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You can very easily see the structural similarities and variations in the
external morphology of the larger living organism, both plants and
animals. Similarly, if we were to study the internal structure, one also
finds several similarities as well as differences. This chapter introduces
you to the internal structure and functional organisation of higher plants.
Study of internal structure of plants is called anatomy. Plants have cells
as the basic unit, cells are organised into tissues and in turn the tissues
are organised into organs. Different organs in a plant show differences in
their internal structure. Within angiosperms, the monocots and dicots
are also seen to be anatomically different. Internal structures also show
adaptations to diverse environments.
6.1 THE TISSUES
A tissue is a group of cells having a common origin and usually performing
a common function. A plant is made up of different kinds of tissues. Tissues
are classified into two main groups, namely, meristematic and permanent
tissues based on whether the cells being formed are capable of dividing
or not.
6.1.1 Meristematic Tissues
Growth in plants is largely restricted to specialised regions of active cell division
called meristems (Gk. meristos: divided). Plants have different kinds of
meristems. The meristems which occur at the tips of roots and shoots and
produce primary tissues are called apical meristems (Figure 6.1).
A
NATOMY OF
F
LOWERING
P
LANTS
C
HAPTER
6
6.1 The Tissues
6.2 The Tissue
System
6.3 Anatomy of
Dicotyledonous
and
Monocotyledonous
Plants
6.4 Secondary
Growth
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Root apical meristem occupies the tip of a root while the shoot apical
meristem occupies the distant most 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. Such buds are present
in the axils of leaves and are capable of forming a branch or a flower. 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 meristems 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.
The meristem that occurs in the mature regions of roots and shoots of
many plants, particularly those that produce woody axis and appear
later than primary meristem is called the secondary or lateral meristem.
They are cylindrical meristems. Fascicular vascular cambium,
interfascicular cambium and cork-cambium are examples of lateral
meristems. These are responsible for producing the secondary tissues.
Following divisions of cells in both primary and as well as secondary
meristems, the newly formed cells become structurally and functionally
specialised and lose the ability to divide. Such cells are termed permanent
or mature cells and constitute the permanent tissues. During the
formation of the primary plant body, specific regions of the apical meristem
produce dermal tissues, ground tissues and vascular tissues.
Central cylinder
Cortex
Protoderm
Initials of central
cylinder
and cortex
Initials of
root cap
Root cap
Root apical
meristem
Leaf primordium
Shoot apical
Meristematic zone
Axillary bud
Differentiating
vascular tissue
Figure 6.1 Apical meristem: (a) Root (b) Shoot
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86 BIOLOGY
6.1.2 Permanent Tissues
The cells of the permanent tissues do not generally
divide further. Permanent tissues having all cells
similar in structure and function are called simple
tissues. Permanent tissues having many different
types of cells are called complex tissues.
6.1.2.1 Simple Tissues
A simple tissue is made of only one type of cells.
The various simple tissues in plants are
parenchyma, collenchyma and sclerenchyma
(Figure 6.2). Parenchyma forms the major
component within organs. The cells of the
parenchyma are generally isodiametric. They
may be spherical, oval, round, polygonal or
elongated in shape. Their walls are thin and made
up of cellulose. They may either be closely packed
or have small intercellular spaces. The
parenchyma performs various functions like
photosynthesis, storage, secretion.
The collenchyma occurs in layers below the
epidermis in most of the dicotyledonous plants. It is
found either as a homogeneous layer or in patches.
It consists of cells which are much thickened at the
corners due to a deposition of cellulose,
hemicellulose and pectin. Collenchymatous cells
may be oval, spherical or polygonal and often
contain chloroplasts. These cells assimilate food
when they contain chloroplasts. Intercellular spaces
are absent. They provide mechanical support to the
growing parts of the plant such as young stem and
petiole of a leaf.
Sclerenchyma consists of long, narrow cells
with thick and lignified cell walls having a few or
numerous pits. They are usually dead and without
protoplasts. On the basis of variation in form,
structure, origin and development, sclerenchyma
may be either fibres or sclereids. The fibres are
thick-walled, elongated and pointed cells,
generally occuring in groups, in various parts of
the plant. The
sclereids are spherical, oval or
cylindrical, highly thickened dead cells with very
Intercelluar space
Figure 6.2 Simple tissues :
(a) Parenchyma
(b) Collenchyma
(c) Sclerenchyma
A fibre
A sclereid
(c)
Lumen
Thick
cell wall
Lumen
Pits
Thick
cell wall
(b)
Thickened corners
Protoplasm
Vacuole
Cell wall
(a)
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