Simple permanent tissues are groups of homogenous cells which perform the same function.
These are categorised into three types namely: 1. Parenchyma 2. Collenchyma 3. Sclerenchyma.
1. Parenchyma:
(Gr., para = beside; enchein = to pour) Parenchyma is a living simple permanent tissue with thin walled cells. Parenchyma is literally universal in distribution in the plant body and is a part of “ground tissue”. Epidermis, cortex, pith etc. are made up of different types of parenchyma. It is also found in mesophyll of leaves, rind of fruits and endosperm of seeds. It is also associated with xylem and phloem.
Structure:
i. Parenchyma cells are more or less isodiametric, with intercellular spaces.
ii. Cell wall is thin and is made up of cellulose.
iii. Protoplasm has functional cell organelles and reserve food materials. Cells store substances like food, secretory and excretory products.
iv. Cells have a large central vacuole, peripheral cytoplasm with a nucleus.
Types of Parenchyma:
Parenchyma can be classified into the following groups:
a. Chlorenchyma:
This category of parenchyma is found to contain chloroplasts and is specialised for photosynthesis. It is found in the mesophyll of leaves which has two types of chlorenchyma – palisade parenchyma and spongy parenchyma. Cells in palisade parenchyma are elongated and placed parallel to each other without leaving intercellular spaces. Cells of spongy parenchyma have intercellular spaces. Chlorenchyma carries out photosynthesis.
b. Aerenchyma:
Here the intercellular spaces have a lot of air. The cells of this tissue are usually oval. Aerenchyma is present in hydrophytes and helps in buoyancy and respiration. E.g. – Eichhornia, Hydrilla.
c. Storage Parenchyma:
Here the cells store sugars, protein granules, oil drops etc. E.g. – cells in the cotyledons, endosperm of seeds, pulp of fruits etc.
d. Xylem Parenchyma:
Parenchyma cells associated with the xylem help in the conduction of water. Such cells are called xylem parenchyma.
e. Phloem Parenchyma:
Parenchyma cells associated with the phloem help in the translocation of food. Such cells are called phloem parenchyma.
f. Stellate Parenchyma:
Star shaped parenchymatous cells with large are spaces are called stellate parenchyma. E.g. – Petiole of Carina and Banana.
Functions of Parenchyma:
i. Parenchyma helps in storage of food, water and air.
ii. It carries on vital activities like respiration, photosynthesis and conduction.
iii. It is capable of meristematic activity and can give rise to secondary meristems.
iv. Aerenchyma gives buoyancy to aquatic plants.
v. Parenchyma cells associated with xylem and phloem help in conduction of water and food materials respectively.
vi. Parenchyma cells can dedifferentiate; acquire the power of division to form secondary meristem which produces secondary tissues.
2. Collenchyma (Gr., Kolla = glue):
Collenchyma is a simple living mechanical tissue. It is composed of more or less elongated cells with thick primary non-lignified walls. It discharges the functions of living tissues and also performs the functions of dead tissues like providing mechanical support.
Collenchyma is highly restricted in distribution and is present only in the aerial parts of the plant body. Collenchyma usually forms a compact patch of cortical layer called hypodermis in dicot stems. It is found in the stalks of leaves, fruits and flowers. It is seen along the midrib regions of the dicot leaves. Collenchyma is absent in monocots and roots with few exceptions.
Structure:
i. Collenchyma cells are elongated and their ends are blunt.
ii. Cell wall is made up of cellulose with localized thickenings of pectin and hemicellulose, lignin is absent. The cell walls are unevenly thick.
iii. Cells may or may not contain intercellular spaces.
iv. Protoplasm includes a very large central vacuole surrounded by a thin layer of cytoplasm. The cells may contain chloroplasts.
v. Nucleus is prominent. Cell organelles are found in a functional state.
vi. Primary pits are present in those collenchyma cells where the walls are almost uniform in thickness.
Types of Collenchyma:
There are three types of collenchyma based on pectinisation of the cell wall:
i. Angular Collenchyma:
The corners of the cells are thick due to the accumulation of pectin, whereas the sides are thin.
E.g. – Hypodermis of stems of Datura and Solarium tuberosum (potato).
ii. Lamellar or Plate Collenchyma:
The cell wall thickenings are found in the form of plate like depositions. These depositions are present on the inner and outer tangential walls.
