What is Meristematic Tissue?
Meristematic tissue, also called meristem, may be defined as the embryonic tissue in the mature plant body, the cells of which continue to divide indefinitely and as a result new cells are added continuously to the plant body.
Meristematic tissue is composed of immature and less undifferentiated cells which possess the power of division.
Cells of meristematic tissue are usually isodiametric in shape and their size varies greatly i.e. in general they are small.
They are compactly situated without showing any intercellular spaces, but intercellular spaces may appear very early among the still dividing derivatives.
All the meristematic cells are characterized by the presence of large prominent nuclei, dense cytoplasm with small vacuoles scattered throughout the cytoplasm or the vacuoles may be absent altogether; ergastic matters i.e. reserve food materials, excretory materials and secretory materials are absent; plastids remain in proplastid stage.
Cell of meristematic tissue have a smaller amount of endoplasmic reticulum and a less elaborate internal structure in their mitochondria than the cells of permanent tissue. Cell wall is thin and homogeneous, composed of cellulose.
According to Kaplan (1937) typical meristematic cells having structural features as stated above should be termed as eumeristem or true meristem.
Certain zones of the apical meristems and the cambial initials have thick walls with primary pitfields ; starch, tannins may be present within such cells; sometimes cells may be long, narrow, fusiform, polygonal in shape ; intercellular spaces may appear very early among the still dividing derivatives.
Some cells of the apical meristems contain large vacuoles. Biochemically, meristematic cells differ from non-meristematic cells; in relation to their high rate of metabolic activity the meristematic cells exhibit strong reaction for the enzyme peroxidase—this enzyme occurs in the cells before and during the division stage and declines after the completion of divisions.
Meristematic Tissue Function
Being meristematic i.e. divisible, the meristems increase the number of cells in the plant body through division; and also initiate the growth and development of the plant organs i.e. the plant body as a whole.
Meristematic Tissue Location
Meristematic tissues in plants
Meristems occur at the growing apices of the main and lateral shoots and roots.
They are also found at the bases of internodes between mature tissues. Besides, meristems are situated parallel to the circumference of the organs in which they are found e.g. the vascular i.e. fascicular cambium of the vascular bundle and the phellogen (cork-cambiun) in the cortex.
Cell division may also occur in tissues other than meristems, e.g, in the cortex of the stem and in young developing vascular tissues. But the number of division is limited in these tissues.
On the other hand, the cells of the meristems continue to divide indefinitely and as a result new cells are continually added to the plant body.
The cells of meristematic tissue that remain active throughout are called initiating cells and the cells derived or formed by them are called derivatives.
These new cells i.e. derivatives gradually enlarge, change their shape, lose the power of division and are ultimately converted into mature cells having definite shape and functions—these changes of one type of cells to another type taking place through such processes may be referred to as differentiation.
Due to this differentiation, formation of permanent tissue from meristematic tissues takes place.
Sometimes, cells of the permanent tissue may regain the power of division (e.g. secondary meristem)—this phenomenon is called dedifferentiation by some workers.
Types of Meristematic Tissue
Meristems are classified according to their method of development, their origin, their position in the plant body, their function and their plane of division.
A. Classification of meristem according to their stage or method of development —
It is the region of new growth in plant body where the foundation of new organs or their parts is initiated—it is also known as “primordial meristem,” “embryonic meristem” etc.
This part comprises the apical initials and their immediate derivatives.
Promeristem is composed of all young thin-walled cells which are alike in shape, full of non-vacuolate or vacuolate active cytoplasm, and large nuclei ; intercellular spaces absent or minute.
It is to be mentioned that whenever the cells of promeristem begin to change in size, shape, character of wall and cytoplasm, they are then no longer a part of typical promeristem because they have passed beyond that earliest stage.
For example, if a meristem of some length is present at the tip of an organ, then only the youngest part of this i.e. a small apical portion constitute the promeristem.
B. Classification of meristem according to origin —
Meristems are classified into primary and secondary according to the nature of the cells that give origin to such meristems.
(i) PRIMARY MERISTEMS
Primary meristems are those whose cells originate directly from the embryonic cells and thus constitute a direct continuation of the embryo.
Such meristems practically consist in part of promeristem.
In primary meristems, promeristem is always the earliest stage and transition stages to permanent (mature) tissues constitute the remainder of the meristem.
Primary meristems continue to divide or retain the power of division and build up the fundamental i.e. primary part of the plant.
The main primary meristems are the apices of stems, roots and the primordia of leaves and similar appendages.
(ii) SECONDARY MERISTEMS
Secondary meristems may be defined as those meristems that develop from mature (i.e. permanent) tissues which have already undergone differentiation.
Secondary meristems are so called because they arise as new meristems in permanent tissue which is not meristematic.
Like primary meristems, they have no typical promeristem, though their initiating layers may often resemble such tissue.
