PERMANENT TISSUES
The meristematic cells gradually differentiate and loose the
ability to divide. Such cells are termed as mature or
permanant cells. The permanent tissues actually composed
of cells in which the growth has stopped.
Permanent tissues can be divided into two types
I. Simple Permanent Tissues
The permanent tissues having all cells similar in structure
and function are called simple tissues. These are groups of
homogenous cells which perform the same function.
i. Parenchyma
It is a living, simple permanent tissue composed of
thin-walled cells. Parenchyma (Para—beside; enchein — to
pour) is also called primary tissue or ground tissue. It is
present in cortex, pith, palisade, mesophyll and some other
parts of flower. It is mostly produced by the ground tissue.
The parenchyma forms the major part of the plant organs.
The characteristic features of parenchyma are as follows
(a) The cells are thin-walled, less spherical and
polyhedral in shape, these are generally isodiametric.
(b) The cell wall composed of cellulose, hemicellulose
and pectin.
(c) Cells have a large central vacuole, peripheral
cytoplasm with a conspicuous big nucleus.
(d) The cells may be either closely packed or have small
intercellular spaces.
The parenchyma can be further classified as
Chlorenchyma specialised for photosynthesis.
Aerenchyma forms a connected air system throughout
the entire plant.
Storage parenchyma stores sugars, protein granules, oil
drops, etc.
Xylem parenchyma helps in the conduction of water.
Phloem parenchyma helps in the translocation of food.
Stellate parenchyma star-shaped parenchymatous
tissue with large air spaces.
Different functions performed by parenchyma are
(a) These help in storage of food, water and air.
(b) The vital activities like photosynthesis, respiration
and conduction are carried out inside parenchyma
tissue.
(c) It helps in wound healing, grafting, etc., and also
provides buoyancy in aquatic plants.
(d) Parenchyma cells associated with xylem and phloem
help in conduction of water and food materials.
(e) These cells can dedifferentiate, acquire the power of
division to form secondary meristem which produce
secondary tissues.
ii. Collenchyma
Collenchyma (Gr. Colla — glue; enchyma — an infusion) is a
simple, living mechanical tissue. Its cells composed of more
or less elongated cells with thick, primary non-lignified
walls. Intercellular spaces are found to be absent.
The characteristic features of collenchymatous tissues are
listed below
(a) It is present in layer below the epidermis in
dicotyledonous plants.
(b) It is found either as a homogeneous layer or in patches.
(c) Collenchyma consists of cells which are much
thickened at the corners due to a deposition o
cellulose, hemicellulose and pectin.
(d) The cells may be oval, spherical or polygonal and
often contain chloroplasts.
(e) These cells assimilate food, when they contain
chloroplasts.
Based on pectinisation of the cell wall, there are three types
of collenchyma
(a) Angular collenchyma
(b) Lamellar collenchyma
(c) Lacunar collenchyma
Different functions performed by collenchyma are
(a) It provides mechanical support to the growing parts
of the plant, such as young stem and petiole of a leaf.
(b) Collenchyma cells are capable of photosynthesis, as
they contain chloroplasts.
iii. Sclerenchyma
The sclerenchyma (Gr. Sclerous—hard; enchyma—an
infusion) consists of long, narrow cells with thick and
lignified cell walls having a few or numerous pits.
The characteristic features of sclerenchymatous cells are
(a) These are dead cells and do not perform any
metabolic function.
(b) They possess hard and extremely thick secondary
walls due to uniform deposition of lignin.
(c) Cells are long or short, narrow, thick-walled and
lignified.
(d) They show different types of lignin depositions
and also have pits.
The sclerenchymatous cells are of two types
(a) Fibres These are thick-walled, elongated and
pointed cells, generally occurring in groups, in
various parts of the plant.
(b) Sclereids These are short or irregular, spherical,
oval or cylindrical sclerenchymatous cells. The
walls are very thick, irregular and the lumen is very
narrow. The walls show simple pits. These are
commonly found in the fruit wall of nuts, pulp of
the fruits, like guava, pear and sapota, seed coats of
legumes and leaves of tea.
Different functions performed by sclerenchyma are
(a) It provides mechanical strength and support.
(b) Surface fibres help in dispersal of seeds.
2. Complex Permanent Tissues
Complex permanent tissues are a group of more than one
type of cells having common origin and working together
as a unit.
The main complex tissues in vascular plants are xylem
and phloem.
i. Xylem
Xylem (Gr. Xylos — wood) is a complex permanent tissue
which conducts water and mineral nutrients upwards
from the root to the all aerial parts of the plant.
The xylem tissues are composed of four components
(a) Tracheids
These are elongated, tubular and primitive cells with tapering
end walls. They are dead cells and do not contain protoplasts
The inner layers of the cell walls have thickenings which vary
in form. The end of the tracheids are tapering, blunt or
chisel-like. These are constituents of xylem of primitive
plants.
Note
These are found in pteridophytes and gymnosperm, tracheids may be
classified as annular or helical, spiral and scalariform or pitted
depending on type of cell well depositions.
