THE TISSUE SYSTEM
The tissues also vary, depending upon their location in the
plant body. Their structure and function would also be
dependent on location. Thus, on the basis of their structure
and location, there are three types of tissue system, i.e.,
epidermal tissue system, ground or fundamental tissue
system and vascular or conducting tissue system.
l. Epidermal Tissue System
The epidermal tissue system forms the outermost covering
of the whole plant body. Its various components are
epidermal cells, stomata and the epidermal appendages, i.e.,
trichomes and emergences.
i. Epidermis
The epidermis (Gr. Epi—upon; derma —skin) is the
outermost layer of the main plant body. The epidermal cells
are elongated and compactly arranged to form a continuous
layer. This layer is interrupted by stomata. Sometimes they
are separated by intercellular spaces.
It is usually single-layered, but is also multilayered in the
aerial roots of orchids and leaves of Nerium and Ficus elastica.
The cells are parenchymatous and living. Each cell has a
large central vacuole and a peripheral thin cytoplasm. It is
thicker in xerophytic plants. In roots, epidermis has
tubular, unicellular, projections called root hairs. The other
substances deposited on cuticle surface may be oil, resin,
silicon and salts (calcium oxalate or calcium carbonate).
ii. Stomata
The stomata (sing. stoma) are openings in the epidermis 0f
most of the aerial parts of the plants, especially the leaves
Each stomata is composed of two bean-shaped cells called as
guard cells, which enclose stomatal pore. The guard cells
are generally much smaller in size as compared to Other
epidermal cells. They are sensitive to even a small change in
turgor pressure. The dimension of stomatal pore varies
from species to species, but it measures about 20 um long
and about 10-20 gm wide when fully open.
In some species, the guard cells are surrounded by
subsidiary cells or accessory cells which differ.
morphologically from the other epidermal cells. The guard
cell walls have special elastic properties. The adjoining cell
walls of two guard cells around pore are free and not
attached with each other. These properties help them to
stretch laterally during stomatal opening. The stomatal
aperture, guard cells and the surrounding subsidiary cells
are together called stomatal apparatus.
In most monocots, the guard cells are dumb-bell-shaped,
The stomata are mostly found on the upper epidermis of
the leaves. In some hydrophytes, the stomata occur on the
upper surface to avoid water contact.
Based on their distribution, stomata are of following types
(a) Apple Type Present on the underside of a leaf,
e.g., apple, mulberry.
(b) Oat Type Stomata are almost equal on the two
surfaces, e.g., maize, oat.
(c) Potato Type These are more on the under
surface, e.g., cabbage, potato, bean.
(d) Water lily Type These stomata are more on the
upper surface, e.g., many aquatic plants.
(e) Potamogeton Type Stomata vestigial or absent, e.g.
Potamogeton.
iii. Epidermal Appendages (Outgrowths)
The epidermis of most plants often bears outgrowth known
as epidermal appendages or epidermal outgrowths. They
are of following two types
(a) Trichomes The epidermal hairs present on the stem
are called trichomes. These are epidermal
outgrowths present temporarily or permanently on
almost all plant parts. They are multicellular
structures which may be branched or unbranched.
They may be secretory in nature.
They also help in checking the rate of transpiration
from aerial plant surfaces.
(b) Emergences (Prickles) They are multicellular, stiff
and sharp epidermal outgrowths containing some
Inner tissues.
Functions of Epidermal Appendages
They protect the plant against excessive loss of water and
grazing. They also help in climbing in some plants, e.g.,
rose.
Functions of Epidermal Tissue System
Epidermal tissue system serves the following important
functions
(a) It provides a protective covering all over the plant
parts.
(b) It helps in gas exchange through stomata and
lenticles present on the surface.
(c) The presence of cuticle helps in the reduction of
evaporation of water (epidermis).
(d) The glandular trichomes excrete various useful plant
products for the plant function.
(e) In some monocot leaves, the bulliform cells help in
the rolling and unrolling of leaves. The rolling of
leaves in peak summer midday helps to reduce
transpiration in plants.
2. Ground Tissue System
All tissues, except epidermis and vascular bundles constitute
the ground tissue system. It mainly forms the bulk of the
plant body. It consists of simple permanent tissues.
It’s various components are hypodermis, cortex,
endodermis, pericycle, medullary rays and pith.
In leaves, the ground tissue consists of thin-walled
chloroplast containing cells is called mesophyll.
i. Hypodermis
This is the region situated just below the epidermis and as
an outer region of cortex. It consits of one, two or few
continuous or discontinuous layers of collenchyma (in
dicots) or sclerenchyma (in monocots). It is protective and
mechanical in function.
ii. Cortex
The cortex lies between epidermis and endodermis.
