants are divisible,  so they do not show apical growth. Thus such type of growth in these plants, is called diffused growth. Diffused growth also occur in animals. 

• In some algae (eg. Fucus, Dictyota & Sargassum), Bryophytes and Some Pteridophytes (eg. Selaginella) apical meristem is consist of single cell. This cell is known as apical cell. This apical cell is pyramid in shape and divide into two  

In Ferns, Gymnosperms and Angiosperms apical meristem consist of many cells.

Several views have been available in relation to structure and growth habit of apical meristem.

[1] Apical Cell theory :

• This theory proposed by Karl Nageli and Hofmeister and supported by wolff 

•  According to Nageli and Hofmeister (mainly), the apical meristem is composed of single apical cell. This view is applicable on Bryophytes and some Pteridophytes and some algae (Ex. Fucus, Dictyota & Sargassum)

 [2] Histogen Theory : It was proposed by Hanstein (1870).

According to him, the root and shoot apices are distinguished  into three meristematic regions or three layers of histogen cells. These are as follows. (i) Dermatogen :- This is the outermost single layer of cells. These cells forms uniseriate epidermis by anticlinal divisions.

(ii) Periblem :- This region is situated just below the dermatogen. It forms cortex (Hypodermis, general cortex and endodermis).

(iii) Plerome :- This is the innermost region. Stele formation takes place by division of these cells. It means formation of pericycle, vascular bundles, pith rays or medullary rays and pith .

• This theory is only true for root apex. It is not applicable for shoot apex of higher plants because in most  of the gymnosperms and angiosperms, shoot apex is not differentiated into three histogens.

• Including above described three histogens, a fourth type of histogen is also present in monocotyledon root apex. This is known as Calyptrogen. Root cap is produced by Calyptrogen in monocots. Root cap & epiblema is produced by dermatogen in dicotyledons.

Exception : One histogen present in Ranunculus and two histogens occurs in Casuarina.

Note :

1. In hydrophytes root cap is absent eg. Pistia

2. Generally, root cap is single layered but in pandanus (screw pine) root cap is multilayered.

3. Root cap contain more amount of golgi body which secrete mucilage which makes the root slimy.

QUIESCENT CENTRE

•  A group of inactive or less active cells  present between the dermatogen and calyptrogen or between active meristem  and rootcap is called quiescent Centre. These cells contain less amount of DNA, light Cytoplasm, small nuclei and synthesis of protein is also less. Quiescent centre name coined by "Clowes". Quiescent centre was discovered in Maize root with the help of autoradiography.

•   Calyptrogen name proposed by Janczewski.

Note: The quiescent centre in the root meristem serves as a reserve for replenishment of damaged  cells of the meristem

[3] TUNICA CORPUS THEORY

This theory was proposed by Schmidt (1924). This theory is applicable on shoot apex. This theory is based planes of division. According to this theory two types of layers are found in the shoot apex :-

(I) TUNICA :

• This is peripheral layer. Epidermis is formed by this layer. In tunica cells,Anticlinal division takes place only in one plane.

•  Surface growth of the plant organs increases due to anticlinal divisions.

Note:1 

•  Anticlinal division occurs at right angle to longitudinal axis (tangential plane) of cell.

Note:2

•  When anticlinal division occurs in one plane, number of layers are not increased.

Note:3 

• Generally, tunica is single layered, but some times it is multilayered, then the outer most layer forms the epidermis and remaining layers form rest types of the tissue system with the association of corpus. (II) CORPUS : The mass of cells present below the tunica is called Corpus. The cells of this zone divides in all directions (many planes) due to which volume increases. The cells of corpus usually larger than the cells of tunica.

Function : Formation of ground tissue system and vascular tissue system.

[4] MANTLE CORE THEORY

This theory was proposed by "Popham and Chan"for shoot apex. They compared mantle with the tunica and core with the corpus. Mantle forms epidermis. According to them corpus or core is distinguished into three zones –

(I) Sub-Apical Meristem :- This is present just below the mantle. It's function is re-establishment of mantle if mantle gets damaged.

(II) Central-zone Meristem :- This is the inner most zone. This zone is responsible only for formation of pith.

(III) Peripheral Meristem :- This region is only responsible for the formation of cortex, pericycle and vascular tissues.

[5] NEWMAN'S THEORY :

According to this theory meristematic tissues of shoot apex are three types -

(I) Monoplex :- Such type of shoot apex in which meristematic cells are arranged in groups in place of meristematic layers. Such type of shoot apex is found in Ferns.

(II) Simplex :- Such type of shoot apex is formed by single layer of meristematic cells. It is found in Gymnosperms.(Except-Gnetales)

(III) Duplex :- Such type of shoot apex is formed by two layers of  meristematic cells. Peripheral layer is called tunica and inner layer is called corpus. Such types of shoot apex is found in Gnetales and Angiosperms. Tunica-corpus organization occurs in duplex type of shoot apex.

Note - According to Newman, the tunica and Corpus of shoot apex is not distinguished in Ferns and Gymnosperm (except Gnetales). 

[6] KORPER-KAPPE THEORY :

It was proposed by Schuepp (1917). According to this theory, the cells of central and peripheral part of the root apex exhibit differences in planes of cell divisions.

In peripheral region each cell first divides transversely and there after the lower daughter cell divides longitudinally thus forming the shape of 'T'. Such divisions are called the Kappe divisions. In the central region T is inverted (^) as the second division takes place in the upper daughter cell. Such divisions are called the Korper divisions.

As a result of these 'T' or '^' divisions, the cells in root apex remain arranged in rows.

By  Kappe divisions, the number of rows increases downwardly and by Korper divisions upwardly.