The following points highlight the three main types of vegetative shoot apex found in plants. The types are: 1. Monoplex Apex 2. Simplex Apex 3. Duplex Apex.
Type # 1. Monoplex Apex:
Ex. cryptogamic vascular plants. Continuing meristematic residue occurs in the superficial layer only. It may consist of more than one cell provided a common wall is present between the two cells.
It may also consist of 3 or more cells and the cells have a common angle. During mitosis division walls lie parallel to the inclined walls. With inclined walls any one division in c.m.r. contributes to increase in both length and breadth, that is, one division provides for increase in bulk growth.
Type # 2. Simplex Apex:
Ex. Abies balsamea (gymnosperm). In simplex apex continuing meristematic residue is in the superficial layer alone. Continuing meristematic residue may consist of more than one cell provided a common wall is present between the two cells. It may also consist of 3 or more cells and the cells have a common angle. Continuing meristematic residue is parallel-sided as viewed in longitudinal section.
Cell division is restricted to one layer. Continuing meristematic residue divides by both anticlinal and periclinal divisions. Anticlinal division contributes to increase in breadth. Increase in length occurs as a result of periclinal divisions. So for bulk growth both periclinal and anticlinal divisions are needed.
Type # 3. Duplex Apex:
Ex. angiosperms. In duplex apex c.m.r. consists of more than one layer and the cells composing them are parallel-sided as viewed in longitudinal section. The peripheral layer of c.m.r. forms the surface layer. Continuing meristematic residue may consist of more than one cell per layer provided a common wall is present between the two cells.
It may also consist of 3 or more cells that have a common angle. The surface layer of c.m.r. divides by anticlinal divisions only. As a result increase in breadth or surface occurs. The inner layer of c.m.r. divides by both anticlinal and periclinal division. As a result bulk growth, i.e. increase in both length and breadth occurs.
Duplex apex thus exhibits two contrasted mode of cell division in different layers:
(i) Peripheral layer of c.m.r. has anticlinal plane of cell division, and
(ii) Inner layer of c.m.r. has both anticlinal and periclinal divisions.
Hence the prefix ‘duplex’ was introduced to recognize two modes of activity in c.m.r. For bulk growth anticlinal division in peripheral layer of c.m.r. and both anticlinal and periclinal divisions in the inner layer(s) of c.m.r. are necessary.
Previously the shoot apex was neatly labeled by Hanstein, Schmidt, Buvat and Popham.
The concept of continuing meristematic residue of Newman and its classification into monoplex, simplex and duplex appear to be simplest in comparison to previous views due to the followings:
i. The classification is applicable to all groups of vascular plant and it includes stem and root.
ii. This classification is not rigid like histogen theory regarding the differentiation of derivatives.
iii. The classification does not take into account whether the shoot apex possesses tunica-corpus – or dermatogen-periblem-plerome organization.
iv. The concept of meristeme d’attente of Buvat has not received universal acceptance and it is not as prominent as quiescent centre of root.
v. The classification of shoot apex according to Popham into seven types has limited usefulness. The meristem fluctuates in structure during ontogeny at different stages of plastochron.