Mechanical Tissue in Plants (With Diagram) | Botany

In this article we will discuss about the mechanical tissue in plants. This will also help you to draw the structure and diagram of mechanical tissues in plants.

The tissue that supports a plant and their growing organs against any deformation and provides mechanical strength is termed as mechanical tissue. Haberlandt (1914) called the mechanical tissue as stereome. Schwendener in 1874 termed the mechanical cells (e.g. collenchyma, bast fibres and libriform fibres) as stereids. Schwendener is of opinion that stereids collectively constitute the stereome or the mechanical tissue system of plants.

Plant organs are to withstand various strains like stretching due to presence of large fruits, bending due to natural calamities like high wind and passing animals and downpour, heavy snow etc.

The stems are compressed due to presence of large number of branches and leaves at their top; branches are to withstand bending as they lie either in oblique or horizontal position; the fruit stalks tend to be extended due to weight of fruits; the roots are also subjected to extension when the stem bends due to high wind.

In high wind the leaves lacerate. To withstand these strains the cell wall provides mechanical strength in all cells- un-thickened or thickened, lignified or non-lignified. The cell walls of parenchyma, collenchyma and sclerenchyma provide mechanical rigidity to the plant.

The large size and structural strength of a woody plant are achieved by the cell wall. In these plants the constituents of cell wall contribute 95% of the dry weight of the wood. The strength and rigidity of the entire plant are due to cell wall.

The non lignified wall derives strength from the cellulose microfibrils. It is believed that the mechanical strength of plant cell walls is due to the presence of skeletal framework formed by cellulosic microfibrils. Cellulose is the major component of paper, cotton etc. The cellulosic wall of wood is further strengthened by lignifications.

In some species where cellulose is absent, other polysaccharides assume the strengthening role and may form microfibril. In algae the microfibril are composed of either xylose or mannose.

Chitin is the strengthening materials of fungi where it forms microfibril. In some higher plants, the hemicellulosic xylans form long strands, which lie parallel to microfibrils. The lignin contributes strength in lignified walls. The strength is greatest in the direction parallel to the microfibrils in both types of cell walls.

There are several layers in a cell wall and the orientation of microfibrils is different in each layer. The construction of the wall can be compared to plywood where the orientation of grains is analogous to the direction of orientation of microfibrils in the cell walls. So, a cell wall is enough strong to resist forces from any direction.

In un-thickened cells, the wall gives limited support but the strength is greatly increased when the outward pressure of the turgid cytoplasm supplements it. The hydrophytes, some herbs and the growing seedlings gain the mechanical rigidity from turgid parenchyma cells.

The orientation of microfibrils in the wall is of great importance to withstand the major mechanical stresses imposed by environment. In the stem parenchyma, the microfibrils are oriented transversely on the vertical walls so that the cells bend without breaking. In roots the microfibrils have steeply-pitched and helical orientation to resist extension forces.

Maximum strength is obtained from collenchyma and sclerenchyma. The wall of collenchyma cells is thickened by pectin, hemicellulose, protein and cellulose. Lignin is completely absent. The thickening materials are deposited mainly at the corners or at the tangential walls. In growing cells also, deposition occurs and the microfibrils show transverse and longitudinal orientation at different alternate layers.

Collenchyma is living cell and retains its protoplast even when mature. So, it can regulate the deposition and orientation of wall materials according to the need of developing organs. The collenchyma cells, in addition to mechanical strength, also provide elasticity to the cell due to the presence of hydrated pectin on the wall.

Collenchyma (Figs. 13.1, 13.6) is one of the important mechanical cells of the growing organs and the mature organs of herbaceous plants. In stems it usually occurs just beneath the epidermis (e.g. Cucurbita). In Tilia stem it is separated from epidermis by parenchyma cells that may be one or two layered.

It occurs as a continuous cylinder in Sambucus, Helianthus etc. It is present as individual bundle in Cucurbita, Pastinaca etc. Collenchyma cells are very conspicuous below the ridges in the petioles and stems with projecting ribs (e.g. Chenopodium). Collenchyma may occur as bundle cap (e.g. Apium graveolens).

It may form a sheath around the entire vascular bundle of many plants (Esau, 1965). In leaves collenchyma occurs in the petiole and blade. In the latter it may be present in one or two sides of the vascular bundles. In the leaf of Sambucus it occurs below the phloem of large vascular bundles and forms a prominent cap under the phloem.

These caps are formed in large vascular bundles that protrude on the underside of the leaf as ribs. Collenchyma cells also occur along the margins of a leaf thus making it tough and resistant to tearing. The rinds of some fruit also gain mechanical rigidity from collenchyma, e.g. Vitis, Cassia etc.

The other important mechanical cell is sclerenchyma. It may be non-­conducting sclerenchyma and conducting sclerenchyma. The former includes sclereids and fibres.

Sclereids give strength, resistance and inflexible protection in the organs where they occur (Fig. 13.2). In stems they may occur just below the epidermis (e.g. Trichocereus chilensis), on the periphery of vascular region (e.g. Hoya carnosa) and in the pith of Hoya and Podocarpus. In Trichocereus chilensis the epidermis is thin walled. Below the epidermis there occur six or seven layers of sclereids. They are very thick walled.

Astrosclereids are present in the cortex of Trochodendron, Pseudotsuga etc. Sclereids are also present in the leaves of Trochodendron, Olea, Pseudotsuga, Camellia, Mouriria etc. In Mouriria the sclereids are branched or fibre like and are restricted to the ends of veinlet.

Foster (1947) called these as terminal sclereids. Sclereids are also present in fruits (Pyrus, Cydonia etc.) where they are present either singly or in clusters. The epidermis of seed coat of Phaseolus, Pisum, Glycine etc. contains sclereids.

Fibres (Fig. 13.3) add mechanical rigidity to the organ where they occur. The fibres may be extraxylary and xylary. The former fibres may occur uninterrupted just beneath the epidermis (e.g. Zea mays stem) or may be present in the ground tissue (e.g. Asparagus stem). They may be present in groups as isolated patches in the cortex (e.g. Pandanus root).

A group of fibres may be present above the vascular bundles as bundle cap (e.g. Xanthium stem) or they may surround each vascular bundle forming bundle sheath (e.g. Maize stem). They may occur above and below of each vascular bundle (e.g. Canna stem). Fibres may be present beneath the endodermis in the form of an uninterrupted wavy cylinder (e.g. Aristolochia and Tinospora stem).

Phloem fibres occur in the stem of Sambucus, Tilia, Robinia etc. These fibres may also occur in the secondary phloem (e.g. Sequoia, Thuja, Tilia etc.). Fibres are conspicuous in monocotyledonous leaves. They may form bundle sheath and connect the vascular bundles with the upper and lower epidermis, e.g. Datepalm leaf. In this leaf sub-epidermal strands of fibres are very common and they are not associated with the vascular bundles.

The xylary fibres and the conducting sclerenchyma, i.e. tracheary elements also add mechanical rigidity to the organ where they are present.

Some tracheary elements that are present in branches and stems show special adaptation to resist the force of gravity. This “reaction wood” differs in structure and location from ordinary wood. In conifers, this wood is located at the lower side of branches to resist compression while in dicots it is present on the upper side to resist tension. In conifer reaction wood, the innermost layer of the secondary wall is usually absent.