In this article we will discuss about:- 1. Distribution and Arrangement of Fibre 2. Shape, Structure and Content of Fibre 3. Types 4. Functions 5. Sources and Uses.
Fibre is narrow, elongated cell with thick lignified wall and narrow lumen tapering to a wedge-shape at both ends.
Distribution and Arrangement of Fibre:
Fibre occurs in the ground and vascular tissues of all plant organs like leaves, stems, roots, fruits etc. in association with other tissues. It occurs either as bands or an uninterrupted cylinder (e.g. Linum usitatissimum) or single as idioblasts (ex. Cycas leaflet). They are most commonly associated with xylem and phloem. They are found in groups above the vascular bundle as bundle cap (ex. sunflower stem) and around the vascular bundle as bundle sheath (ex. maize stem).
They are also located encircling the vascular cylinder—termed pericyclic fibre. They also occur between the vascular bundles (ex. leaf of Agave) extending between upper and lower epidermis. They are also associated with the parenchyma cells of pith and cortex. The fibres usually lie parallel to the long axis of organ in which they occur. They may be interlocked at the ends (Fig. 8.6).
Shape, Structure and Content of Fibre:
They are much-elongated cells with many times longer than the breadth, tapering to a wedge-shape at both ends. These cells have very thick secondary walls made up of lignin or cellulose (ex. Linum usitatissimum).
Sometimes the deposition of lignin is so thick as to obliterate the central lumen. The fibres appear more or less angular in cross sectional view with little intercellular spaces (Fig. 8.6). The cell wall is pitted and may show slight indentation (ex. Zingiber).
The pits may be cross-pit, i.e. pits of a pit pair have crossed apertures. Usually fibres are considered as dead cells and contain no living protoplast at maturity. But the works of Fahn and Leshem (1963) reveal that the wood fibre can retain living protoplast for several years. The fibres of Vitis, Zingiber etc. are septate and the transverse wall is very thin. Septate fibres contain starch, oils, calcium oxalate crystals etc.
Types of Fibre:
Fibres are classified into two groups according to the tissues in which they are associated:
(i) Xylem or xylary fibre, and
(ii) Extraxylary fibre.
(i) Xylary fibre:
This fibre is associated with the xylem as xylem fibre.
Origin of Xylary Fibre:
These fibres are present in the primary and secondary xylem and accordingly their origin differs. They are developed from the procambium when associated with primary xylem and originate from cambium when associated with secondary xylem. The fusiform initials of cambium give rise to fibres.
The xylary fibres are classified into:
(1) Libriform fibre, and
(2) Fibre-tracheid on the basis of pit and thickness of the wall.
1. Libriforn fibre (Fig. 9.1F):
These fibres are longer in length, have thicker cell wall with simple pit in contrast to tracheids of the plant in which they occur. The pit canal is longer than that of tracheids. The inner aperture of a pit-pair appears slit-like due to wall thickening and is usually formed at right angles to outer aperture.
2. Fibre-tracheid (Fig. 9.1E):
This fibre is longer in length, has thicker cell wall with bordered pit than tracheids but shorter in length, has thinner cell wall than libriform fibre of the plant in which they occur. The border of the bordered pit is much reduced than those of tracheids.
As a result the pit chamber becomes smaller than tracheids. The inner aperture appears as slit due to thickening and is usually formed at right angles to outer aperture. Thus the fibre-tracheids are intermediate in all respects between tracheids and libriform fibres.
Gelatinous or mucilaginous fibre:
These fibres are characterized by either absence or presence of lignin in little amount on the cell wall. Thus gelatinous or mucilaginous fibres differ from other xylary fibres in having less lignin and much a-cellulose on their cell wall, α-cellulose molecules are deposited on inner secondary wall of these fibres instead of lignin.
Gelatinous fibres are found in tension wood of dicotyledon. (Tension wood is one type of reaction wood that is formed in leaning branches or trunk of dicots. The tension wood, i.e. secondary xylem is formed on the upper side of the branch or trunk when they lean due to gravity. These woods usually show eccentric growth rings).
(ii) Extraxylary fibre:
These fibres occur in the cortex, pith and in association with phloem as phloem or bast fibre. They never associate with xylem elements. In monocotyledonous stems the extraxylary fibre may surround the individual bundles-termed bundle-sheath; it may form an uninterrupted cylinder on the ground tissue. These fibres originate partly from ground meristem and the rest from the procambium.
In some dicotyledons (e.g. Aristolochia, Cucurbita etc.) fibres occur as bands or uninterrupted cylinder on the peripheral side of vascular cylinder and at the innermost layers of cortex. These fibres originate from the ground meristem and termed as pericyclic fibres. The structure, content and shape of these fibres are more or less like xylary fibres.
Function of Fibre:
(i) Fibres are mechanical cells that support the organs in which they occur;
(ii) They protect the inner tissues due to the presence of thick wall;
(iii) Septate fibres are the storage tissue where starch and oils are present;
(iv) Fibres exhibit elastic properties and so can withstand tension.
Source and Use of Commercial Fibres:
Fibres are used in textile industry, as filling fibres and in the preparation of ropes, brushes, brooms etc. The textile fibres are soft fibres that may or may not contain lignin on the cell wall. They are obtained from the dicotyledonous plants like Gossxjpium sp. (cotton), Corchorus sp. (jute), Boehmeria nivea (ramie), Linum usitatissimum (flax), Cannabis sativa (hemp) etc.
Cordage fibres are hard fibres due to the presence of lignin on the cell wall. They are used in the preparation of ropes and binder twines and are obtained from the monocotyledonous plants like Musa textiles (abaca), Agave sisalana (sisal) etc. Agave fibres are used in the preparation of brushes and brooms. Cotton, jute etc. are used as filling fibres.