The Mechanical Tissue: Types, Distribution, and Function

No plant can flourish, or even maintain its existence for long unless it is provided with arrangements that ensure that the plant body as a whole is firmly knit together and that each of its organs is possessed of the requisite degree of mechanical strength. The plants which are made up of very numerous and varied parts are subject to all kinds of mechanical injury. An insufficiently strengthened organ is liable, to be broken across, to be torn pieces, to be bruised or crushed, and so forth. Every Plant must safeguard itself again all the possible forms of injury by which its different organs are threatened. The plant organs must therefore be constructed so as to withstand, in some cases a transverse to the welfare of all plants, from the most insignificant of Algae to the tallest or bulkiest of trees. The trunk of a tree has to support the weight of the massive crown, with its large branches and foliage, it must therefore be built like a pillar or column so as to be capable of resisting longitudinal compression. The branches also have to bear a heavy load.

In order to maintain the mechanical stability necessary to their welfare, large plants are forced to provide themselves with more reliable mechanical arrangements. This can be done only by applying the principle of division of labour, or in other words, by assigning the task of maintaining stability to special tissues. Such mechanical tissue must of course be more or less perfectly adapted to their special function.

The most important types of mechanical tissue have been discussed in the following:

Collenchyma

• 

  Collenchyma is a living tissue composed of more or less elongated cells with thick primary non lignified walls.

• From the standpoint of morphology, collenchyma is a permanent and simple tissue as it consists of one type of cell.
• The collenchyma is commonly interpreted as thick-walled kind of parenchyma structurally specialized as a supporting tissue.

Position in the Plant Body

• It occurs chiefly in the peripheral parts of the stems, petioles and leaf mid-ribs.
• It is very commonly found in the ridges and angles of the plant organs.
• It may also occur in the roots particularly exposed to the light.
• However, it is absent from the leaves and stems of majority of the monocotyledons.
• Usually, the collenchyma occurs just beneath the epidermis but sometimes one or more layers of parenchyma may develop in between the epidermis and tissue.

Structure

• The cells of collenchyma vary in length. The shortest cells are like parenchymatous cells whereas the longest ones are fibre like in appearance.
• The longer cells are tapering at their ends whereas the shorter ones are prismatic.
• In the transverse sections, both kinds of these cells appear polygonal in their structure.
• The strands of collenchymatous cells result by repeated longitudinal divisions.
• The cells are living with persistent protoplasts.
• The cell walls consist of cellulose and pectin and are thickened in a highly characteristic manner.
• The wall materials are usually found to be deposited in the angles where many cells join together (e.g., Ficus, Rumex, Polygonum, Boehmeria, Begonia etc.).
• The degree of wall thickenings to the angles is related to the amount of wall thickenings found on other wall parts. Muller had described the three forms of collenchyma as angular (Eckencollenchym), lamellar (Plattenocollenchym), and tubular or lacunate ( Luckencollenchym).
• The end walls of the collenchymatous cells are usually thin and the pointed ends appear thick due to the deposition of wall material.
• The simple, large or small pits with rounded or silt like apertures are found in collenchyma cells. Such pits are found both in thick and thin walls of collenchyma. In certain cases, the collenchyma walls become modified in older parts of plant.
• Collenchyma is a compact tissue having no intercellular spaces.
• Chloroplasts are found in numerous collenchyma.
• Tannin may also be present in these cells.

Function

• Collenchyma is a mechanical tissue which gives support to the growing organs of the plant body. The compactness and the thick walls make it a strong tissue.
• Collenchyma tissue possesses tensile strength.
• The plastic nature of collenchyma walls is important from standpoint that much of the elongation of the internodes occurs after walls of collenchyma have been thickened. The plastic nature of collenchyma changes with age.
• The old tissue is harder and more brittle than young. The parts of plant which have ceased to elongate possess hard collenchyma.
• The collenchyma strand is much stronger than the vascular tissue.

Sclerenchyma

The sclerenchyma tissue consists of thick-walled cells which are very often lignified. The principle function of this tissue is mechanical and therefore, this is known as one of the mechanical tissues.

As regards their classification the sclerenchyma cells are grouped into fibres and sclereids.