E.g. – hypoderms of sunflower stem.
iii. Lacunar Collenchyma:
The deposition of pectin is restricted to the walls of the regions bordering the intercellular spaces. The thickening leaves a small inter-cellular space between the cells. E.g. – hypodermis of Cucurbita stems.
Functions:
i. Collenchyma provides tensile support to the young stems and leaves during development. It provides elasticity to the plant organs, therefore it prevents tearing action of wind on dicot leaves.
ii. Due to the presence of chloroplasts it can conduct photosynthesis.
iii. It can also perform other vital functions such as respiration and storage.
iv. Collenchyma cells have active protoplasts capable of removing the extra wall thickenings during dedifferentiation. This happens during the formation of cork cambium in dicot stems and in response to wound healing.
3. Sclerenchyma (Gr., scleros – hard; enchyma = an infusion):
Sclerenchyma is a simple permanent mechanical tissue. These cells are compactly arranged without intercellular spaces. These cells become dead on maturity i.e., they do not possess living protoplast. After primary growth sclerenchyma cells show the deposition of lignin in their secondary walls.
Sclerenchyma cells are found in all parts of a plant body. These cells form a distinct structure called hypodermis in monocot stem.
Sclerenchyma cells are grouped into two types:
(a) Fibres.
(b) Sclereids.
(a) Fibres:
These are elongated sclerenchyma cells usually with pointed ends. Fibres have thick lignified walls and are found in groups. As their walls are very thick, the lumen or cell cavity is very much reduced. They do not possess living protoplasts. These walls have depressions or cavities termed as pits. Pit is a region in cell wall where secondary wall material is not deposited. The fibres are compactly arranged without intercellular spaces.
Classification:
Fibres are classified into two types:
(i) Xylary fibres.
(ii) Extra-xylary fibres.
(i) Xylary Fibres:
These are also called wood fibres and are present in the secondary xylem. They develop from the same meristematic tissue from which other components of the xylem differentiate. Hence, these fibres form an integral part of the xylem.
(ii) Extra-Xylary Fibres:
These are found outside the xylem such as the cortex, pericycle and phloem. These fibres vary in length and are also referred as bast fibres.
Functions:
I. Fibres provide strength and rigidity to the various organs of plants and enable them to withstand various strains caused by external agencies without any damage to the inner thin-walled cells. Hence, they are purely mechanical in function.
II. Sclerenchyma fibres, in the form of sheaths provide protection to internal parts of plant organs.
Economic Importance of Fibres:
Fibres of various plants are commercially important:
I. They are used in manufacture of cords, ropes, mats and textiles. E.g. – Cannabis sativa (hemp) and Linum (flax) and Cocos nucifera (coconut).
II. Cotton fibres are peculiar because they do not have lignin. They are made up of only cellulose. Cotton fibres are useful in dispersal of seeds and fruits. They are chiefly used in textile industry.
III. Leaf fibres of monocotyledons serve as raw material for making paper.
(b) Sclereids:
Sclereids are widely distributed in the plant body. These are thick walled, strongly lignified hard cells of sclerenchyma. They show a great variation in their shape and size. They are commonly found in the cortex and pith of gymnosperms and dicots, arranged singly or in groups. Sclereids are also common in fruits and seeds.
The secondary walls of the sclereids are lignified and vary in thickness. Lignification occurs in a lamellar fashion on the primary wall and the secondary wall shows long tubular pits.
Classification:
The important types of sclereids are:
a. Brachysclereids (Stone Cells):
These are more or less isodiametric sclereids. They are usually found in bark, pith, phloem, cortex, hard endocarp of coconut and fleshy portions of many fruits like apple, pear, guava, etc.
b. Macrosclereids (Rod Cells):
These are rod-like cells forming the palisade like epidermal layer of leguminous seed coats. They are also present in the bark of dicot stems and in protective scales of onion, garlic, etc.
c. Osteosclereids (Bone Cells):
These are bone shaped sclereids i.e., columnar cells with enlarged ends. They are commonly found in the hypodermal layers of many seeds and fruits and also in xerophytic leaves. E.g. – Pea.
d. Astrosclereids (Star Cells):
These are star shaped sclereids and are commonly found in the leaves of dicot plants. E.g. – Thea chinensis (tea). They are branched in an irregular way, with radiating arms of varying length.
Functions:
The important functions of sclereids are as follows:
I. They provide hardness and mechanical strength to the plant organ where they are present.
II. They provide toughness to the otherwise soft pulp of fruits and provide efficient protection to the seeds in the form of shells.
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