Role of meRistems in plant gRowth
• Meristems are the regions of growth in plants. Cells of
the meristematic region have the capacity to divide and self
perpetuate. They produce cells which lose the capacity to
divide and undergo differentiation to form particular tissues
and organs.
• The meristem which is consumed in the formation of an
organ is called determinate meristem. The meristem
which continues its activity throughout life of the plant is
called indeterminate meristem.
• Depending on their origin, meristems are of two types:
primary and secondary.
• Primary meristems are those meristematic tissues which
are derived directly from the meristems of the embryo. They
are divided into apical meristems, intercalary meristems and
lateral meristems depending upon their position in body.
• Apical meristems are of two types: root apical meristem
present on root apex and shoot apical meristem present on
shoot apex.
• Root apical meristem (RAM) and shoot apical
meristem (SAM) contribute cells for elongation of plant
parallel to its axis. It is primary growth. Another meristem
contributing to primary growth is intercalary meristem
located above the nodes in grasses and related plants. It
increases length of the plant organs.
• Lateral meristems contribute
tissues for growth in girth. Only
one type of primary lateral
meristem is found in plants
i.e., intrafascicular cambium.
Lateral meristems also include
secondary meristems which
cause secondary growth of
plant.
• Secondary meristems are formed
secondarily from the permanent tissues as some of
the permanent cells acquire the power of division. This
phenomenon is called dedifferentiation. The secondary
meristems are usually lateral. They are cylindrical meristems.
They give rise to secondary tissues e.g., vascular cambium
of the root, interfascicular vascular cambium of stem,
cork cambium or phellogen (from an outer layer of
cortex) and wound cambium (from the cells surrounding
an area of injury or wound).pRimaRy gRowth
• Primary growth is the first form of growth to occur. A whole
plant can be built up by primary growth, and in most
monocotyledonous plants and herbaceous dicotyledons it is
the only type of growth. It is a result of the activity of the
apical, and sometimes intercalary meristems.
• The seedling undergoes primary growth to form the
herbaceous (non-woody) green plant. Primary growth is
initiated by divisions of meristematic cells at the apex of
stem and root, the apical meristems.primary growth in shoots
• The primary growth of the shoot is concentrated at the shoot
tip. All tissues of the stem and leaves arise from terminal
meristem. The shoot apical meristem (SAM) is conical
or dome shaped in outline in seed plants. Leaf primordia
originate on the lateral flanks of the shoot apical meristem.
• Shoot apical meristem and the young primordia are
commonly termed the bud. The leaf primordia subsequently
grow and differentiate into leaves and lateral buds. The point
of attachment of the leaf and lateral bud on the stem is the
node. The part of stem between the nodes is referred to as
the internode.
• Growth of the stem axis is by cell enlargement in the
internodal region caused by intercalary meristem. There is
a progressive increase in the distance between the young
nodes and therefore, the apex of the shoot is pushed upward.
Branch primordia develop in the axils of old leaves.
• According to tunica-corpus theory of Schmidt (1924),
the shoot apex has two parts, outer mantle like tunica and
inner cellular mass known as corpus. Cells of
tunica undergo anticlinal divisions and
form protoderm (gives rise to
epidermis). Cells of corpus
divide in different planes to
form procambium and ground
meristem. Procambium gives
rise to primary vascular
tissues (xylem and phloem)
and intrafascicular cambium.
Ground meristem gives rise to