TISSUE:
A group of cells which is having similar or dissimilar in shape, having a common origin and usually performing a common function is called tissue.
The term tissue was coined by Nehemiah Grew.
Tissues are mainly divided into three categories :
(1) Meristematic tissues or Meristems
(2) Permanent tissue
(3) Secretory tissue
Meristematic tissues or Meristems
The word “Meristem” originated from “Meristos” (Greek = continuous division) and the term meristem was introduced by Nageli (1858). A group of cells which are much active and capable of showing continuous divisions and redivisions, is called as meristematic tissue.
The various characteristic features of the meristems are discussed below
Meristematic tissues or meristem is an undifferentiated tissue.
Cell cycle of meristem is in continuous state of division. It means they have the capacity to divide. So 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 isodiametirc.
They have dense cytoplasm.
Normally vacuoles are absent in meristematic cells but if present then small.
They have prominent and large nucleus.
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 state.
They do not have intercellular spaces. Cells are closely fitted (packed) together, so it is a compact tissue
CLASSIFICATOIN OF MERISTEMATIC TISSUE:
MERISTEMATIC TISSUE BASED ON ORIGN AND DEVELOPMENT
On the basis of origin and development meristems can be divided into following three types :-
(I) PROMERISTEM/EMBYONIC MERISTEM
This meristem develops in beginning during embryonic stage.
They develops primary meristem.
(i) Primary meristem
Meristematic cells developed from promeristem are known as primary meristem.
These cells are always in division phase and form primary permanent tissue.
(ii) Secondary Meristem
These are the meristem developed from primary permanent tissues. They are not present in the embryonic stage of the plant.
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 or Formation of undifferentiated tissue from any permanent tissue is called dedifferentiation.
MERISTEMATIC TISSUES BASED ON LOCATIOIN (POSITOIN) IN PALNT BODY
On the basis of position, meristematic tissues are divided into three types :
(a) Apical Meristem :
The meristems which occur are the tips of roots and shoots and produce primary tissues are called apical meristem. They are responsible for increase in the length of plant organs, Example : - Root apex, shoot apex. They are responsible for primary growth. Apical meristem has two regions sat embryonic stage-First – Promeristem or primordial Meristem -
Which develops in embryonic stage.
Second - Eumeristem -
It is formed from the division of primordial meristem.
According to Haberlandt, Eumeristem is divided into three regions on the basis of function):-
(i) Protoderm : It is the outer most layer of Eumeristem. By the activity of Protoderm epidermal tissue system is formed. It includes Epidermis, Root hair, stem hair etc.
(ii) Procambium : These cells are long and it gives rise to the vascular tissue system. It includes Xylem phloem.
(iii) Ground Meristem : The cells of this region are large, thin walled and isodiametirc. Ground tissue system is formed by the activity of these cells. It includes hypodermis, cortex, endodermis, pericycle, pith-rays and pith.
(b) Intercalary Meristem :
The meristem which occurs between mature tissues.
This is the separated region of apical meristem. This meristem is found between plant organs.
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, lengths of leaves increases.
Note : They are short lived and covert into permanent tissue.
(c) 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 width of plant organ so it is responsible for secondary growth.
Lateral meristem are both primary and secondary in origin (mostly secondary in origin). There are two examples of primary lateral meristem
1. Marginal meristem: - It occurs at the margin of leaf , its activity increases the width of leaf so total growth of leaf is called intercalary marginal growth.
2. Intra fascicular cambium or fascicular cambium : This cambium occurs inside the vascular bundle. Except intra fascicular cambium all cambia are secondary in origin.
Note : Lateral meristems are cylindrical.
CLASSIFICATION 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 rig-meristem.
Note : Formation of some cells of cortex and pith takes place by this meristem.
(II) Plate-meristem : The meristem which divided 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 in the increase in the volume of plant body (organ).
Example : The formation of embryo and endosperm takes place by this kind of meristem.