Fibres

• The long cells are called fibres.
• The pits are less conspicuous in the fibre walls.
• Fibre originates from the fibre meristematic cells.
• Fibres are the most important mechanical cells.
• They are recognized for their great tensile strength, flexibility and elasticity and because of these characteristics, they enable the plant organs to withstand various types of strains and tensions which result from the action of wind, gravity etc.

Occurrence of Fibres

• The fibres may occur in patches, in continuous bands in the cortical region and the phloem, as bundle sheaths or bundle caps with vascular bundles, and sometimes singly among other cells.
• They also occur in the xylem and phloem either in groups or scattered.
• In many hollow stems of Gramineae (e.g., Triticum), the fibres are found to be arranged in the form of a peripheral ribbed cylinder.
• In solid stems of Zea, Saccharum, Sorghum, etc., the vascular bundles possess prominent fibre sheaths. These fibre sheaths are more conspicuous in the bundles of peripheral region.
• The fibres are also common in the leaves of monocot plants. Here they form the bundle caps and bundle sheath.
• In many dicotyledonous stems, the fibres form the tangential plates in the outermost part of the primary phloem. In some plants, Nicotiana, Boehmeria, Magnolia, etc., the fibres develop in the secondary phloem.
• In many dicotyledonous plants such as Pelargonium, Aristolochia, Cucurbita, the complete cylinders of fibres are found.
• In Polygonum, the fibre strands are found on both inner and outer sides of vascular bundles.
• In Nicotiana, where the phloem is internal to the xylem, the fibres are associated to this phloem.
• The fibres are found to be arranged in characteristic patterns in the primary and secondary xylem of the angiosperm. The fibres are also found in the primary and secondary body of the roots of angiosperms.

Classfications of Fibres

From the point of view of their morphology, there are two types of fibres.

1. Xylem fibres: Found in the xylem have bordered pits.
2. Extraxylary fibres: Found in the cortical, pericyclic and phloic regions possess simple pits.

Sometimes the fibres are subdivided into two classes known as

1. Bast fibres
2. Wood fibres

Most of the workers, however, classified the fibres as follows-

Phloem Fibres: The fibres originating in primary or secondary phloem.

Cortical fibres: The fibres originate from the cortical region.

Pericycle fibres: The fibres originating in the pericycle.

Wood fibres: The fibres originating in wood.

Function

• The fibres of various plants have been used economically since ancient times.
• The flax is known to have been cultivated early as 3000 years B.C. in Europe and Egypt.
• Flax, Hemp, Ramie and Jute, where the commercial fibres develop in their phloem, the term fibre stands for a fibre strand.
• The commercial fibres are separated from the plants by means of a process known as retting. In this process, the plants are kept underwater for a considerable time for bacterial and fungal activity.

Sclereids

The sclereids occur in a wide range of positions in the plant body. They are very frequently found, either single or in groups in the cortical and pith regions of many dicotyledonous plants. They are also found in the xylem and Phloem. Some regions or the tissues of the plant body are almost exclusively composed of sclereids, such as the hard shells of many fruits and hard coats of many seeds. In many plants, the parenchyma cells found in between the primary phloem strands convert into the sclereids by development of lignified secondary walls, and thus they form a continuous sclerenchyma cylinder together with the fibres. In many plants, the sclereids develop in the leaves. The sclereids also develop in the epidermis of some protective scales, e.g., Allium sativum.

Classifications of Sclereids

According to the form and structure of the cell four main categories of sclereids have been proposed. They are-

1. Brachysclereids
2. Macrosclereids
3. Esteosclereids
4. Astrosclereids.

Structure

• The secondary walls of sclereids are thick and lignified.
• Sometimes the wall is suberized or cutinized.
• They possess small pits with round apertures.
• Commonly pits are simple.
• The secondary walls of sclereids become concentrically lamellated.
• The secondary walls of sclereids of some species contain crystals.
• The lumen of the cell is very much reduced and almost filled up with wall deposits.
• According to Eames and MacDaniels (1947), the sclereids are regarded as dead cells when they mature and sometimes they contain the shrivelled remains of protoplasm and inclusions of protoplast, e.g., tannin and mucilage.

Xylem

• The xylem is a complex tissue and consists of many types of living and non-living cells.
• The presence of tracheary elements, that conduct water is the characteristic of xylem.
• The xylem contains many specialized supporting elements known as fibres and because of its mechanical function, the tissue is included in mechanical tissue.
• The primary xylem originates from the procambium.
• The secondary xylem develops by the meristematic activity of the vascular cambium.
• Xylem consists of tracheid and vessel.

