Simple permanent tissue (plants)

March 18, 2019

Introduction

A permanent tissue is a group of cells that have lost the power of cell division and growth.
These cells have attained their definite form and size.
These cells may be dead or living, thin walled or thick walled.
They are formed by the differentiation of the meristematic cells.
The permanent tissue can be classified as follows:

A) Simple permanent tissue B) Complex permanent tissue

C) Special or secretory tissue

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A) Simple permanent tissue

It is a group of cells which are alike in origin, form and function.
It is made up of one type of cells forming a homogeneous or uniform mass.
The main types of simple permanent tissues are:

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I) Parenchyma

Parenchyma are the most common permanent tissues in plants.
It is an aggregate or collection of living and isodiametric cells which are oval, spherical or polygonal in shape.
Their walls are thin and made of cellulose or calcium pectate and they contain large vacuole.
The cells have intercellular spaces in between them.
These spaces may be small or very big and conspicuous.
The tissue is generally present in almost all the organs, e.g. in roots, stems, leaves, flowers and seeds.
It constitutes the ground tissue of all the parts of a plant and is generally found in epidermis, cortex, pith, mesophyll of leaves, and the pulps of fruits, endosperm of seeds and even in the complex tissues like xylem and phloem.

Types of parenchyma

Parenchyma tissues are further classified according to the shape of cells, their contents and size of intercellular spaces into following types:

i) Prosenchymatous tissue or prosenchyma

In some cases the parenchymatous cells become long and taper at either end, e.g. in the pericycle of some plants.
Such a parenchymatous tissue is called prosenchyma.

ii) Aerenchyma ( aeriferous parenchyma)

Some cells have wide air spaces between them and may be called aerenchyma.

iii) Chlorenchyma

In the leaves, the palisade cells are long and contain chloroplast in them.
They help plant to manufacture its food by photosynthesis and are essentially parenchymatous due to their contents chlorophyll.
For this they are called chorenchyma.
Due to their palisade like arrangement they may be called palisade parenchyma and due to their function, they may be called photosynthetic or assimilatory tissues.
Chlorenchyma of leaves is called mesophyll.

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Functions of parenchyma

The parenchyma performs following different functions.

Parenchymatous cells that contain chloroplasts are the main seats of photosynthesis, e.g. palisade cells of leaf.
It serves mainly for storage of food material in the forms of starches, proteins, oils and fats. The parenchymatous tissues in the root and stem tubers are good examples.
In aquatic plants, parenchyma cells have lot of spaces that store air to keep up the buoyancy which facilitate gaseous exchange.
In the fleshy stems and leaves, the parenchyma cells store water, e.g. Opuntia, Euphorbia sp., Aloe, etc.
Various secretory organs of plants are made up of parenchymatous cells that secrete various useful products like oils, nectar, resin, etc.
In many aquatic plants, parenchyma forms a honey comb like system that is used to withstand some mechanical stress.

II) Collenchyma

The cells of collenchyma are some-what elongated, living and often contain some chloroplasts.
The cells of this tissue possess localized thickening on the cell walls, particularly at the corner or intercellular spaces.
The thickening is due to the deposition of cellulose and pectin.
Their walls are provided with simple pits.
Collenchyma occurs in the stem of herbaceous dicotyledons (e.g. sunflower, gourd, etc.) in the form of two layers below epidermis and forms a layer, called the hypodermis.
It is absent in the roots and monocotyledons except in some special cases.
The presence of cellulose and pectic substances in the thickened walls endow tissue with capacity to expand and give a tensile strength to the organs.
Based on thickening and arrangement of cells, the collenchyma tissues are classified into three types. They are:

a) Angular collenchyma

The cell wall possesses the thickening at the corner or angle of the cells.
The cells are compact and show irregular arrangement without intercellular spaces.
It occurs in the stem of Datura, Lycopersicon, Soalnum, Nicotiana tabacum, etc.

b) Lacunar collenchyma

The thickening is present on the walls bordering the intercellular spaces or air spaces.
The cells contain large intercellular spaces.
They are found in the stem of Salvia, Malva, Althaea, Cucurbita, etc.

c) Plate or lamellar collenchyma

The thickening occurs at the tangential walls due to which cells appear like plates, bands or lamellae.
It is found in hypodermis of sunflower, Sambucus, Rheum, Eupatorium stem.
The cells contain no intercellular spaces.

Functions

It gives mechanical strength to the organs and due to its peripheral positions in the stem, resists the bending and pulling action of wind.
In some leaves the hypodermal tissue is located at the margin of lamina and saves it from tearing effect of wind.
It also manufactures sugar and starch by the process photosynthesis as it contains chloroplast.
In some plants, collenchyma cells become thin-walled and regain the power to divide.

III) Sclerenchyma

It forms the main strengthening tissue of the plant.
It consists of long, narrow, thick walled and lignified dead cells.
They are usually pointed at both ends and are fibre like in appearance and hence called sclerenchymatous fibres or simple fibres.
They have simple, often oblique pits in their walls.
The middle lamella is conspicuous in sclerenchyma. They are found abundantly in plants, and occur in patches or in definite layers.

Functions

They serve purely mechanical function, i.e., they give strength and rigidity to plant and thus enable it to withstand mechanical strains.

Types of sclerenchyma

These are of two types. They are: a) fibres and b) sclereids or stone cells.

a) Fibres

These are much elongated, narrow, thick walled structure usually with pointed end.
They are usually fibre like in appearance and hence called sclerenchymatous fibres or simply fibres.
At maturity, the cells lose protoplast and become dead and empty.
Cell walls are hard, uniformly thickened and lignified.
Usually, the secondary wall becomes highly thickened and the lumen or cell cavity is reduced a lot or altogether obliterated.
The pits of fibres are simple, small, round or slit-like.
These fibres lack intercellular spaces where there length is 1 to 3 mm.
However, in the fibre yielding plants their length range from 20mm to 550mm.
These fibres provide raw materials for textile and are used for manufacturing bags, sacks, twine, ropes, etc.
Fibres are abundantly found in various parts of both monocot and dicot plants.
They may occur in groups, in continuous layer as in the hypodermis of monocot stems and sometimes singly among other cells.
The sclerenchymatous fibres are of two types. They are: Bast fibres which are found in the pericycle and phloem. They have simple pits in their walls. Wood fibres or xylem fibres which are present in xylem having bordered pits.

b) Sclereids

These structures consist of highly thickened, dead sclerenchyma cells.
They are commonly called stone cells which are much broader and usually isodiametric.
They are commonly found in the cortex and pith of gymnosperms and dicotyledons.
They occur singly or in groups which are more common in fruits, seeds and barks of plants.
They provide strength and hardness to the stony fruits (e.g. almond, coconut) and seed coat (e.g. date).
The secondary walls are strongly lignified and vary in thickness, which commonly possess simple pits.
They are classified into five categories, on the basis of their size, shape, nature of cell wall and mode of deposition of secondary wall materials.
They are: i) brachysclereids ii) macrosclereids iii) osteosclereids iv) astrosclereids v) trichosclereids.

They provide firmness, stiffness, and mechanical strength to the part of plant where they are found.
They are present in seed coat and endocarp of fruits that produce a hard texture and protect the internal parts.

References:

i) https://www.toppr.com/guides/biology/tissues/permanent-tissue/

ii) https://www.aakash.ac.in/important-concepts/biology/simple-permanent-tissue

iii) https://www.pmfias.com/plant-tissue-meristematic-simple-complex-permanent-tissue/

iv) https://www.learninsta.com/permanent-tissues/#google_vignette