The below mentioned article provides an overview on Apical Meristems. After reading this article you will learn about: 1. Introduction to Apical Meristems 2. Initials and Derivatives of Apical Meristems 3. Vegetative Shoot Apex 4. Root Apex 5. Theories of Structural Development and Differentiation.
Introduction to Apical Meristems:
The apical meristem includes 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 developed 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 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 terms 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-called growing point but occurs abundantly — and may be even more intense — at some distance from the apical meristem (Wardlaw, 1945; Goodwin and Stepka, 1945).
On the other hand, 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.
Initials and Derivatives of Apical Meristems:
An initial or initiating cell is a cell that remains within the meristem indefinitely with the addition of cells to the plant body by combining self-perpetuation. The concept regarding meristematic initials implies that a cell is an initial, not because of its inherent characteristics, but simply because of its particular position in the meristem, a position that cannot be treated as permanent.
The number of initials in root and shoot apices is variable. In most of pteridophytes a single initial cell occurs at the apex. In the lower vascular plants, as well as in the higher, several initials are present. The single initial in its morphology is quite distinct from its derivatives and is commonly known as the apical cell. If the initials are numerous, they are called apical initials.
Usually the apical initials occur in one or more tiers. If there is only one tier, all cells of a plant body are ultimately derived from it. On the other hand, different parts of a plant body are derived from different groups of initials.
Apical Meristems –Vegetative Shoot Apex:
The vegetative shoot apices vary in shape, size and cytohistologic structure, and in their relation to the lateral organs. The shoot apex of Pinus and other conifers are commonly narrow and conical in form. In Cycas (cycads) and Ginkgo they are usually broad and flat on the other hand the apical meristem of a grass and some other monocotyledons remains elevated above the youngest leaf primordium.
In many dicotyledons the apical meristem rises above the primordia, and in other cases it appears to be sunken beneath them. The diameters of apices range from 90µ in some angiosperms to 3.5 mm in Cycas revoluta (Foster, 1949). The size and shape of the apex marked by change during the plant development.
There is tunica- corpus organization in the shoot apex of angiosperms. One to five layers of tunica have been observed in the dicotyledons, and one to three layered in the monocotyledons. However, tunica-corpus organization is not found in Saccharum officinarum.
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 may vary during the ontogeny of the plant body and under the influence of seasonal growth changes.
In the angiosperms the segregation of apical-meristem zones is more definite than in lower groups. There are two sets of initials, one above the other, which give rise to 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 may vary even in an individual plant.
Other Theories of Shoot Apex Organization:
In support of shoot apex organization other theories have also been propounded. Dermen (1947) put forth his Histogenic Layer Concent. According to him there is no distinct layer of apical meristems. He named the different layers of apical meristem as L1, L2, L3, etc. He recognized these layers on the basis of their origin.
However, this concept did not get any support. Popham and Chan (1950) put forth Mantle Core Concept. This concept is comparable to tunica corpus theory. They used the term mantle instead of tunica and core in place of corpus. Plantefol (1947, 1950), Buvat (1955) and Amefort (1956) propounded the Concept of French School and recognized three distinct regions in the apical meristem.
According to these authors, peripheral active zone was known as Anneu initial, the zone next to it Meristeme de attente and the central zone was termed Meristem medullaire. Newman (1961) put forth his concept and recognized three kinds of shoot apices.
According to him, Monoplex type is found in vascular cryptogams and ferns; here the shoot apex is denoted by one or more cells which divide by walls parallel to the inclined walls in the stem.
The Simplex type is found in gymnosperms; it consists of one or more initial cells arranged in a single layer; these cells divide anticlinally and periclinally. The Duplex type is found in the shoot apex of angiosperms; it consists of atleast two successive layers of cells; the cells of surface layer divide anticlinally and that of inner layer divide in more than one plane.
Apical Meristems –Root Apex:
During the later stages of development 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 a number of these cells of the apical meristem, which are termed initials.
In contrast to the apical meristem of the shoot, that of the root produces 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 located beneath the root cap.
The root apex also differs from the shoot meristem in that it forms no lateral appendages comparable to the leaves, and no branches. The root branches are usually initiated beyond the region of most active growth, and they arise endogenously. It also produces 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.
Apical Cell Theory:
This theory was put forth by Nageli. In the roots of vascular cryptogams (pteridophytes), e.g., Dryopteris, a single tetrahedral apical cell is present; it is generally thought that by its division this gives rise to all the tissues of the root. However, the apical cell theory was super-ceded by the histogen theory.