Functions of Tracheids
The tracheids conduct water and dissolved mineral element
from roots to leaves. They also provide mechanical support
(b) Vessels
These are long, cylindrical, tube-like structures made up
many cells called vessel members, each with lignified an
a large central cavity.
The vessel cells are also devoid of protoplasm. The vessel
members are interconnected through perforations in their
common walls. The presence of vessels is a characteristic feature
of angiosperms. Gymnosperms lack vessels in their xylem.
(c) Xylem Fibres
The sclerenchymatous fibres associated with the xylem
called xylem fibres. These fibres have lignified cell walls. Th:
thickness of the walls varies considerably, but these are usually
thicker than the walls of the tracheids in the same wood.
These are found in both primary and secondary xylem. The
xylem fibres provide mechanical strength.
(d) Xylem Parenchyma
These cells form the only living component of the
Xylem parenchyma cells are thin-walled and their cell walls
are made up of cellulose. Xylem parenchyma stores in
the form of starch or fat and other substances like tannins.
These cells assist directly or indirectly in the conduction of
upward through the vessels and tracheids.
water upward though the vessels and tracheids.
The xylem parenchyma can be subdivided into two types
Primary Xylem The xylem which appears first in the plant
is the primary xylem. The primary source of this xylem is
the procambium. The primary xylem is of two types, i.e.,
protoxylem and metaxylem.
The first formed primary xylem elements are called
protoxylem. The latter formed primary xylem is called

metaxylem.
In stems, the protoxylem lies towards the centre (pith) and
the metaxylem lies towards the periphery of the organ. This
type of primary xylem is called endarch.
In roots, the protoxylem lies towards periphery and
metaxylem lies towards the centre. Such arrangement of
primary xylem is called exarch.
Secondary Xylem is composed of tracheary elements. rays,
fibres and interspersed axial parenchyma cells. The cell formed
toward inside of cambium tissue are called secondary sydern or
wod The primary function of secondary xylem is to provide
mechanical support to plants.
ii. Phloem
Phlcxrn (Gk. Ph/ois—bark) is a food conducting complex
permanent tissue. The term ‘phloem’ was coined by Nageli
(1958). In angiosperms, it is also called bast. In gymnosperms,
albuminous cells and sieve cells are present.
Phloem consists of four types of cellular components,
(a) Sieve Elements
e sieve tube elements are long, tube-like structures
arranged longitudinally and are associated with the
companion cells. Their end walls are perforated in a
sieve-like manner to form the sieve plates. A mature sieve
element possesses a peripheral cytoplasm and a large
Vacuole, but lacks a nucleus, Golgi body and most cytosol.
Sieve elements are of following two types
• Sieve cell It is a special kind of cell which posseses
sieve areas in its lateral walls. There is no specialised
plate in it. Sieve cells are usually found in
pteridophytes and gymnosperms.
Sieve tube members In this type, the sieve areas are
localised on its end walls. Sieve tube members are
placed one above the other forming a tube called sieve
tube. The end walls are perforated (sieve pores) like a
sieve. These are found in angiosperms.
The uniqueness of the sieve tube is that even without
nucleus, it is living and the nucleus of the companion cell
controls its cellular functions.
Functions of Sieve Elements
The main function of sieve element is trans- location of
organic solutes. The callose (a plant polysaccharide) is
present in the perforations in the sieve plates.
It is soluble and disappears when the solute is dilute so that
the solute can pass from one cell to another cell through
the pores. Callose reappears and sometimes closes the
pores when solute is less dilute, thus stopping the
movement.
(b) Companion Cells
These are specialised parenchymatous cells, which are
closely associated with the sieve tube elements. Usually, a
single companion cell is found associated with a sieve tube
member.
The cytoplasm of the sieve tube element and companion
cells are connected by thin cytoplasmic strands called
plasmodesmata, passing through the pit membranes in
their walls. Companion cells are absent in the phloem of
pteridophytes and gymnosperms. They have albuminous
cells in abundance which contain stored food materials.
Functions of Companion Cells
The companion cells in association with phloem
parenchyma play an important role in the maintenance of
a pressure gradient in sieve tubes during food transport.
They form a link between sieve tube cells and other cells
and regulate the passage of materials.
(c) Phloem Parenchyma
The phloem parenchyma is made up of elongated,
tapering cylindrical cells which have dense cytoplasm and
nucleus. The cell wall is composed of cellulose and has pits
through the plasmodesmatal connections, which exist
between the cells. The phloem parenchyma is absent in
most of the monocotyledons.
Functions of phloem parenchyma
They store food materials and other substances like resins,
latex and mucilage.
(d) Phloem Fibres
The phloem fibres (bast fibres) are made up of
sclerenchymatous cells. These are generally absent in the
primary phloem, but are found in secondary phloem. The
cell wall of phlcrm fibres is very thick. are much
elongated, unbranched, have apices. At maturity, these
lose their protoplasm and become dead. The phlCRtn
fibres of jute. flax and hemp have important economic uses.
The first formed primary phloem consists of narrow sieve
tubes and is referred to as protophloem and later formed
phloem has bigger sieve tubes and is referred to as
metaphloem.