Explain different theories with diagram in relation to organisation of shoot apex and root apex ?
Answers
Answer:
Apical Cell Theory:
Nageli in 1944 advocated this theory. According to this theory the apical meristem consists of a single apical cell (also called apical initial) and this cell is interpreted as the ‘structural and functional unit of apical meristem’. The cell is very large and is shaped like an inverted pyramid.
The apical cell is tetrahedral in shape and has three or four cutting faces among which single face is directed upward and the rest points downward. The side of apical cell that is directed upward is triangular or square in shape and forms a part of the outer surface of the shoot apex (Fig. 7.7).


The cutting faces of apical cell divide in an orderly fashion that is in helical succession. The cell divides by an asymmetric division; as a result a narrow and flat cell is formed. The next division of the apical cell is also asymmetric. This type of asymmetric division is repeated in the downwardly pointed faces of the apical cell.
As a result all cutting faces have their daughter cells. The daughter cells also divide and form large packet of cells. The packet of cells differentiates and forms different segments of shoot. So the apical cell is regarded as ‘a reserve of one genetically sound cell’.
A single apical cell composing an apical meristem is present in vascular cryptogams. After the discovery of solitary apical cell in vascular cryptogams, it was supposed that such apical meristem might exist in higher plants as well.
Later extensive investigations refuted the universal occurrence of solitary apical cell in a meristem. In higher plants the apical cell theory was replaced by the concept that the different parts of a plant body have independent origin. So the apical cell theory was later superseded by histogen theory.
2. Histogen Theory:
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Hanstein in 1868 put forward histogen theory (histogen means tissue builder).

According to this theory the tissues of a plant body originate from a mass of meristem where the following three (histogens) can be distinguished (Fig. 7.8):

(a) Dermatogen:
(In Greek meaning skin). It is the outermost layer of the meristem. It gives rise to epidermises of root and stem.
(b) Periblem:
(In Greek meaning clothing). This region occurs internal to dermatogen but peripheral to plerome. This histogen is destined to form cortex of root and shoot and inner tissues of leaves. It surrounds plerome.
(c) Plerome:
(In Greek meaning that this fills). This region gives rise to vascular cylinder of stem and root including pith. It is the central core of stem and root and the cells composing this zone are very irregular. This region is enveloped by a variable number of mantle-like layers which are represented by dermatogen and periblem.
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According to Hanstein dermatogen, periblem and plerome arise from independent initials of the apical meristem.
Later investigations reveal that the sub-divisions — dermatogen, periblem and plerome have no universal application due to the following two reasons:
(i) In gymnosperm and angiosperm there exists no clear distinction between periblem and plerome.
(ii) The respective roles of the three histogens cannot be demonstrated.