MERISTEMATIC TISSUES
The growth in plants is mainly restricted to specialised regions of active cell
division called meristems (Gk. Merisms—divided). A meristematic tissue is an
undifferentiated mass of cells, that is in a continuous state of divisions and have
retained their power of division. These tissues divide to form new cells which
differentiate to give rise to permanent tissues.
Characteristics of Meristematic Tissue
The characteristics of meristematic tissue are listed below
(i) They are living and contain undifferentiated mass of rapidly dividing cells.
(ii) The shape of cells is spherical, polygonal or rectangular.
(iii) The cells are compactly arranged without intercellular spaces and are
interconnected by plasmodesmata.
(iv) Nucleus is large and present either in interphase or in subphases of
divisions.
(v) Cell wall is thin with only a primary wall made up of
cellulose. Secondary wall is absent.
Classification of Meristematic Tissue
Meristem can be classified based on their position in the
plant body, functions and origin.
Classification Based on Position
Meristems can be divided into three types, based on their
position in the plant body. These are as given below
(a) Apical Meristems The meristems which occur at
the tips of root and shoot produce primary tissues
are called apical meristems. The Root Apical
Meristem (RAM) occupies the tip of a root while,
the Shoot Apical Meristem (SAM) occupies the
most distinct region of the stem axis.
During the formation of leaves and elongation of
stem, some cells, left behind from shoot apical
meristem, constitute the axillary bud. These buds
are present in the axil of leaves and are capable of
forming a branch or a flower. Specific regions of the
apical meristem produce dermal tissues, ground
tissues and vascular tissues.
(b) Intercalary Meristems The meristem which occurs
between mature tissues is known as intercalary
meristem. They occur in grasses and regenerate parts
removed by the grazing herbivores.
Both apical and intercalary meristems are primary
meristems because they appear early in life of a plant
and contribute to the formation of the primary plant
body. These meristems are usually responsible fot
growth in length and present mostly at the base of
node (e.g., mint), base of internode (e.g., stem of
wheat and grasses) or at the base of leaf (e.g., Pinus).
(c) Lateral Meristems The meristems that occur in the
mature regions of roots and shoots of many plants.
These meristems produce woody axis. These appear
later than primary meristem so are also called the
secondary meristem.
They are cylindrical meristems. Some examples of
lateral meristems are fascicular vascular cambium,
interfascicular cambium and cork cambium. These
are responsible for producing the secondary tissues.
Classification Based on Functions
The meristems are also classified on the basis of their
functions as
(a) Protoderm It is the outermost portion of the
primary meristem found at the apex of the stem and
root. It develops into epidermis.
(b) Procambium It develops into primary vascular
tissues. It forms the isolated strands of elongated
cells, very near to the central region.
(c) Ground Meristem It develops into the ground
tissue. The cells are thin-walled, living and
isodiametric. In the later stages of growth, they
become differentiated into hypodermis, cortex,
endodermis, pericycle, medullary rays and pith.
Classification Based on Origin
The meristems are grouped on the basis of origin as
(a) Primary Meristems These meristems are derived
during the early embryonic stages. They divide
rapidly and differentiated into primary permanent
tissues which make the fundamental structure of the
plant body. They are mainly found in the growing
apical regions of the root and shoot.
(b) Secondary Meristems These meristems appear in
later stage of development in the plant body. They
lie lateral in position in both the stem and root.
Some primary permanent tissues acquire the power
of division and become meristematic.
These tissues can get dedifferentiated to form
secondary meristems as per need of the plant. Some
examples of secondary meristems are cork embium
and interfascicular cambium. They allow secondar
growth in tissues.