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The group of young cells that have the capacity of active cell division is called Meristem. In plants growth is not generalized. It is restricted to only certain specific regions which have the meristematic tissue. Meristems are variously classified on the basis of different characters like origin, stage of development, position in plant and function different theories have been put forward from time to time to explain the zonation of the shoot apices.

(1)        APICAL CELL THEORY
Nageli in 1858 stated that in large number of cryptogam’s single solitary cell has been found to form the growing point and it was supposed that similar condition exists in higher plants too. The presence of complex growing apices in higher plants cannot be explained by this theory.

(2)        HISTOGEN THEORY
Henstein in 1868 a divided the shoot apex into various zones. Each zone consists of a group of initial cells and called Histogen or a tissue builder. The major histogens of the stem and roots are (a) the dermatogen, a meristematic external layer (b) the plerome, a central core and (c) the periblem, the region between the two. The dermatogen gives rise to the epidermis, the prerome to the primary vascular bundles and the periblem develops into the cortex. The dermatogen in the root cuts some small cells at the apex called calyptrogen that give rise to root cap. These layers in some plants are not distinguished and in some have no morphological significance as the plerome sometimes forms only the pith or it may form even the cortex, even the histogen sometimes form different levers in homologous axis of the same plant.

(3)        TUNICA CORPUS THEORY
Schmcdt in 1924 proposed this theory for the organization of shoot apex. According to this theory the shoot apex can be differentiated into two regions of unlike structure and appearance, a central core, the corpus with large cells dividing irregularly to result in volume growth. It is surrounded by an outer enveloping layer the tunica. Tunica cells are smaller and divide mainly anticlinically to result in surface growth. The tunica may be one to many layered.

The work of Foster in 1939, 1941 and Gifford in 1954 has furthered the concept of particularly with regard to tunica some workers named tunica to only those layers which never show periclinical division in median position. The additional parallel layers according to them are part of corpus. Other workers treat tunica as fluctuating in number of layers. According to this view one or more inner layers of tunica may divide periclinically and may become part of corpus (clower in 1961). According to Guttenburg in 1960 tunica could not consist more than two layers, dermatogen and subdermatogen. Sometimes the latter one is not well differentiated. It is summarized as

(1) The tunica layers are variable in number in various taxon.
(2) During ontogeny and also under seasonal fluctuations of growth, the number of periclinical layers may vary in same species.
(3) Tunica corpus organization is even absent in some cases.
It is thus clear that this concept is quite flexible and the tunica and corpus may be treated as morphological identities with fluctuating and not clearly demarked limits. Some workers reject this concept entirely because it does not relate the apical activity to the origin of tissues. Still there is a general feeling that this concept is useful for characterizing growth of the shoot apex of angiosperms.

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