It consists of parenchyma, collenchyma and sclerenchyma
tissues. The cortex is very distinct layer in dicotyledons, but
not in monocotyledons. The cells of cortex contain starch
grains, oil, tannins and crystals. Sometimes, cortical cells
may develop chloroplasts become photosynthetic and are
called chlorenchyma.
In hydrophytes, the cortex may be aerenchymatous
(spongy tissue with large air spaces found between the cells
of the stems and leaves of aquatic plants). The special types
of cells like sclereids, resin ducts, oil glands laticifers are
found in this region. The cortex helps in performing vital
functions, such as storage, etc.
iii. Endodermis
This is the innermost layer of the cortex. It is
single-layered, barrel-shaped and arranged without
intercellular spaces. The cells are parenchymatous. The
presence of bands of suberin on the radial and transverse
wall is the characteristic feature. These bands are called
casparian strips.
The endodermal cells of roots usually have thick, radial and
inner tangential walls.
These thick-walled cells form a continuous ring which is
interrupted at certain places by passage cells, which are
thin-walled and usually present opposite to the protoxylem
region.
A well-developed endodermis is present in all types of roots,
aerial stems of woody dicotyledons and gymnosperms with
characteristic casparian thickenings.
Functions of Endodermis
The endodermis helps to control the outward movement of
water and air between the cortex and xylem. It also helps to
maintain the root pressure and conducts water to the
protoxylem.
iv. Pericycle
It is made up of a single layer or many layers of cells present
between endodermis and vascular tissue. In roots, pericycle
comprises cells of parenchyma. Initiation of laternal roots
and vascular cambium during the secondary growth takes
place in these cells. The pericycle is absent in roots and
stems of some aquatic plants.
v. Medullary Rays
The medullary rays are non-vascular areas which occur
between vascular bundles in dicot stems for lateral
conduction. These are made up of parenchyma cells. These
originate from the apical meristem. They serve the function
of lateral transport.
vi. Pith
The central portion of root and stem is occupied by pith. It
contains parenchymatous cells and also sclerenchymatous
cells, laticifers, medullary vascular bundles, in some cases.
In leaves, the ground tissue is parenchymatous and
possesses chloroplast.
Functions of Pith
The main function of pith is storage of water and food
materials.
vii. Ground Tissue of Leaves
In leaves, the ground tissue of petiole is made up of
parenchymatous cells with distinct intercellular spaces. In
the lamina, the bulk of ground tissue is called mesophyll,
which is usually differentiated into palisade and spongy
parenchyma.
These cells are thin-walled and possess chloroplasts. The
main function of mesophyll is in photosynthesis.
3. Vascular Tissue System
A vascular bundle is a strand of conducting tissue, which is
generally composed of xylem and phloem in monocots and
xylem, phloem and cambium in dicots. These tissues originate
from the procambium and apical meristems.
The arrangement of xylem and phloem is the characteristic
to particular plant organs. However, a few exceptions are
also there.
On the basis of arrangement of xylem and phloem in the
vascular bundles, there are three types Of bundles,
i.e., radial, conjoint and concentric.
i. Radial
The xylem and phloem alternate with each other separated
by parenchymatous cells. This types of vascular bundles art
called radial and is found mainly in roots.
ii. Conjoint
The xylem and phloem are present together in the
bundle on the same radius. Conjoint bundles are of
types, i.e., collateral and bicollateral.
(a) Collateral The xylem and phloem lie together on
same radius. The xylem lies inwards and the phloem
outwards.
They are of two types
• In a dicot stem, the cambium is found to be present
in between the xylem and phloem, such bundles are
called open, e.g., Helianthus (sunflower).
• In monocots, the cambium is absent, the vascular
bundle is called as a closed bundle, e.g., Zea may
(maize).
(b) Bicollateral This is the conjoint vascular bundle with
two groups or patches of phloem, one on each side
of the centrally located xylem.
The various components are arranged in sequence of
outer phloem, ougr cambium, xylem, inner
cambium and inner phloem. Such bundles are
commonly found in the members of Cucurbitaceae.
Such bundles are always open.
iii. Concentric
A vascular bundle in which one tissue is completely
surrounded by the other is called concentric. The
concentric bundles are of two types, i.e., amphibasal
(phloem lies in the centre and remains completely
surrounded by xylem) and amphicribal (xylem lies in the
centre and remains completely surrounded by phloem).