Apical Meristem Theories
Apical cell theory
Presence of a single tetrahedral apical cell in the shoot apex of most vascular cryptogams prompted Nageli (1878) to postulate the apical cell theory. According to this theory, a single apical cell is the structural and functional unit of the apical meristem and it governs the whole process of growth single apical cell occurs in algae and majority of bryophytes and pteridophytes. A single apical cell was also believed to be present in seed plants. The apical cell of the pteridophytes is usually four-sided with three cutting faces but in Salvia and Azolla it is three-sided with two cutting faces.
Histogen theory
According to this theory, which was proposed by Hanstein (1868), three distinct meristematic zones (layers) can be recognized in the shoot apex of angiosperms. These layers, each of which has its separate set of initials, were termed as histogens. The outermost histogen was designated as dermatogen the middle one as periblem and the innermost as plerome. Definite functions were assigned to each histogen. The dermatogen is destined to give rise to the epidermis, the periblem to the cortex and the plerome to the central cylinder. Subsequent studies revealed that the above layers have no morphological significance as they are not specific in their function. However, in most of the gymnosperms and angiosperms, it was not possible to distinguish these layers. Hence this theory which held ground for a long time was later dropped.
Tunica-corpus theory
Schmidt (1924) proposed the tunica-corpus theory, according to which two distinct zones occur in the shoot apex of the angiosperms. The peripheral zone consisting of one or more layers of regularly arranged cells was termed as tunica and the inner core of cells surrounded by tunica as corpus ). These two zones can be distinguished on the basis of the plane of cell division. Tunica, which appears as a discrete unit or sheet of cells, is characterized by only anticlinal divisions, while the cells of corpus divide in various planes. Unlike histogen theory, the tunica-corpus theory does not imply any relationship between apical and subsequent formation of tissues.
Tunica and corpus have separates initial which are contiguous with one another at the tip of the apex. These cells can usually be identified by their larger size and more vacuolated contents which take a lighter stain. The number of initials in these two zones varies in different species. This zone was identified as ‘apical initial group (Foster, 1938) or zone of initial cells’ (Stant, 1952).
Variations in the number of tunica layers have been reported even in the same species during different stages of development of the shoot apex. According to Schmidt (1924), Boke (1940) and Reeve (1942), these variations are due to plastochron periodicity. Howeverm Hara (1962) who observed three to seven tunica layers in Daphne pseudo-mezerum attributes them to the influence of seasonal growth changes. Sometimes, a few outer layers of corpus are very inform and resemble very much to those of tunica. But these layers do not always show anticlinal divisions. Such stratifications have been observed in several dicotyledonous (e.g., Malvacese, Lauraceae, Bignoniaceae) as well as monocotyledonous (e.g., Iridaceae, Zingiberaceae) families.
Some workers consider all mantle like layers as part of tunica, while other define tunica in a very strict sense and interpret and other stratified layers which occasionally so priclinal divisions as part of the corpus. The tunica-corpus concept. As thus become more flexible and the two reason which are credit as morphological entities are subjected to fluctuations. To accommodade these fluctuations in tunica and corpus, Popham and Chan (1950) introduct mental-score hypothesis. They differentiated the shoot apex into two histological zones without taking into account the planes of division. Their mental included all the outer layers of the apex and tunica was confined only to layers of mental which divide anticlinally. The mass of cells surrounded by mental was termed a score.
Histogenic layer theory
According to Dermen (1947), the terms tunica and corpus to not have fixed values and they for short of having rare meaning, histogenetically are ontogenetically. On the basis of his studies on naturally occurring periclinal cytochineras in apple and periclinal cytochineras in cran berry and peach induced by colchicine treatment, he suggested that tunica-corpus designations in these plants be replaced by reference to primary histogenic layers. He recognized three basic histogenic layer in all angiospermic plants which where refered to L-I, L-II and L-III layers. These layers exist in various combinations of ploidy in the shoot apex. For example, the cells of L-I may me 2x, those of L-Il 2x, and those of L-III 4x, expressed in terms ploidy, constitution would be 2, 2, 4. Besides this, there exists possibilities of various other combinations, such as 2-2-2, 4-2-2, 2-4-4, 4-4-4 and 4-4-2. The epidermis of the stem and leaves is derived from L-I. The L-II layer forms 1-3 layered hypodermis of stems, but in some cases inner cortical issues are also derived from it and in still others it may contribute to a portion of vascular tissue. In general, the vascular tissue and pith originate from L-III. The histogenic layer theory is an a sense modified version of the Hanstein’s histogen theory without implication of absolute tissue pre-distinction.
Anneau initial and meristem d’attente theory
This interpretation of apical organization, based on Plantefol’s theory of phyllotaxis was put forward by Bubat (1952, 1955) and subsequently supported by several France workers. According to this view, peripheral and sub- terminal reason are real initiating zones and the distal group of cells is relatively inert and without any histogenic function during the vegetative phase of development. The three distinct reasons recognized in the apical meristem are: anneau initial (the peripheral active zone), meristem d’attente (the waiting meristem that becomes active only during the formation of inflorescence or terminal flower) and meristem medullaire (the central pith region). The correlation of these zones with tunica and corpus is shown below:-
- Tunica (anneu initial promeristem sporogene) meristem d’attente
- Corpus (promeristem receptaculaire) meristem medullaire pith meristem
- Cytological observation relating to nuclear and nucleolar size, degree of vacuolation, chondriosome counts of mitosis and location and synthesis of protein and DNA, corborate the presence of a digital inactive zone.
Cyto-histological zonation
The shoot apices of gymnosperms, with the exception of Gnetales, do not have a surface layer which divides anticlinally. Thus they can not be interpreted in terms of tunica-corpus theory. Foster (1939) described the shoot apex of Ginkgo biloba in terms of certain regions or zones which have distinct cytological characteristic and react differently to staining. These zones were considered to have a dynamic relationship with growth processes in the apex. The four interrelated growth zones recognized are as follows:
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Apical initial group-
It includes a group of initials along the apical surface and their lateral and sub-adjacent derivatives.
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Central mother cell zone-
This zone lies below the apical initial group and is derived from them. This entire distal group of cells which usually takes a lighter stain is conspicuously vacuolated and has a relatively low rate of mitotic activity. The primary walls of central mother cells are thickened and distantly pitted.
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Rib meristem-
It consists of centrally situated derivatives of central mother cells which give rise to the pith. The cells of this zone are considerably vacuolated and divide transversely to form vertical files. This zone is also called file meristem or pith meristem.
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Peripheral meristem-
This zone which is also called flank meristem, originates partly from the lateral derivatives of the apical initial and partly from the central mother cell.
Important links
- Meristem & Meristematic Tissue- Definition, Characteristics, Types
- Economic Importance of Pteridophytes
- Origin & Evolution of Pteridophytes (Vascular Cryptogams)
- Alternations of Generation in Pteridophytes (Origin, Theories)
- Apogamy and Apospory
- Stele in Pteridophytes: Meaning, Types & Its Evolution
- Lycopodium: Distribution, Steles, Morphology and Anatomy
- Economic importance of Lichens
- Reproduction in Lichens (Various Types)
- ‘Telome Theory’ of Sporophyte Evolution in Pteridophytes
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