In this article we will discuss about:- 1. Definition of Meristems 2. Classification of Meristems 3. Theories of Structural Development and Differentiation.
Definition of Meristems:
Beginning with the division of the oospore, the vasular plant usually produces new cells and forms new organs until dies. In the beginning of the development of the plant embryo cell division occurs throughout the young organism. But as soon as the embryo evolves and converts into an independent plant the addition of new cells is gradually restricted to certain parts of the plant body, while the other parts of the plant remain concerned with activities other than growth.
This visible that, the portions of embryonic tissue persists in the plant throughout its life, and the mature plant is a composite of adult and juvenile tissues. These juvenile tissues are called the meristems.
The presence of meristems remarkably differentiates the plant from the animal. In the plant, growth resulting from meristematic activity is possible throughout the life of the organisms, whereas in the animal body the multiplication of the cells generally ceases when the organism attains adult size and the number of organs is fixed.
The term meristem (Greek meristos, meaning divisible) emphasizes the cell-division activity characteristic of the tissue which bears this name. It is obvious that the synthesis of new living substance is a fundamental part of the method of the formation of new cells by division.
The living tissues other than the meristems may also produce new cells, but the meristems carry on such activity indefinitely, since they not only add cells to the plant body, but also perpetuate themselves, that is, some of the products of division in the meristems do not develop into adult cells but remain meristematic.
The meristems generally occur at the apices of all main and lateral shoots and roots and thus their number in a single plant becomes quite large. In addition, plants bearing secondary increase in thickness contain extensive meristems, the vascular and cork cambia, responsible for the secondary growth.
The combined activities of all apical meristems, expands the plant body and forms the reproductive parts. On the other hand, the cambia, aid in maintenance of the expanding body by increasing the volume of the conducting system and producing supporting protecting cells.
Classification of Meristems:
Various systems of classifying meristems have been proposed by several eminent workers which are based on the characteristics such as, stage of development, position in plant body, origin, function and topography. No system is exclusive and rigid.
A few important kinds have been discussed below:
I. Meristems Based on Origin of Initiating Cells:
Primary and Secondary Meristems:
The meristems are classified as primary and secondary, on the basis of type of tissue in which origin exists.
The primary meristems are those that build up the primary part of the plant and consist in parts of promeristem. In primary meristems, promeristem is always the earliest stage. The possession of promeristem continuously from an early embryonic origin is properly of primary meristems. The main primary stems are the apices of roots, stems, leaves and similar appendages.
The secondary meristem appears later at a stage of evolution of organ of a plant body. Secondary meristems always arise in permanent tissues and they are always found lying lateral along the side of the stem and root. Sometimes certain of the primary permanent tissues acquire the power of division and become meristematic.
These tissues build up the secondary meristem. Secondary meristems are so termed because they arise as new meristems in tissue which is not meristematic. The most striking example of secondary meristem is phellogen or cork cambium. It is produced from mature cells—cortical, epidermal or phloem cells.
II. Meristems Based on Stage of Development:
Promeristem or Primordial Meristem:
Promeristem is the region of new growth in a plant body where the foundation of new organs or parts of organs is started. Sometimes it is also known as primordial meristem, urmeristem and embryonic meristem. From the viewpoint of its structure, this region consists of the initials and their immediate derivatives.
The cells of this region are isodiametric, thin-walled, vacuolate or non-vacuolate, with active cytoplasm and early stages of pits. Prominent nuclei and inconspicuous intercelluar spaces can be seen. As soon as the cells of this region begin to change in size, shape, and character of wall and cytoplasm setting off the beginning of tissue differentiation, they are no longer a part of typical meristem; they have entered beyond that earlier stage.
III. Meristems Based on Function:
Regarding their function, a system of classification of meristems has been proposed. The primary meristem at the apex of the stem and root is distinguished into three tissues— protoderm, procambium and ground or fundamental meristem. The protoderm comprises the outermost tissues which evolve into epidermis.
The procambium develops into primary vascular tissues. It forms isolated strands of elongated cells very near to the central region; in cross-section each procambium appears as a small group of cells in the ground or fundamental meristem, but in longitudinal section the cells appear to be long and pointed.
The ground or fundamental meristem evolves into ground tissue and pith; the cells of such region are large, thin walled, living and isodiametric. In later stages, they become differentiated into hypodermis, cortex, endodermis, pericycle pith rays and pith.
IV. Meristems Based on Position in Plant Body:
Regarding their position in plant body, the meristems can be classified into three groups— apical meristem, intercalary meristem and lateral meristem.
i. Intercalary Meristems:
The intercalary meristems are merely portions of apical meristems that have become separated from the apex during evaluation by layers or more mature or permanent tissues and left behind as the apical meristem moves on the growth.