Tracheid

• The fundamental cell type in the composition of xylem is the tracheid.
• The mature tracheid is non-living and without a protoplast.
• The tracheids possess wall.
• In transverse section, the tracheids appear to be polygonal or sometimes rounded.
• The end of the tracheid of secondary wood is chicel-like.
• They possess pit on their common walls.
• The lumen of a tracheid is large and free of contents.
• The tracheids are primarily meant for water conduction and secondarily for mechanical support.
• The tracheids consist of long, empty thick-walled tubes running parallel to long axis of the organ of the plant body.
• In most plants, the tracheids possess bordered pits with characteristics of shape, torus and border.
• In the tracheids, the secondary thickenings are deposited as rings, continuous spirals as helices of a ladder, giving the wall a ladder-like appearance. Such thickenings are called annular, spiral, and scalariform respectively.
• The secondary wall of tracheids with reticulate thickenings appears like a net, and sometimes the meshes of net are elongated transversely. Such thickenings are known as reticulate and scalariform-reticulate respectively.
• Besides the secondary walls are interrupted by means of pits and thickening is known as pitted.

Function of Tracheid

• The main function of the tracheid is the conduction of water.
• As they possess thick and firm walls, they also aid in mechanical support.
• When the plant organs do not have fibres or other mechanical tissue, they play an important role in the support of plant organs.
• The tracheids are overlapped and interlocked and united into strands and cylinders. These characters make the tracheids more fit for mechanical support to plant organs.

Wood Fibres and Fibres Tracheids

• The fibres and fibres tracheids aid to the mechanical strength of the xylem and various organ of plant body.

Mechanical Strength of Wood

• The mechanical strength, physical properties and suitability for commercial uses of the wood are determined by the composition of xylem tissue and structures and arrangement of its component elements.
• In dicotyledonous wood, the length, and thickness of xylem elements, distribution of pits, libriform fibres, and fibre tracheids are mainly responsible for the strength of the wood.
• These cells have been proved to be more influential when found in dense masses.
• The wood fibres are very important from the viewpoint of mechanical support and strength to the various plant organs.
• In dicotyledonous woods, vessel elements are relatively weak, because their diameters are large and the walls are thin.
• The woods with aggregated vessels are less resistant to certain stresses than the woods with less and evenly distributed vessels.
• The late wood is generally stronger than the early wood.

Phloem

The phloem as a whole is not mechanical tissue. It is meant for the translocation of solutes. However, the phloem fibres are thick-walled and aid in mechanical support. These fibres are considered as mechanical tissue.

Phloem Fibres

• In many angiosperms, the fibres form the main part of both primary and secondary phloem.
• The phloem fibres possess simple pits with small rounded or linear apertures.
• The walls of these fibres are lignified.
• In their development, the long tapering ends of the fibres are being interlocked forming the strong fibre-strands.
• The tangential sheets or cylinders of the fibres are being formed to protect inner tissues.
• In certain plants, e.g., Dirca palustris, they form more important supporting tissue to give mechanical support to the stem than xylem cylinder.
• The fibres of the protophloem prominently aid in mechanical support, especially in the early stages of development of the stem.
• The fibres may be arranged in continuous, uniform or irregular bands. In other species, they may be arranged as scattered or isolated strands.
• In certain species, the fibres form the caps of primary phloem strands.
• The fibres may be lignified (e.g., Cannabis) or non-lignified and composed of cellulose as in Linum.
• In certain species, the secondary phloem fibres mature early and function as mechanical elements (e.g., Tilia).
• These fibres, other fibres and vascular bundles form commercial ‘bast’ and therefore, sometimes the phloem fibres are very strong and because of this strength, they are employed in the making of ropes, cords, cloth and mats.

Function of Phloem Fibres

• The function of fibres and sclereids of phloem is mechanical support.
• They did in support of plant organs and protect the thin-walled soft tissues.
• Flax fibres are remarkable for their great tensile strength.
• They are used in manufacture of linen cloth and thread.
• Hemp fibre is a bast fibre which develops in the pericyclic region.
• The fibres of hemp are lignified and therefore they lack elasticity and flexibility as formed in the flax fibres.

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References

• Plant Anatomy by B.P. Pandey.