Plant world reveals a striking array of simple and complicated forms. The simplest type of plant body is the unicellular type in which the cells separate soon after division. In such forms, the simple cell perform all the vital functions of life i.e., it grows, prepares food, undergoes metabolism, reproduces and completes its span of life.
The next step towards elaboration is the colonial type of plant body (Volvox etc.,) in which a smaller or a larger number of cells live together but perform their own functions or in some cases the functions may be distributed. This is a step towards division of labour.
In filamentous forms like Ulothrix, we come across a slight differentiation in the cells into a basal holdfast and vegetative cells that also perform functions like reproduction and growth.
Then in Bryophytes, multicellular plant shows distinct division of labour and differentiation of cells both in gametophytes and the sporophytes (Marchantia, Anthoceros, Funaria). This differentiation of cells results in the formation of mature cells that perform their own functions. A collection of cells may develop chloroplast and help in photosynthesis called photosynthetic cells or assimilatory tissue and another collection of cells stores food material – called storage tissue. So these collections of cells are called the tissues.
If we look at the higher plants eg., vascular cryptogams and the spermatophyte we find that they have highly complex bodies. One or two types of tissues cannot cope with their requirements. A study of their internal structure reveals that they have various kinds of cells that cluster together to form various kinds of tissues, and 2 or more types of tissues form tissue systems and the tissue systems form organs.
Each tissue in a tissue system perform the particular function, thereby helping a tissue system to perform the function allotted to it. The tissue and the tissue system coordinate and permit the organ to function well.
If we take leaf as an example – it has
i) Epidermal tissue system – lower and upper epidermis made up of epidermal cells, stomata and hairs. Epidermal cells protect the inner tissue, stomata help in gaseous exchange and transpiration, and hairs perform many functions including protection, secretion, etc.
ii)j Ground tissue system – made up of pallisade tissue and spongy parenchyma tissue. The former is green and helps in photosynthesis, and the latter helps in gaseous exchange.
iii) Vascular tissue system – is made up of xylem tissue and phloem tissue. The former helps in conduction of water and gives support to the organs; whereas the latter conducts prepared food.
In the absence of any of these tissue and tissue systems, the leaf will not be able to perform its function of photosynthesis and transpiration. Similar is the case with stems and the roots.
In its broader sense, a tissue may be defined as a group of similar or dissimilar cells that perform or help to perform a common function and have a common origin.
The various types of tissues can be conveniently grouped under –
i) Meristems or meristematic tissue
ii) Mature or permanent tissue
iii) Secretory tissue.
Meristems or Meristematic Tissue
All the cells of a plant embryo are capable of divisions but with the growth of plant this feature becomes restricted to only certain regions. Such embryonic regions of theoretically unlimited growth and cell divisions are referred to as meristems or meristematic tissues.
The term meristem was coined by Nageli (1858) and is derived from a Greek word “meristos”which means “divisible”. A meristem is a localised region in which active cell divisions occur or the cells have potentiality to become active in response to appropriate stimuli.
A meristematic tissue or meristem can be defined as- “a group of cells that are in a continuous state of division or retain their power of division”. Some examples of meristems are apices of stem and root, leaf primordial and vascular cambium. The derivatives of the meristems get transformed gradually into permanent tissues.
The chief characteristics of the cells comprising a meristematic tissue are :-
1. Cells are living and thin walled. Their walls are chiefly made up of cellulose.
2. Cells may be rounded, oval, polygonal or rectangular in shape.
3. There are no intercellular spaces between the cells.
4. The cells have abundant protoplasm. They have large conspicuous nuclei (1 in each cell)
5. The vacuoles may be very small or altogether absent.
6. The cells do not store reserve food material and are on an active state of metabolism. The plastids are present in proplastid stage. However, the plastids may be present in the secondary meristems. Endoplasmic reticulum is poorly developed
The cells may divide in a particular plant depending upon the positions of the meristem or they may divide in all directions. Cambial cells divide by walls that are parallel to the surface. The cells that gives rise to dermatogen divide only in an anticlinal plane (i.e., right angles to the surface).
The cells are, as a rule, devoid of ergastic susbstances.
Classification of Meristems:-
Various systems of classifying meristems have been proposed by many eminent workers. The meristems can be classified on the basis of origin and development of initiating cells, plane of divisions, functions and position in the plant body.
Meristems based on origin and development of initiating cells:-
On the basis of origin and development of initiating cells, the following 3 classes of meristems have been recognised:-
1. Promeristems (Primordial meristem):-
The meristem where foundation of new organs or of their parts is laid down is referred to as promeristem or primordial meristem or embryonic meristem.
