In this article we will discuss about the characteristics and types of Meristem Tissue of Plants.
Characteristics of Meristem Tissue:
(i) Ability to grow and divide,
(ii) Small immature cells,
(iii) Isodiametric, rounded, oval or polygonal,
(iv) Absence of intercellular spaces.
(v) Walls are thin, elastic and made of cellulose,
(vi) Nucleus conspicuous,
(vii) Cytoplasm dense,
(viii) Vacuoles absent or very small,
(ix) Crystals absent,
(x) Endoplasmic reticulum small,
(xi) Pro-plastids are present instead of plastids.
(xii) Mitochondria have simple structure.
(xiii) Rate of respiration is very high,
(xiv) There is large scale synthetic activity,
(xv) There is little reserve food, and
(xvi) Cells of the cambium are, however, slightly different.
They possess large vacuoles and are elongated.
Pro-meristem (Gk. pro— before, meristos—divided):
It is part of apical meristem having actively dividing cells and their most recent derivatives. On the basis of plane of division meristems are of three types— mass, plate and rib meristems.
Mass Meristem:
All the cells of the body are meristematic and divide in different planes, e.g., early embryo.
Plate Meristem:
A flat meristem where cells divide anticlinally in two planes as during formation of leaves.
Rib Meristem (File Meristem):
Meristem with only anticlinal divisions or division perpendicular to longitudinal axis in one plane.
Pro-Meristem Derivatives:
They differentiate into three regions. Hanstein (1870) has called them histogens (Gk. histos-tissue, gennaein — to produce). Histogens are tissue producing definite zones or regions. They are dermatogen, periblem and plerome.
(i) Dermatogen (Gk. dermaskin, gene birth):
It is the region or histogen of single layer of outermost cells formed from the apical meristem. Dermatogen gives rise to epidermis of stem and other aerial parts. In root it gives rise to epiblema and root cap or calyptrogen (Gk. kalyptra-covering, gennaein—to produce). Calyptrogen is meristematic and forms root cap.
(ii) Periblem (Gk. Peri-around, blema-covering):
It is middle histogen which forms cortex of stem and roots.
(iii) Plerome (Gk. Pleroma a filling).
It is the central histogen which forms stele or part of stem and root inner to endodermis. Part of plerome that forms vascular tissues is called pro-cambium. Haberlandt (1914) has proposed a different nomenclature of protoderm, ground meristem and pro-cambium.
(a) Protoderm (Gk. protos—first, derma—skin):
It is the outer layer of apical meristem that gives rise to epidermis of stem and epiblema of root.
(b) Ground Meristem:
It is primary meristem formed from apical meristem which gives rise to ground tissues of the plant body. Ground tissues comprise all tissues except epidermis and vascular strands.
(c) Proeambium (L. pro—before, cambium—change):
It is part of meristem which gives rise to vascular tissues. The meristematic cells are parent ceils from which all other types of cells are formed.
Types of Meristem Tissue:
Depending upon their origin, meristems are of two types, primary and secondary.
1. Primary Meristems:
They are those meristematic tissues which are derived directly from the meristems of the embryo. Depending upon their position, primary meristems are of three types: apical, intercalary and lateral (Fig. 6.1).
(a) Apical Meristems:
The apical meristems are present at the tips of stem, root and their branches. They produce growth in length.
(i) Vegetative Shoot Apex (Shoot Apical Meristem):
It is derived from meristem present in plumule of embryo. Shoot apex 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. Shoot apex is conical or dome shaped in outline.
It is covered over and protected by means of young leaves formed by it. The meristem consists of a single apical cell in many teridophytes. In seed plants the apical meristem is a dome-shaped mass of meristematic cells. Leaf primordia are produced periodically on the flanks.
The period between the appearance of two successive leaf primordia is called plastochron (= plastochrone, Fig. 6.2). During a plastochron the shoot apex goes through a cycle of changes. It gives rise to derivatives in the basal region as well. They add new tissues and cause elongation of shoot.
Some cells of shoot apical meristem are left behind during the formation of leaves and elongation of the stem. They constitute axillary buds. Derivatives of apical meristem produce the primary plant body. Specific regions of apical meristem give rise to specific tissues-dermal, ground and vascular.
According to tunica-corpus theory of Schmidt (1924), the shoot apex has two parts, outer mantle like tunica and inner cellular mass known as corpus (Fig. 6.3). Cells of tunica are small. They undergo anticlinal divisions and form surface meristem called protoderm.
Protoderm gives rise to epidermis of both stem as well as leaves. If tunica is more than one layer in thickness, the outer layer differentiates into protoderm while the inner layers contribute to the formation of leaf interior and cortical tissues.