CLASSIFICATION BASED ON RATE OF DIVISION
Cytohistological zonation theory :- According to Forster, meristem is classified into two region not he basis or 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 is this region. This region lies behind the summit and leaf primordial are formed by this region. This period between initiation of two successive leaf primordial is called “Plastochron”.
Shape of vegetative shoot apex ⇒ Conical or dome shaped
Shape of Reproductive shoot apex ⇒ Broad & flat
Reproeductive shoot apex :During reproductive phase i.e., at the time of flowering vegetative shoot apex transforms into reproductive shoot apes. This change of shoot apex is induced by florigen & light.
Summit zone of reproductive shoot apex is more active i.e., rate of cell division is greater and it forms statement & Carpels and flank zone is less active in reproductive shoot apex and it forms sepals and petals.
COMPOSITION OF APICAL MERISTEM IN DIFFERENT PLANTS
Apical meristems is absent is 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, in called diffused growth. Diffused growth also occur in animals.
Apical meristem in higher algae (eg. Fucus, Dictyota & Sargassum), Bryophytes and Some Pteridophytes (eg. Salaginella) is consist of single cell. This cell is known is known as apical cell. This apical cell is pyramid in shape and divide into two lateral planes.
Apical meristem in Ferns, Gymnosperms And Angiosperms consists of many cells. Several views have been available in relation to structure and growth habit of apical meristem.
Apical Cell theory :
This theory proposed by Karl Nageli and Hofmeister and supported by wolff
According to Nageli and Hofmeister, the apical meristem is composed of single apical cell. This view is only applicable on Bryophytes and some Pteridophytes and some higher algae (Fucus, Dictyota & Sargassum)
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) Dermatoge:- This is the outermost single layer of cells. These cells forms uniseriate epidermis by anticlinal division.
(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 medulallry 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 angioperms, shoot apex does not have distinct differentiation of three layers.
Except above described three histogens, a fourth type of histogen is present in monocotyledons. This is known as Calyptrogen. Root cap is produced by Calyptrogen in monocots. Root cap is produced by dermatogen in dcotyledons.
Exception : There is only one histogen present in Rannuculus. Two histogen occurs in Casuarina.
Due to presence of root cap position of root apex is sub terminal. so maximum growth in root takes place behind the apex.
Note :
1. In hydrophytes root cap is absent
2. Generally root cap is single layered by in pandanus (screw pine) root cap is multilayered.
3. Root cap contain more amount of golgi body which secrete mucilage which make the root slimy.
Quiescent centre
A group of inactive cells present between the dermatogen and calyptrogen is called quiescent Centre. These cells contain less amount of DNA 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.
If calyptrogen get damaged, this zone becomes active to form new cells of calyptrogen.
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
Tunica corpus theory
This theory was proposed by Schmidt (1924). This theory is applicable on shot 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.
Anticlinal division occurs at right angle to longitudinal axis of cell. Surface area of the plant organs increases due to anticlinal divisions. When division occurs in single anticlinal plane they do not increase the number of layers.
Note : Generally, tunica represent only single layer, 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 divided in all directions (many planes) due to which volume increases. The mass of these cells mainly forms the cortex. The cells of corpus usually larger than the cells of tunica. It forms rest of the tissue system.
Mantle core theory
This theory was proposed by “Popham and Chan”. They compared the mantle to 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.
Newmans’ 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 layers is called tunica and inner layer is called corpus. Such type 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).
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 division.
In peripheral region each cell first divides transversely and there after the lower daughter cell divided longitudinally thus forming the shape of T. Such divisions are called the Kappe division. In the central region T is inverted as the second division takes place in the upper daughter cell. Such divisions are called the Korper division. As a result of these ‘T’ or inverted T divisions, the cells in root apex remain arranged in rows. By Kappe division, the number of rows increases downwardly and by Korper divisions upwardly.
Note : this is rejected theory because there is not relation of arrangement of cells to tissue formation