The intercalary meristems are internodal in their position. In early stages in internode is wholly or partially meristematic, but later on certain of its part, becomes mature more rapidly than the rest and in the internode a definite continuous sequence of development is maintained. The intercalary meristems are found lying in between masses of permanent tissues either at the leaf base or at the base of internode.
These meristems are commonly observed in the stems of grasses and other monocotyledonous plants and horsetails, where they are basal. Leaves of many monocotyledons (grasses) and certain other plants, such as Pinus, have basal meristematic region. These meristematic regions are short living and ultimately disappear, to become permanent tissues.
ii. Lateral Meristems:
The lateral meristems are made up of such initials which divide mainly in one plane (periclinally) and increase the diameter of an organ. They add to the bulk of existing tissues or give rise to new tissues. These tissues are responsible for growth in thickness of plant body. The cambium and the cork cambium are examples of this type.
iii. Apical Meristems:
The apical meristem lies at the apex of the stem and the root of vascular plants. Very often they are also observed at the apices of the leaves. Due to the activity of these meristems, the organs increase in length.
The initiation of growth takes place by one or more cells located at the tip of the organ. These cells always maintain their individuality and position and are known as ‘apical cells’ or ‘apical initials’. Solitary apical cells occur in pteridophytes, whereas in higher vascular plants they exist in groups which may be terminal and sub-terminal in position.
The apical meristem consists of the meristematic initials and their immediate derivatives at the apex of a shoot or root. The apical meristem thus delimited corresponds approximately to the promeristem, and to contrast with the partly evolution derivatives of the promeristem, i.e., the protoderm, the ground meristem, and the procambium.
This seems quite impracticable to think of the apical meristem as consisting of the initiating cells only because this cells may be poorly differentiated from their most recent derivatives.
The terms shoot apex and root apex are more convenient to use instead of apical meristem of the shoot and apical meristem of the root, respectively. In the similar way the called shoot apex and root apex are more conveniently used as the substitutes of growing points. Growth in the sense of cell division, which is characteristic of the meristematic state, is not restricted to the so termed growing point but occurs abundantly—and may be even more intense—at some distance from the apical meristem. In other word, growth in the sense of increase in size of cells, tissues, and organs is most pronounced, not in the apical meristem, but in its derivatives.
a. Vegetative Shoot Apex:
The vegetative shoot apices vary in shape, size and cytohistologic structure, an in their relation to the lateral organs. The apical meristem of a grass and certain other monocotyledons remains elevated above the youngest leaf primordium. In several dictotyledons the apical meristem rises above the primordia, and in other cases it appears to be sunken beneath them.
There is tunica-corpus organization in the shoot apex of angiosperms. One to five layered tunicas have been found in the dicotyledons, and one to three-layered in the monocotyledons. To draw a clear-cut demarcation line in between tunica and corpus is not simple matter. In angiosperms the number of parallel periclinal layers in the shoot apex can vary during the ontogeny of the plant body and under the influence of seasonal growth changes.
There are two sets of initials, one above the other, which produce tunica and corpus. The tunica has no or only rare periclinal divisions and ranges in thickness from several layers to one with two or three layers probably most frequent. The number of layers in the tunica can even in an individual plant.
b. Root Apex:
During the later stages of evolution of embryo, the cells at the root pole become arranged, in a pattern characteristic of the species. This group of cells comprises the apical meristem of the primary root. The cells of this region are all relatively undifferentiated and meristematic, densely protoplasmic and with large nuclei and they all undergo active division.
The tissues of the mature root are eventually derived from numerous these cells of the apical meristem, which are termed initials. In contrast to the apical meristem of the shoot, that of the root forms cells not only toward the axis but also away from it, for it initiates the root cap and because of the presence of root cap the root meristem is not terminal but sub-terminal in its position, in the sense that it is situated beneath the root cap.
The root apex also differs from the shoot meristem in that it forms not lateral appendages comparable to the leaves, and no branches. The root branches are generally initiated beyond the region of most active growth and they arise endogenously. It also forms no nodes and internodes, and therefore, the root grows more uniformly in length than the shoot, in which the internodes elongate much more than the nodes.
In the angiosperms there are three, rarely four, groups of initials. In the dicotyledons the distal group produces the cap and the dermatogen; the median group, the periblem; the innermost, the plerome. The most characteristic is the common origin of cap and dermatogen. In monocotyledons, there are three groups of initials which produce four zones, but the outermost, independently, form the cap, and that next beneath, the dermatogen and periblem.
The most characteristic of such type is that the origin and structure of cap is independent. Moreover the two zones that are formed by one group of initials (dermatogen and periblem) are different from those (cap dermatogen) likewise produced in the dicotyledons.
The Quiescent Centre:
In the apical meristem of root of Zea mays, and other plants with a regular arrangement of cells in the apical meristem, it is possible to conclude from the study of cell lineages that there is a central region of cells which divide rarely or not at all. There inactive or passive cells constitute the quiescent centre.
