Theories of structural development and differentiation of meristems:
Several theories have been put forward to explain the activity of apical meristem.
Apical cell theory:
- This theory was proposed by Nageli in 1858.
- According to this theory the root and the shoot apex consist of a single meristematic apical cell which divides and forms the plant body. So the activity of single apical cell leads to the development of entire plant body.
- Such type of single cell is found in higher algal forms like Dictyota, Sargassum and Fucus, Bryophytes and Pteridophytes.
- In Gymnosperms and Angiosperms(Phenerogams) this theory is not applicable.
Histogen theory:
This theory was proposed by Hanstein(1868-1870)
According to this theory the root and shoot apices of plants consist of 3 meristematic regions or histogens.
a) Dermatogen:
Outermost single layered. It divide anticlinally give rise to epidermis.
b) Periblem:
It is next to the dermatogen. It is single layered but gradually becomes multilayered. This histogen give rise to hypodermis, cortex and endodermis.
c) Plerome:
It is the innermost histogen situated in central region. It give rise to entire vascular cylinder including pith that is pericycle, medullary rays, xylem, phloem, pith.
In case of root apex Hanstein proposed one more histogen.
Calyptrogen- which is responsible for the formation of root cap(in monocot).
Tunica Corpus theory:
This theory was proposed by Schmid 1924. This theory is only applicable to shoot apex. This theory is concerned with planes of cell division in the apex.
According to this theory the apical region of the shoot consist of 2 zones.
a) Tunica:
It comprises two outer layers designated as L1 and L2. It divide only anticlinally (I, e in a plane perpendicular to the surface).
Tunica contributes only to surface growth.
Tunica may be single layered or multilayered when it is multilayered then the outermost Layer(L1) will form epidermis and the other layer will generate internal tissue.
b) Corpus:
A mass of cells enclosed by tunica. The cells of corpus are larger. The cells of Corpus divides in various planes I, e periclinally and anticlinally leading to increase in tissue volume.
Corpus results in the development of Cortex and stele.
Gymnosperms do not usually show a tunica corpus type of organization in their SAM(Shoot Apical Meristem).
Korper kappe theory:
This theory was proposed by Schuepp(1917).
According to this theory cell group present in root Apex(different in plane of division).
First a cell divides into two by a transverse division and then one of the daughter cells divides by longitudinal division and because of which a T shaped structure is formed. The sequence was termed as T-division.
On the basis of such devision he diffrentiation two different regions in root.
T-shaped cell are Kappe cells
Inverted T-shaped cells are Karper type of cells.