Shoot Apex
The shoot apical meristem is the terminal meristem of the shoot which is the continuing embryonic region of the plant. It continuously gives rise to new cells and tissues from which new organs are formed. It lies immediately above the uppermost leaf primordium. It is extremely variable in shape and size, even in the same plant at different stages of development.
It is usually radially symmetrical and appears more or less convex in a median longitudinal section. It is, however, a narrow cone with rounded tip in grasses and slightly concave in Drimys and Hibiscus syriacus.
The apical meristem becomes broader before the initiation of each leaf and after giving off the leaves it again narrows down. This rhythmic phenomenon gives the shoot apex an appearance of broad and narrow zones. Schmidt (1924) termed these areas as maximal and minimal areas of the shoot apex. The time that elapses between the successive initiations of two leaves or pairs of leaves is termed as “plastochoran”.
The size of the shoot apex varies considerably among the spermatophytes.
The shoot apex has been extensively studied by a number of workers and several theories have been put forward from time to time to explain and interpret the mode of growth of the shoot apical meristem. Some important ones are discussed below:-
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 apical meristem and it governs the whole process of growth. Such a 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 some, eg., Salvia and Azolla, it is three-sided with two cutting faces.
Thus, according to the apical cell theory, it was considered that the plant body in plants, including the seeds plants, arises from a single cell land its derivatives. This theory may hold good for higher algal groups, Bryophytes, and some Pteridophytes, but is certainly not applicable to the seed plants.
Further investigations have refuted the occurrence of apical cells in all plants and regarded that different parts of plant body arise independently.
Histogen Theory
According to this theory, which was proposed by Hanstein (1868), three distinct meristematic zones or layers can be recognised in the shoot apex of angiospersm. These layers, each of which has its separate set of initials were termed as histogens.
The outermost histogen was designated as dermatogens, the middle one as periblem and the innermost as plerome. Definite functions were assigned to each histogen:-
i. the dermatogens (Greek word – skin and to bring forth) gives rise to the epidermis which is generally a single layered (sometimes more than one – layered) outer covering in all organs of the plant body.
ii. the periblem (Greek – clothing) gives rise to all the tissues lying between the epidermis and the vascular cylinder, eg., cortex and endodermis.
iii. the plerome (Greek – that which fills) gives rise to the entire vascular cylinder including the pith.
Haberlandt (1914) proposed the terms protoderm, ground meristem and procambium for dermatogens, periblem and plerome respectively.
Recent experiments have however, revealed that there is no strict relationships between the development of the histogens and various regions of plant body. This theory has the following defect :-
i. there is no distinction into periblem and plerome in the apical parts of many angiosperms and in gymnosperms on the whole.
ii. it is not possible to assign to the histogens the origin of the various regions of plant body as determined by Hanstein.
Hence, this theory which held ground for a long time, was later dropped.
Tunica – Corpus Theory:
This theory was propounded by Schmidt(1924) to describe the angiosperm shoot apices. This theory recongises only 2 zones of tissues in the apical meristems. There are – 1. the tunica and 2. the corpus apex in which the plane of division is generally anticlinal. It consists of one or more peripheral or outer layers of cells which envelope a central mass of tissue.
In tunica, the cells are smaller than those of corpus. The corpus represents the central core with cells dividing in different planes and are larger in size as compared to the cells in the tunica.
The two layer are thus distinguished from each other by their different modes of cell division, which are predominantly anticlinal in tunica and are, therefore, undergoing surface growth. The layers in the corpus divide in various directions to add to the volume of the apical meristem. The tunica and the corpus originate from a separate set of initials.
As compared to the Histogen theory, which sets apart 3 distinct apical layers (dermatogens, periblem, plerome) each of which gives rise to a set of distinct tissues or tissue systems of the body,Tunica – corpus theory does not believe in any such tissue below except for the epidermis, which arises from the outermost layer of the tunica, other tissues like cortex, endodermis, pith and the vascular elements may have their origin from the tunica or the corpus or from both.
Subsequent researchers, especially by Foster and his team of workers have show that:
1. the number of tunica layers varies greatly in various species of genera, 2. The limits of demarcation between tunica and corpus varies greatly even within the same species at different phases of its antogeny and 3. In some cases their boundaries are not clear.
According to these observations the tunica-corpus concept has become more flexible and the two regions which are treated as morphological entities are subjected to fluctuations. To accommodate the fluctuations in tunica and corpus, Popham and Chan (1950) introduced Mantle-core hypothesis.
Mantle Core Concept:
It was proposed by Popham and Chan in 1950. They used the term mantle for the outer dome-shaped layers that covered the central part called the core. This is just a substitute for the tunica and corpus concept. They regarded tunica as mantle and corpus a core.
Popham recongnised 2 main types of corpus (core) in the angiosperms:
1. the usual angiosperm type: In this type 3 zones can be distinguished in the corpus
a) zone of central mother cells – located below the apical portion of the tunica or the tunica initials
b) the rib meristem – which appears to be continuation of the central mother cells and gives rise to the to the vascular tissues.
c) the flank meristem – which is peripheral in position and gives rise to the cortex
2. the Optunia type : in this type a cambium like transition zone can be recognised in addition to the above mentioned 3 zones. It is shaped like a cup. This type is also found in Xanthium, Phoenix, Ficus and Optunia species.
Concept of Newman:
Newman believed that there is nothing like a group of permanent initial cells in the shoot apex, instead there is a sequence of meristematic cells which function as initials over a period of time. He called them as continuing meristematic residue.
On basis of the form of meristem, he recognised 3 types of shoot apices:-
1. Monoplex: this type is found in ferns and their related genera. The meristematic residue is in the superficial layer. Any kind of division contributes to growth in length and breadth.
2. Simplex : this type is found in Gymnosperms. The meristematic residue is present in a single superficial layer. The cells divide by anticlinal and periclinal divisions and growth takes place.
3. Duplex : this type is found in Angiosperms. The meristematic residue occurs in at least two surface layers with two contrasting mode of growth. The superficial layer divides anticlinally and the inner layer of cells divide anticlinally and periclinally.
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. The root meristem is not terminal like that of shoot meristem, it is subterminal in its position, as it is located beneath the root cap. The root apex also differs from the shoot meristem in that it forms no lateral appendages like leaves and no branches. The root branches arise endogenously. The root also has 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.
Korper – Kappe Theory:
According to this theory, the cells at the root apex divide in two planes. The first division is transverse and one of the daughter cells then divide by a longitudinal division. This is known as ‘T’ division., because the cell walls from a T-shaped structure.
In some zones of the root, mainly in the centre, the bar of the T faces the root apex i.e., the letter ‘T’ is straight whereas in the peripheral zone, the bar of the ‘T’ faces away from the apex i.e., the letter ‘T’ is inverted( ┴). These two zones of the root apex delimited were named as Kopper (body) and Kappe (cap) respectively.
This type of demarcation between Kopper and Kappe region can be seen in some grasses, e.g., Zea mays.
Clowes proposed this concept. While making auto radiographic studies of DNA synthesis in the root tip of Zea mays, Clowes noted a central cap-like region, lying between the root cap and active meristematic region. The cells in this region have fewer mitochondria and ER, very small nuclei and low DNA and protein synthesis. He referred this apparent inactive region as Quiescent centre.
The quiescent centre provides a reserve block of cells within the root. The centre may be the site of hormone synthesis.
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