The cells on the periphery of such hemishperical or cup-shaped region are meristematic and may be regarded as the constituents of the promeristem. By various techniques, the existence of quiescent centre has now been demonstrated in the root apices of a considerable numerous species.
The quiescent centre develops during the ontogeny of the root. A quiescent centre is not observed in the roots with a single apical cell. In 1956, Clowes was able to show that there was a central region (quiescent centre) in the roots of Zea where the cytoplasm had the lower content of RNA and where the cells had smaller nucleoli. He was also capable to demonstrate that the cells in the quiescent centre did not actively synthesize DNA.
The physiological and cytological characteristics of the cells in the quiescent centre have now been studied in a number of species. The cells in this region have a lower concentration of DNA, RNA and protein than any other cells in the root apex.
The cells of quiescent centre also have fewer mitochondria, little endoplasmic reticulum, and the smallest dictysomes, nuclei and nucleoli. They are less sensitive to radiation damage than other cells of the meristem.
The function of the quiescent centre can be to provide a reserve block of diploid cells within the root. The quiescent centre can be side of hormone synthesis.
Theories of Structural Development and Differentiation in Meristems:
Views regarding the number, the arrangement, and the activity of the initial cells and their derivatives in the apical meristems has undergone several changes since the shoot apex was recognized as an undeveloped region from which the growth of the plant proceeded.
The Histogen Theory:
Such was introduced in 1870 by Hanstein who considered that the primordial meristem was sharply separable into three distinct zones or histogens. According to such theory the apical meristem or growing region of the stem and root are composed of small mass of cells which are all alike and are in a state of division.
These meristematic cells constitute promeristem. The cells of the promeristem soon differentiate into three regions Dermatogen; Periblem and Pleronie. Every zone includes a group of initials and is called a histogen, or a tissue builder.
Dermatogen this is the single outermost layer of the cells which later produce the epidermis of the stem. In the root it is also single layered, but at the apex it merges into the periblem and just outside the periblem the dermatogen cuts off several new cells resulting into a small called tissue, the calyptrogen, which is also meristematic and gives rise to the root cap.
Periblem—This region is observed internal to the dermatogen, and is the middle region of the apical meristem. It is single layered at the apex but in central part in becomes multilayered. It develops into the cortex of the stem. In the roots it is also single layered at apex and several layered in the central portion. In the case of root, it also develops into cortex.
Plerome—It is the central meristematic region of stem apex and lies internal to the periblem. It is also made-up of thin walled isodiametric cells. Ultimately it develops and differentiates into the central stele consisting of primary vascular tissues and ground tissues such as, pericycle, medullary rays, and medulla. In the roots the function of plerome is practically same as in stem. At a little distance behind the apex some strands of cells show a, tendency to elongate.
These strands of elongated cells make the procamium. The procambial strands ultimately become differentiated, into vascular bundles. A portion, therefore, remains undifferentiated, and it forms the cambium of the vascular bundle.
The investigations have revealed that there is no strict relationship between the development of the histogens and different regions of plant body and the segmentation and layering of the cells in the apical meristem. Therefore, the distinction of these histogens in an apex cannot be made in some plants, and in others the regions have no morphological significance.
The Tunica-Corpus Theory:
This theory was proposed by Schmidt in 1924. The apical cell theory and the histogen theory were evolved with reference to both the root apex and the shoot apex. Later attention became centred largely on shoot apices, and with the result the tunica- corpus theory was developed.
According to such theory, there are two zones of tissue in the apical meristems—the tunica consisting of one or more peripheral layers of cells, and the corpus, a mass of cells enclosed by the tunica. According to such theory different rates and method of growth in the apex set apart two regions. The layers of the tunica show predominantly anticlinal divisions, that is, they are undergoing surface growth. In the corpus the cells are large, with arrangement and planes of cells division irregular, and the entire mass grows in volume.
Each layer of the tunica arises from a group of separate initials, and the corpus has one layer of this initials. In the tunica the number of layers of initials is equal to the number of layers of tunica, that is, each layer of tunica has its own layer of initials. The corpus arises from a single tier of initials which divide first periclinally to produce a group of derivatives, which divide in various planes resulting in the formation of the inner mass of cells.
The number of initials differs from few to many. For example, in small very slender apices, such as those of grass seedlings, there can be only one or two in the tunica and about two in the corpus.
Significance of the Tunica-Corpus Theory:
The tunica-corpus theory served well in the establishment of meristematic patterns of the shoot apices of seed plants. The position, number and behaviour of the initiating cells in seed plant stems, and early stages in the evolution primary body of the shoot are now much better understood. The tunica-corpus theory is of topographical stem, i.e., leaves branches and floral organs arise near the apex and studies of tunica and corpus have added greatly to knowledge of origin and early evolution of these organs.
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