Various theories related to organization of Root apical meristem

Root Apical Meristem:- Several theories have been presented regarding its organization –

1. Apical Cell Theory

2. Histogen Theory

3. Korper – Kappe Theory

4. Quiscent Nature Concept

1. Apical Cell Theory:-

·    This theory was presented by Nageli in 1845.

·    According to this theory, the meristem at the top of the root is made up of only one cell. This cell is tetrahedral shaped. This cell forms the root cap by a cut at the lower side.

·    Example:- Such apical cell is found only in Pteridophytes. Hence, this theory applies only to Pteridophytes.

2. Histogen Theory:-

·    This theory was presented by Hanstein in 1868.

·    According to this theory the apical meristem differentiates into four layers called histogen layers. The future of these four layers is predetermined.

·    These four layers are:-

i. Calyptrogen:- Its cells show only anticlinal divisions. It forms the root cap which is also known as Calyptra.

ii. Dermatogen:- In this, the cells show only anticlinal divisions. It makes epidermis.

iii. Periblem:- In this, the cells show both anticlinal and periclinal divisions. It forms the ground tissue.

iv. Pleurome:- n this, cells divide in all planes. It forms stellar tissue.

·  Example:- This theory is applicable only to some gymnosperms and some angiosperms. Most of the gymnosperms and angiosperms either do not have these layers and if they have, their future is not determined.

Note:- Hanstein observed these layers only in one plant Capsella and presented this theory.

3. Korper – Kappe Theory:-

·    This theory was presented by Shueep in 1926.

·    According to this theory, the apical meristem of the root is differentiated into two regions:-

i. Korper

ii. Kappe

i. Korper:- In this, first cell division takes place is anticlinal, and after that the periclinal and anticlinal division takes place alternately. Such cell divisions are called as inverted T-shaped divisions.

ii. Kappe:- In this, the first cell division takes place is periclinal, and after that the anticlinal and periclinal divisions take place alternately. Such cell divisions are called as T-shaped divisions.

·    Example:- These regions are found in almost all gymnosperms and angiosperms. Therefore it is most accepted theory.

4. Quiscent Nature Concept:-

·    This theory was presented by Clowes.

·    According to this theory, there are two areas in the root apex:-

i. Central zone (Quiscent zone):- This region is inactive i.e. its cells are not dividing.

ii. Peripheral zone :- This area is active i.e. its cells are dividing.

·    This theory is also based on Histochemical test and Mitotic index.

 Note:- This is also a controversial theory because these regions are not found in plants of other countries except France.