It is made up of thin walled isodiametric cells with dense cytoplasm and large nuclei. Promeristem differentiates into primary meristem.
2. Primary Meristem:-
It is the first derivative of the promeristem and forms the fundamental parts of the plant. The chief meristems are apices of roots and shoots, primordial of leaves and fascicular cambium of dicot stems. The cells of primary meristem divide in all possible planes.
The primary meristem persists from the embryonic stage of development of the plant, or of an organ of a plant. Such meristems continue to divide or retain their power of division and give rise to the primary permanent tissues of primary plant body.
In some monocots, where secondary growth is absent (eg., palms), the apical portions of the stem are concerned in adding more vascular tissue and thus add to the thickness. It is affected by the activity of embryonic cells of the apex and is, therefore, regarded as primary in nature. Such a meristem is called primary thickening meristem.
3. Secondary Meristem:
This meristem which appears after a certain stage of development of plant organ is referred to as secondary meristems. These meristems develop from mature or permanent tissues which have already undergone differentiation. They are not present from the beginning of the formation of an organ but develop at a later stage.
They are not present from the beginning of the formation of an organ but develop at a later stage. They develop at the time of emergency (healing of wounds) or when there is a need of meristematic activity, eg., to affect secondary growth or to form cork – cork cambium, interfascicular cambium and cambium of root are excellent examples of secondary meristems.
They are always lateral in position and give rise to secondary tissues which add to the girth of the plant organ.
Meristems based on plane of divisions:
The plane of division of cells constituting the meristem is of considerable importance in determining the growth patterns on the basis of plane of cell division, the following 3 types of meristems have been recognised
1. Rib or file meristem:
The rib meristem divides by divisions perpendicular to the longitudinal axis of the plant organs resulting in paralled longitudinal rows of cells in cylindrical plant parts. Ex:- cortex and pith of stem and cortex of root.
2. Plate meristem:
The plate meristems divides mostly by anticlinal divisions (divisions perpendicular to the surface) resulting in surface growth as in the leaf blade of angiosperms and epidermis in particular.
3. Mass meristem:
In mass meristem, cell divisions occur in all planes resulting in the formation of a massive plant body or organ. This meristem is involved in early development of embryo, endosperm etc.
Meristems based on functions:-
The primary meristems in the root and shoot apices have been placed in the following 3 distinguished categories on the basis of their functions:-
1. Protoderm :
It is the outermost meristematic layer of young growing region, which subsequently develops into epidermis (Epidermal tissue system).
2. Procambium :
It is composed of narrow, elongated prosenchymatous meristematic cells that develop into primary vascular tissue system (primary xylem and phloem)
3. Ground Meristem
It is precursor of ground or fundamental tissue system, cortex and pith and has large and thin walled cells which in later stages of development become differentiated into hypodermis, cortex, pericycle, pith and medullary rays.
Meristems based on position in the plant body:
On the basis of their position in the plant body, meristems have been classified into the following 3 groups:
Apical Meristem:
The apical meristem lies at the apex of the stem and the root of vascular plants often they are also found at the apices of the leaves. Apical meristems are also called growing points. The activity of the apical meristems adds to the length of the plants or its parts.
The initiation of growth takes place by one or more cells situated at the tip of the organ. These cells always maintain their individuality and position and are called “apical cells” or “apical initials”. Solitary apical cells occur in pteridophytes, whereas in higher vascular plants they occur in groups.
Intercalary Meristem:
These meristems are intercalated in between the permanent tissues. These are actually the portions of apical meristems which are separated from the apex during the growth of the axis. They may be present wither at the base of the internode as in the stems of various grasses and wheat, or at the base of the leaf as in pinus, or at the base of a node as in Mentha viridis (mint).
The activities of these meristems also add to the length of the plant or its organs. Rapid elongation of intermodal portions takes place due to the activity of the intercalary meristems. Intercalary meristem also occurs at the base of leaf sheaths in grasses.
These meristems are short lived and ultimately disappear i.e., very soon they become permanent tissues.
Lateral meristems:
These meristems are present along the side of the stem. They are arranged parallel to the long axis of the organs in which they are seen chiefly periclinally or radially. They are responsible for the increase in diameter and form secondary permanent tissues. Due to the activity of lateral meristems the thickness of the organs increases. Vascular cambium and cork cambium are the common examples of such meristems.
Functions of Meristems:
Meristems are concerned with growth i.e., increase in mass or size of both of plant body.
The primary growth initiated in the apical meristems expands the plant body, increases its surface and its area of contact with air and soil and eventually produces the reproductive organs.
The cambia aid in maintenance of the expanding body by increasing the volume of the conducting system and forming supporting and protecting cells.
The combined activities of all the meristems give rise to a complex and often large plant body.
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