In number, the initials range from one to many. Where the initials are more than one, they are arranged in one to four fairly distinct, uniseriate groups. In each group there are one to several initials. Where there is more than one group, the groups lie adjacent to one another on the longitudinal axis of the root.
Each of these groups quickly develops one or more growth zones. In many plants these zones appear to represent ‘the histogens’. The terms dermatogen, periblem and plerome are no longer in general use in descriptions of stem ontogeny but they have been continued to indicate general zones in studies of root development.
A fourth histogen, the calyptrogen, is added where the cap has an independent origin. There are basic patterns for the major plant groups. The pattern is determined by the number of initials, the number of groups of these initials, the zones formed by each group, the morphological nature of the cap, and the degree of independence of the cap.
In the angiosperms there are three, rarely four groups of initials. In the dicotyledons the distal group forms 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 form four zones, but the outermost, independently, forms the cap, and that next beneath, the dermatogen and periblem.
The most characteristic of this 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) and different from those (cap dermatogen) similarly formed in the dicotyledons.
This theory was put forth by Schuepp in 1917. Since the root changes in diameter during growth, there are various points at which a single longitudinal file of cells has become a double file as a result of cell division. At these points a cell first divides transversely and thereafter one of its daughter cells divides longitudinally.
This was known as T division, because the cell walls form a T-shaped structure. In some zones of the root, mainly in the centre, the bar of the T faces the root apex, in other it faces away from the apex (1). These zones of the root, delimited by the planes of cell division, were called Korper (body) and Kappe (cap) respectively. This theory may be compared with the tunica-corpus theory in the shoot apex.
Mainly in roots with a very regular arrangement of cells in the apical meristem, such as of Zea mays, 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. The cells on the periphery of this hemispherical or cup- shaped region are meristematic. This inactive or passive region of cells is known as ‘quiescent centre’.
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. These inactive or passive cells constitute the quiescent centre.
The cells on the periphery of this hemispherical or cup-shaped region are meristematic and may be regarded as the constituents of the promeristem (Clowes, 1958). By various techniques, the existence of quiescent centre has now been demonstrated in the root apices of a considerable number of species. The quiescent centre develops during the ontogeny of the root (Clowes, 1958).
A quiescent centre is not found 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 able to demonstrate that the cells in the quiescent centre did not actively synthesize DNA.
The physiological and cytological properties of the cells in the quiescent centre have now been studied in a number of species. The cells in the region have a lower concentration of DNA, RNA and protein than any other cells in the root apex (Clowes, 1958; Jensen, 1958).
The cells of quiescent centre also have fewer mitochondria, little endoplasmic reticutum, and the smallest dictyosomes, nuclei and nucleoli (Clowes, 1964). They are less sensitive to radiation damage than other cells of the meristem (Clowes, 1959, 1964).
The function of the quiescent centre may be to provide a reserve block of diploid cells within the root. The quiescent centre may be the site of hormone synthesis.
Clowes (1961) has defined the promeristem as that part of the root apex which is capable of giving rise to all the tissues of the root. In the roots of vascular cryptogams, for example, the promeristem would consist of apical cell only, and in angiosperms it would comprise the initials of the histogens.
This way there is a tendency to regard the promeristem as a rather small region, situated terminally in the root apical meristem, below the root cap.
On the basis of modern work it has been suggested that in many roots the promeristem is broad and consists of a somewhat cup-shaped group of cells on the periphery of a central inactive region. This grouping of the initial cells of the promeristem was suggested by Clowes (1950) on the basis of an anatomical study of the root apex of Fagus sylvatica.
Theories of Structural Development and Differentiation of Apical Meristems:
By several workers (Foster, 1939, 1941; Sifton, 1944; Wardlaw, 1945), the view regarding the number, the arrangement, and the activity of the initial cells and their derivatives in the apical meristems has undergone many changes since the shoot apex was first recognized by Wolff in 1759 as an undeveloped region from which the growth of the plant proceeded.
The Apical Cell Theory:
This theory was put forth by Nageli in 1858. Solitary apical cells occur in many of algae, bryophytes and vascular cryptogams (pteridophytes). The discovery of the apical cell in cryptogams led to the concept that such cells exist in phanerogams (seed plants) as well. The apical cell was interpreted as a constant structural and functional unit of apical meristems governing the whole process of growth.
However, this was confirmed by later researches that this theory may hold good for cryptogams but is not applicable to the phanerogams. Further researches have refuted the universal occurrence of apical cells and replaced it by a concept of independent origin of different parts of plant body. The apical cell theory was super-ceded by the histogen theory.
