Meristematic Tissues Or Meristems
Category : 11th Class
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 :
(1) They contain immature and young cells and are capable of repeated divisions.
(2) Intercellular spaces are not present in meristematic tissue.
(3) They contain a homogeneous thin cellulosic wall.
(4) They contain large nuclei associated with abundant cytoplasm.
(5) They are metabolically very active but they do not store food material and further no plastids in them.
(6) Vacuoles are small or absent.
(7) Meristematic cells are isodiametric in shape.
(8) Undifferentiated tissue in which cells divides continuously
Types of meristems
The meristems may be classified on the basis of their mode of origin, position or function :
According to origin and development : On the basis of origin, meristematic tissues are of three types :
(1) Promeristem or Primordial meristem : The promeristem originates from embryo and therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. It occupies a small area at the tips of stem and root. The promeristem gives rise to all other meristems including the primary meristem.
(2) Primary meristem : A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia. Primary meristem gives rise to the primary permanent tissue.
(3) Secondary Meristem : They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature. This process in which permanent tissue regains meristematic nature is called dedifferentiation. The secondary meristems are so called because they originate from permanent cells. The phellogen or cork cambium arising from epidermis, cortex or other cells during secondary growth, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and repair.
According to position : On the basis of their position in the plant body meristems are classified into three categories :
(1) Apical meristem : This meristem is located at the growing apices of main and lateral shoots and roots. These cells are responsible for linear growth of an organ. Solitary apical cells occur in ferns and other Pteridophytes while apical initials are found in other vascular plants.
(2) Intercalary meristem : These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. It is present mostly at the base of node (e.g., Mentha viridis, Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophytes like Equisetum) or at the base of the leaf (e.g., Pinus). The intercalary meristems ultimately disappear and give rise to permanent tissues.
(3) Lateral meristem : These meristems occur laterally in the axis, parallel to the sides of stems and roots. This meristem consists of initials which divide mainly in one plane (periclinal) and result increase in the diameter of an organ. The cambium of vascular bundles (Fascicular, interfascicular and extrastelar cambium) and the cork cambium or phellogen belongs to this category and are found in dicotyledons and gymnosperms.
According to function : Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types :
(1) Protoderm : It is the outermost layer of the apical meristem which develops into the epidermis or epidermal tissue system.
(2) Procambium : It occurs inside the protoderm. Some of the cells of young growing region which by their elongation and differentiation give rise to primary vascular tissue, constitute the procambium.
(3) Ground meristem : It constitutes the major part of the apical meristem which develops ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary rays.
According to plane of cell division : On the basis of their plane of cell division meristem are classified into three categories :
(1) Mass meristem : The cells divide anticlinally in all planes, so mass of cells is formed. e.g., formation of spores, cortex, pith, endosperm.
(2) Plate meristem : The cells divide anticlinally in two planes, so plate like area increased. e.g., formation of epidermis and lamina of leaves.
(3) Rib or File meristem : The cells divide anticlinally in one plane, so row or column of cells is formed. e.g., formation of lateral root.
Structure and organisation of apical meristem
(1) Vegetative shoot apex : Shoot apex was first recognized by Wolff (1759) shoot apex is derived from meristem present in plumule of embryo and occurs at the tip of stem and its branches as terminal bud. It also occurs in the inactive state in the axils of leaves as lateral buds. The tip of the shoot apex is dome-shaped and from its flanks at the base of the dome divide to form one or more leaf primordia. This continues throughout the vegetative phase. Many theories have been put forward to explain shoot apex, such as :
(i) Apical cell theory : This theory was proposed by Nageli (1858). According to this theory, shoot apical meristem consists of single apical cell. This theory is applicable in case of higher algae, bryophytes and in many pteridophytes but not in higher plants (i.e., gymnosperms and angiosperms).
(ii) Histogen theory : It was proposed by Hanstein (1870). According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers (or histogens).
(a) Dermatogen : Outermost layer and it forms epidermis and epidermal tissue system.
(b) Periblem : It is the middle layer which gives rise to cortex and endodermis.
(c) Plerome : The innermost layer forms pith and stele.
(iii) Tunica corpus theory : This theory was proposed by Schmidt (1924). According to this theory, the shoot apex consists of two distinct zones.
(a) Tunica : It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus. The tunica shows only anticlinal division and it is responsible for surface growth.
(b) Corpus : It represents the central core with larger cells. Corpus shows divisions in all planes and it is responsible for volume growth.
(2) Root apex : A group of initial cells, present at the subterminal region of the growing root tip, which is protected by a root cap, is called root apical meristem or root apex. It is embryonic in origin and formed from the radicle part of embryo. However, in adventitious roots it is produced from derivatives of root apex. According to Hanstein (1870) root apex of most of the dicotyledons also consists of three meristematic zones - plerome, periblem and dermatogen (fourth meristem calyptrogen to form root cap only in monocots). Regarding the apical organisation of root following theories have been put forward.
Korper-Kappe theory : It was proposed by Schuepp (1917). This theory is comparable with the tunica and corpus theory of shoot apex. Korper means body and Kappe means cap.
Quiescent centre theory : It was proposed by Clowes (1961) in maize. According to him, in addition to actively dividing cells, a zone of inactive cells is present in the central part of the root apex called quiescent centre.
The cells in this region have light cytoplasm, small nuclei, lower concentration of DNA, RNA and protein.
(3) Reproductive apex : During reproductive phase, the vegetative apices are converted into reproductive apices. Before conversion, the apex stops producing leaf primordia. The summit of the apex which remained inactive during the vegetative phase, starts dividing. As a result of cell divisions, the apical meristem undergoes change in shape and increase in size. The apex may develop into a flower or an inflorescence.
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