Shoot Apex:
The terminal meristem present at the apices of stem and its branches is called shoot apex. Formed either at the plumular tip of the embryo or in the axil of a leaf. Lies immediately above the uppermost primordial leaf. Radially symmetrical and appears like a cone with rounded (or convex) apex. Varies in shape and size in different plants or in different branches of the same plant. Not protected by any cap.
However it is protected by the overarching young leaves and often by bud scales. Cell division in the apical meristem continuously adds new cells to stern beneath it. Growth in the shoot apex is not uniform due to presence of leaves, nodes and internodes.
The apex shows alternating broad and narrow zones due to formation of leaf pnmordia which occur at regular intervals and in rctular nattern. (A leaf orimordiurn is on the flank of the apex. It results in the broadening of the apex at this region, The apex becomes narrow again after the formation of leaf primordium).
The interval between the two successive leaf primordia is called plastochron. Several theories have been put forward to describe the structure and organisation of shoot apex. These include
- Apical cell theory
- Histogen theory
- Tunica corpus theory, mantle-core hypothesis, etc.
Apical Cell Theory :
According to this theory, shoot apical meristem consists of single apical cell (Nageli, 1858)
Single apical cell leads to the development of entire plant body. This theory is applicable in case of higher algae, bryophytes and in many pteridophytes. Not applicable to higher plants (i.e. gymnosperms and angiosperms). This theory supported by Hofmeister.
Histogen Theory : This theory was proposed by Hanstein (1870) According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers (or histogens).
- Dermatogen (external layer) forms the outer covering in all organs of the plant i.e epidermis,
- Periblem (middle layer) gives rise to cortex and endodermis, and
- Plerome (the central core) forms the pith and primary vasular tissue.
Haberlandt (1914) proposed the name protoderm for dermatogen, ground meristem for periblem and procamium for plerome.
Tunica Corpus Theory: This theory was proposed by Schmidt (1924) According to this theory, the shoot apex consists of two distinct zones-
(i) The outer tunica, which surrounds and envelops:
Tunica consists of one or more peripheral or outer layer of cells. These cells are comparatively smaller and divide by anticlinal divisions i.e.. with the axis of mitotic spindle paralled to the surface. Thus the cross walls separating the two daughter cells are formed perpendicular to the surface so that the number of lay ers do not increase. Cells derived from tunica differentiate into epidermis
(ii) The inner corpus, which forms the central core:
The corpus occupies the central portion in hich the cells are comparatively larger.These cells divide in all the planes. These cell form the procambium and ground meristem. Procambium gives rise to pr mar xylem, primary phloem and intrafascicular cambium. The ground meristem forms the ground tissue (i.e. cortex, endodermis, pericycle and pith).
Root Apex :
A group of initial cells, present at the subterminal region of the growing root tip, which is protected by a root cap is called root apical meristem or root apex. It is embryonic in origin and formed from the radicle part of embryo. In adventitious roots it is produced from derivatives of root appex. The root apex is short as compared to shoot apex. It is more or less uniform due to complete absence of lateral. Appendages (leaves and branches) and differentiation of nodes and intemodes. Cell division in the apical meristem adds new cells both to the body of the root and to the root cap.
Histogen theory of Hanstein (1870), the root apex of most of the dicotyledons consists of three meristematic zones or layers –
- Plerome forms pith, vascular strands and pericycle,
- Periblem forms endodermis and cortex and
- Dermatocalyptrogen gives rise to protoderm as well as the root cap.
The theory believes that in monocotyledons, the root cap is derived from a separate layer of initials called calyptrogen and the protoderm is derived from dermatogen.
In addition to actively dividing cells, a zone of inactive cells is present in the central part of the root apex called quiescent centre (clowes, 1961). It is a central cap- like or hemispherical region of inactive cells present in between the root cap and the active meristematic region of root apex. This zone contains several hundred cells which are characterized by having lower concentration of DNA, RNA and protein.
These cells also contain fewer number of mitochondria, less endoplasmic reticulum and small dictyosomes. The quiescent centre acts as reservoir of cells and becomes active whenever the initials get damaged or during the establishment of the secondary’roots. However, normally the cells of quiscent centre do not divide and remain inactive.
Mainly in roots with a very regular arrangement of cells in the apical meristem, such as of Zea mays, it is possible to conclude form 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’.
Root apex: pattern of cell lineages in the root apex of Zea mays the cortex,vascular cylinde of stem, root cap quiescent center are indicated.
