Let us make an in-depth study of the shoot apex and root apex of meristematic tissue of plant cell.
The Shoot Apex:
The terminal part of the shoot with the leaf primordia is the shoot apex where primary organisation of the shoot is initiated.
In size and shape it varies enormously, but in a general way it can be said that the shoot apex is more or less convex in longitudinal section.
The apical meristem widens considerably before the initiation of the leaf and it again becomes narrow after the appearance of the leaf primordium, thus exhibiting a rhythmic phenomenon.
The term plastochron has been used for the period between the initiation of two successive leaves. The shoot apex is usually small in angiosperms, but they are much wider in gymnosperms like Gingko and Cycas.
In Pteridophyta (Fig. 532A—1 & 2) one (apical cell) or more (apical initials) are easily distinguishable from others and they give rise to all the cells of the apex.
The apical cell, usually pyramidal in shape, as found in Psilotales, Equisetum and leptosporangiate ferns divide to produce new cells on all sides, whereas the eusporangiate ferns possess two to four apical initials.
It is believed that ferns with apical cells are more advanced from evolutionary point of view than those with apical initials.
In Gymnosperms (Fig. 532A—3 & 4) the cells in the surface of the apex divide both anticlinally and periclinally. As a result an initiating zone of the entire apex, known as surface meristem, is formed which gives rise to the epidermis and other meristems.
Due to periclinal divisions a subsurface zone of central mother cells develop where the cells have large nuclei, prominent vacuoles and often thick wall.
The rate of cell division is more pronounced on the peripheral side of this zone, and the derivatives together with those formed by anticlinal divisions of the apical initials constitute the peripheral zone or flank meristem.
This zone produces the cortex, the procambium and the leaf primordia. The basal part of the subsurface region develops into a zone of rib meristem consisting of vertical series of cells dividing in all planes. Pith originates from this zone.
Three types of gymnospermic apices (Popham, 1952) have been recognised which are nothing but
differences in detail in an organisation that is basically uniform.
In one of the types— the Gingko type another meristematic zone, a narrow transitional one lias been found to occur below the central mother cells. This cup-shaped portion called cambium-like transitional zone consists of cells -which undergo rapid periclinal division.
Unlike other gymnosperms the Gnetales exhibit a definite separation into a surface layer and inner core derived from the initials, which correspond to the tunica and corpus of the angiosperms, and thus suggesting the most advanced type of apical organisation amongst gymnosperms.
As already reported the shoot apex of angiosperms has been the subject of intensive investigation for pretty long time, and a number of theories (discussed) have been enunciated regarding the growth and differentiation of the same.
The generally accepted concept, tunica-corpus theory, demands that the tunica with one or few layers of cells forms an envelope round the centrally-located initials of the corpus.
The number of layers of tunica may vary (one to nine) in the same family, genus and even in the same plant at different stages of growth. Two types of cells have been recognised in the tunica.
One or few initials forming a central apical zone having larger size, more prominent nuclei and conspicuous vacuoles. The cells of this zone take light strain.
The second zone consisting of comparatively smaller and deeply staining cells occur on the sides of the apex between the initials and the leaf primordium.
According to internal arrangement, the corpus may be of two types—(1) the usual angiospermic type with three main zones, viz., (a) a zone of central mother cells representing the corpus initials, just below the apical portion of the tunica, (b) rib meristem, and (c) flank meristem, the last two appearing as continuations of central mother cells; (2) the Opuntia type, in which, a cup-shaped cambium-like transitional zone between the mother cells and other meristems have been recognised (Fig. 532A—5 & 6).
This zone which is always associated with leaf primordium differs from the rest in that the height and diameter vary considerably during the plastochron. It is considered by some (Philipson, 1954) as only a temporary feature, because it disappears towards the end of the plastochron.
Another concept about apical meristem developed on the basis of the rate of mitotic activity suggesting two zones—(1) a central apical zone with tunica and corpus initials having less frequent activity, and (2) a peripheral zone with pronounced mitotic cell division.
Many workers, mostly in France and hence called ‘French school’ (Plantefol, 1947-48; Buvat, 1952) maintained that an inactive zone exists at the shoot apex, which becomes active only during reproductive stage.
The vacuolated distal part of the apical meristem, according to them is of no significance in vegetative growth, the peripheral meristem with actively dividing cell and the rib meristem play the main roles in building the shoot.
The above view was seriously challenged and investigations were carried on from different aspects—histological (Popham, 1958) asserting considerable mitotic activity in the promeristem, in colcichine-induced cytochimeras (Dermen, 1951; Clowes, 1958) claiming ontogenetic continuity between promeristem and mature tissues, in tissue culture experiments suggesting the necessity of promeristem for the restoration of the entire shoot.
The Root Apex:
In comparison to the stem apex the apical meristem of the root is simpler, because of the absence of nodes and internodes and lateral appendages. But it has a protective cap, which acts as the buffer between the root-tip and the soil particles.
As cap occupies the terminal position, the apical meristem is subterminal here. Curiously enough, growth in the root-tip proceeds in two directions opposite to each other—towards the tip in the cap and away from the tip in the root proper.
Though histogen theory has been practically discarded in case of stem apex, it is followed in interpreting the structure and growth of root apex. The apical meristem here is rather short.
Considerable variations exist in the relation between the cap and the tip. In fact, root apices are of a few types depending on the mode of origin of cap and relations between histogens and primary tissue regions of the root proper.
In vascular cryptogams a solitary cell (Fig. 533A) occurs at the root apex. In course of time the root proper and the cap are formed from the apical cell, though the cap and the root become structurally quite distinct.
Two groups of initials occur (Fig. 533B) at the apex of roots of many gymnosperms. Of the two the inner group forms the plerome, and the outer forms the periblem and the cap. Unlike other groups dermatogen does not occur at the apex.
It is formed from the periblem a bit away from the apex as a ‘distal proliferation’. The outstanding features here are the formation of the cap from the cortex and at the same time outer layer of cortex becoming the epidermis—thus both cap and epidermis from the cortex.
In angiosperms three groups of initials usually occur at the root apex. In dicotyledons (Fig. 533C), of the three groups, the terminal one forms the dermatogen and the cap; the median one forms the periblem; and the innermost one gives rise to the plerome.
So here the cap and dermatogen have common origin. Thus cap may be considered a specialised development of the epidermis.
In monocotyledons (Fig. 533D), the outermost group of initials produces the cap; the medium one, the dermatogen and periblem; and the innermost one, the plerome.
The group of initials forming dermatogen and periblem is usually one-layered, but it may be two or three layers in thickness. Independent origin of the cap is a notable feature. The histogen concerned with the formation of the cap is referred to as calyptrogen.
The lateral and adventitious roots show same type of organisation.
In recent years many investigators (Jensen, Clowes, and others) have claimed that a zone of low mitotic activity composed of the cells of the central part of the promeristem occur. It has been called quiscent centre.
A theory—Kőrper-Kappe theory, more or less similar to the tunica-corpus of the shoot apex, was enunciated on the basis of planes of cell division. According to this theory there are two regions, outer (Kappe) and inner (Korper), and the cells divide in a pattern known as T-division.
Kappe cells first divide horizontally and the derivatives divide at right angles to the plane of first division—the planes of two thus forming a T. In the inner region Korper T is inverted as the second division takes place in the upper daugher cells.
