Describe any three theories with reference to organization of the meristem
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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