Cells of corpus are comparatively larger. They divide in different planes. Cells derived from corpus form pro-cambium, and ground meristem. Pro-cambium is slow to differentiate. Initially its cells are narrow, elongated and densely cytoplasmic.
They occur in parallel files. Pro-cambium gives rise to primary phloem, primary xylem and intra-fascicular cambium between the two (in case of dicots and gymnosperms). Ground meristem differentiates into pith in the centre and pericycle, endodermis, cortex and hypodermis respectively towards the outer side.
According to histogen theory of Hanstein, the stem apical meristem is differentiated into three regions or histogens (Fig. 6.4) — dermatogen (forms epidermis), periblem (forms cortex and endodermis) and plerome (forms pericycle, vascular bundles, medullary rays and pith).
(ii) Reproductive Shoot Apex (Fig. 6.5):
During reproductive phase all or some of shoot apices get changed into reproductive apices. The shoot apex stops producing new leaf primordia and axillary buds on the flanks. The cyclicity of divisions comes to an end. The meristem broadens, becomes less conical and increases in size. The summit cells, which were quite inactive in the vegetative shoot apex, begin to divide actively.
Therefore, all parts of the reproductive shoot apex show meristematic activity. It may give rise to an inflorescence or a single flower. When a single flower is to be formed, the meristematic cells give rise to different floral parts in the different regions. The meristematic cells get depleted or consumed in the formation of the various floral parts.
So further growth stops. The different floral parts formed by reproductive apex from below to tip or outside to the centre are sepals, petals, stamens and carpels. Sepal primordia are the first to be formed.
They come to lie at the lower end while other floral organs are formed successively higher up (exceptions found in some perigynous and epigynous flowers). Sepals enlarge rapidly and come to surround and protect the remaining floral organs. Carpels are the last to be formed. During their formation the apex of the meristem is used up.
(iii) Root Apex (Root Apical Meristem):
It is found at the tip of main root and its branches. In case of tap root system, the root apical meristem is formed from radicle part of the embryo or its derivative. In adventitious roots, the root apical meristem is produced from derivatives of shoot apex.
Root apical meristem (Fig. 6.6) is sub-terminal because it is covered by root cap. It does not produce lateral appendages. Root branches develop much behind the apex from the interior of the root (endogenous origin). In many cases, a quiescent centre is found in the centre of the root apex. Cell divisions are very few in the quiescent centre as there is very little synthesis of new proteins, RNAs and DNA.
Quiescent centre may function as reserve meristem. Due to presence of quiescent centre, the root apical meristem appears cup-shaped or hemispherical. Differential divisions in various parts of root apical meristem gives rise to 3-4 regions— protoderm, pro-cambium, ground meristem and calyptrogen. Calyptrogen differentiates only in monocots.
It gives rise to root cap. Protoderm forms epiblema or epidermis. In dicots it also produces root cap. Pro-cambium gives rise to vascular tissues. Ground meristem forms pith (if present), endodermis and cortex.
Histogen theory of Hanstein (1870) believes root apex to have three regions or histogens dermatogen (forms epiblema and root cap in dicots), periblem (forms cortex and endodermis) and plerome (forms pericycle, vascular strand and pith, if any).
(a) Intercalary Meristems:
They are meristematic regions which are derived from the apical meristems and which have been separated from them by the formation of’ permanent tissues in between. Intercalary meristems help in elongation of the organs. They also allow the fallen stems of cereals to become erect.
Intercalary meristems are commonly found at the bases of leaves, above the nodes (e.g., grasses) or below the nodes (e.g., mint). The intercalary meristem present at the base of Pinus leaf (basal meristem) lives almost throughout the life of the leaf. Usually the intercalary meristems differ from other meristems in that they ultimately get fully used up in the formation of permanent tissues.
(b) Lateral Meristem:
The meristem occurs on the sides and takes part in increasing girth of the plant. Only one type of primary lateral meristem is found in plants. It is intra-fascicular cambium. The cambium lies in vascular bundles of dicot and gymnosperm stems in between phloem and xylem.
2. Secondary Meristems:
The meristems are formed secondarily from the permanent tissues. Here, some of the permanent cells acquire the power of division. The phenomenon is called dedifferentiation. The secondary meristems are usually lateral. They are cylindrical meristems. The meristems give rise to secondary tissues that constitute secondary growth.
The common examples are vascular cambium of the root (derived from conjunctive parenchyma), inter-fascicular vascular cambium of stem (formed from medullary ray cells), cork cambium or phellogen (from an outer layer of cortex), wound cambium (from the cells surrounding an area of injury or wound) and accessory cambia of monocots (e.g., Dracaena, Yucca).