The Histogen Theory:
This was introduced in 1870 by Hanstein who considered that the primordial meristem was sharply separable into three distinct zones or histogens. According to this theory the apical meristem or growing region of the stem and root are composed of small mass of ceils 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 plerome. Every zone consists of 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 gives rise to 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 many new cells resulting into a small celled tissue, the calyptrogen, which is also meristematic and gives rise to the root cap. Periblem This region is found internal to the dermatogen, and is the middle region of the apical meristem.
It is single layered at the apex but in central part it becomes multi-layered. It develops into the cortex of the stem. In the roots it is also single layered at the apex and many layered in the central portion. In the case of root, it also develops into the cortex. Plerome — It is the central meristematic region of stem apex and lies internal to the periblem.
It is also composed 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 certain strands of cells show a tendency to elongate. These strands of elongated cells make the procambium. The procambial strands ultimately become differentiated, into vascular bundles. A portion, however, remains undifferentiated, and it forms the cambium of the vascular bundle.
Recent investigations have revealed that there is no strict relationship between the development of the histogens and various regions of plant body and the segmentation and layering of the cells in the apical meristem. However, 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 put forth by Schmidt in 1924. The apical cell theory and the histogen theory were developed 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 this theory, there are two zones of tissues 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 this theory different rates and method or 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 cell division irregular, and the whole mass grows in volume. Each layer of the tunica arises from a group of separate initials, and the corpus has one layer of such 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 give rise to group of derivatives, which divide in various planes resulting in the formation of the inner mass of cells.
The number of initials varies from few to many. For example, in small very slender apices, such as those of grass seedlings, there may be only one or two in the tunica and about two in the corpus.
In vascular plants, the differentiation of the zones of stem-apex follows more or less definite patterns that seem to be characteristic of the major groups. These patterns show increasing complexity from the lower to the higher groups and appear to represent a series in specialization from simplicity to complexity.
As regards the concept of tunica and corpus, there may be several types which are found in the stem apices of several vascular plants.
The types may be as follows:
The primitive type of stem apex having no distinction of tunica and corpus:
Lycopodium, Isoetes, Selaginella (pteridophytes) and cycads (gymnosperms) belong to this group. They have simple apices with surface initials and no distinction of tunica and corpus. In Lycopodium, the initiating layer is weakly defined, having uniseriate surface area which divides freely both anticlinally and periclinally.
Here all the cells of the layer are morphologically alike. The anticlinal divisions increase the area of the surface layer, whereas the periclinal divisions form the inner core.
The stem apices with weak tunica-corpus demarcation:
The demarcation of tunica and corpus layers begins in some of the lower conifers. In Abies and Pinus (Coniferales), the initials make an apical uniseriate group. These initials further give rise to a central core and an enveloping uniseriate layer by both periclinal and anticlinal divisions.
The uniseriate layer that envelops the central core suggests a tunica in appearance, but there is no clear-cut demarcation between the tissues of two regions. However, in the apices of Sequoia sempervirens, the initials are a small group of surface cells in one tier, with both anticlinal and periclinal divisions. These divisions result in the formation of a dermatogen like layer and a central mass.
The outer layer suggests a tunica and the central mass, the corpus. The species of Cryptomeria and Taxodium (Coniferales) have a dermatogen, in which there are no periclinal divisions. There appears to be structural segregate on of tunica and corpus in the apices of many Coniferales, but there is only one tier of initials and no independent meristematic regions are recognized.
The stem apices with distinct tunica and corpus:
In angiosperms the demarcation of meristematic zones of apical region is usually more distinct and definite than in lower groups. There are two sets of initials, one above the other, which give rise to tunica and corpus which seem to be completely independent. The tunica has no or only rare periclinal divisions. It ranges in thickness from one to several layers. Usually there occur two or three layers.
The larger numbers of tunica layers occur more frequently in the dicotyledons. A single-layered tunica occurs in the grasses. However, in monocotyledons the number of tunica layers is one to three. In Zea, tunica divides periclinally, which shows an exceptional condition. The number of the layers in tunica may vary even in an individual plant. The corpus varies from a large complex type to a slender, simple type.
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 development of primary body of the shoot are now much better understood.
The tunica-corpus theory is of topographical value in studies of detailed development. The lateral organs of the stem, i.e., leaves, branches and floral organs, arise near the apex and studies of tunica and corpus have added greatly to a knowledge of origin and early development of these organs.