Examples of the secondary meristems are the phellogen that develops from permanent tissues (parenchyma or collenchyma) that have already gone differentiation, and callose tissue which develops in tissue cultures made from permanent (mature) tissues.
The secondary meristems add new cells to the primary body (formed by permanent meristems) forming supplementary tissues during secondary growth or serve in protection and repair of wounded regions.
The above definitions of primary and secondary meristems are not always accurate — because the apical meristems of real adventitious organs develop secondarily within relatively mature tissues and also within secondary meristematic tisues, but according to their structure and function they are primary meristem.
Again a large portion or even the entire vascular cambium, which is generally considered to be a secondary meristem, often develops at later stage from the apical meristem i.e. from a portion of the procambium.
C. Classification of meristems according to their position in the plant body —
Meristems are classified into three distinct types viz., apical, intercalary, and lateral.
(1) APICAL MERISTEMATIC TISSUE
This type of meristem occurs at the apices of the main and lateral shoots and roots, and often of the leaves of vascular plants and thus represent their growing points.
Apical meristem includes the promeristem and the meristematic zone behind it i.e. primary meristem, in which three basic meristems e.g. the protoderm, the procambium and ground meristem of the tissue system can be distinguished.
Increase in length of the axis is mainly due to activities of the apical meristems — hence apical meristems are also called growing points.
A single apical cell is found in the apical meristem of pteridophytes; but in case of higher plants i.e. spermatophytes, a group or groups of cells constitute the apical meristem, called apical initials or apical cells. Such apical initials may occur in one or more tiers.
(2) INTERCALARY MERISTEMS
As the name indicates, these meristems are inserted between permanent tissues.
Their origin takes place when actively growing primary tissue regions get detached from the apical meristems due to the growth of organs—as a result they finally remain embedded between masses of permanent tissues.
Intercalary meristem are found in different organs of plants, e.g., at the leaf base as in Pinus sp., or at the base of the internode as in the stem of many grasses (monocot) and Equisetum sp. (pteridophyte) or at the base of the node as in Mentha sp. (dicot) etc.
Increase in length of the axis and its branches is the main function of this type of meristem.
Intercalary meristems are short-lived, either they merge with the neighbouring tissues or become converted into permanent tissues.
(3) LATERAL MERISTEMS
Lateral meristems are situated laterally i.e. parallel to the circumference of the organ (the root and the stem of gymnosperms and dicotyledons) in which they occur.
This meristem is known lateral due to its lateral position.
Lateral meristem is always composed of a single layer of rectangular cells which divide mainly in one plane i.e. periclinally and gradually produce new i.e. secondary permanent tissues.
The vascular cambium (fascicular cambium) and the cork cambium (or phellogen) are the examples of lateral meristems.
Increase in diameter of the plant organ is the main function of the lateral meristem.
D. Classification of meristems according to their functions –
In physiological plant anatomy (Haberlandt, 1914) this functional classification of the meristems has been followed, and accordingly meristems are classified into (i) protoderm, (ii) procambium and (iii) fundamental or ground meristem.
According to Haberlandt, the primary meristem at the apex of the axis is gradually differentiated into three zones viz protoderm, procambium, and ground meristem, and they are correlated with the equally and convenient classification of mature tissues into three systems such as epidermal, vascular and fundamental or ground tissue system.
It is the outermost cell layer; cells after radial division give rise to epidermis. In Ficus, multiple epidermis is formed due to tangential division.
The elongated tapering cells of the growing region constitute procambium.
Procambium strands occur in a ring in case of dicotyledonous stem.
The ring is separated into a number of isolated strands, each of which develops into a vascular bundle consisting of xylem, phloem and cambium.
The strands are scattered in monocotyledonous stems.
In roots only one procambium strand is found in the centre.
The procambium strands gradually increase giving rise to pericyle in some stems.
(3) FUNDAMENTAL OR GROUND MERISTEM
The rest of the meristematic tissue other than protoderm and procambium forms the ground meristem. It is gradually differentiated into cortex, medullary rays and the pith.
E. Classification of meristems according to the plane of division —
On the basis of plane of division, mass, plate and rib meristems have been distinguished as “growth-forms” of meristems.
(1) MASS MERISTEM OR BLOCK MERISTEM
In this type, growth is by three-plane or all plane and produces increase in mass.
Early stages of many embryos, developing sporangia, the endosperm of many plants, young pith, and cortex of most plants, etc. are examples of mass meristems.
(2) PLATE MERISTEM
Here division occurs mainly anticlinally in two planes, so that there is plate-like increase in area.
One-layered plate meristem forms epidermis and 2 to several-layered contribute in the leaf development. e.g. growth of flat leaf blade, uniseriate epidermis etc.
(3) RIB MERISTEM OR FILE MERISTEM
Here division occurs continuously (anticlinally) in one plane, as a result columns or rows of cells are produced.
Increase in length of organs takes place as a result of this type of division e.g. formation of young roots and of the pith and cortex of young stems.