Sclerenchyma
Sclerenchyma is a specialized tissue consisting of a group of cells in which secondary walls are often lignified. The term Sclerenchyma is derived from the Greek word ‘skleros’ means ‘hard’ and ‘enchyma’, an ‘infusion’. The term sclerenchyma was coined by Mettenius in 1805 and the cells are known as sclerenchymatous cells. Sclerenchyma cells may or may not retain their protoplast at maturity. On the basis of length, sclerenchyma cells are of two types; fibres and sclereids. Usually the fibres are long while sclereids are short sclerenchyma cells.
Fibres
Fibres are long, spindle shaped cells with thick secondary wall and occur as strands in plant. They exist as overlapping structure and impart strength to the fibre bundles. Rutting is the process for fibres extraction from plant body which separates the fibre bundles from associated non-fibrous cells. It is carried out in pond and this process is hastened and assisted by microorganisms.
Properties of fibres
The fibre cells have presence of high lignin content and absence of pectin and cellulose, their walls are not much hydrated or they have very less affinity for water. This property makes wall more elastic in nature than plastic. Fibres possess simple pits whereas border pits are relatively scarce.
Distribution
Fibres are distributed in plants as separate strands either in cortex and in phloem or as sheath or bundle caps associated with the vascular bundles or may be grouped or scattered in xylem and phloem. Monocot and dicot fibres show several characteristic patterns. In Poaceae, fibres form a system having the shapes of a ribbed hollow cylinder with the ribs connected to the epidermis. Vascular bundle have prominent sheaths of fibres and the peripheral bundles may be irregularly fused with each other or united by sclerefied parenchyma into a sclerenchymatous cylinder for e.g. Zea mays, Saccharaum, Andropogon, Sorghum.
Figure: Showing location of different types of fibres
Type of fibres
In angiosperms fibres are located in stem as outermost part of the primary phloem as an astomosing strands or tangential plates. On the basis of their position in plants they have been kept in two groups, xylary (intraxylary) and extraxylary fibres. The fibres present in xylem are called xylary or intraxylary fibres. Xylary fibres are also known as wood fibre and are of following types:
a) Libriform fibres
b) Fibre tracheids
c) Septate fibres
d) Mucilage fibres
These libriform and fibre tracheids are classified on the basis of type of pits present on their walls. Libriform fibre (Latin Liber means inner bark) is longer and possesses simple pits whereas fibre-tracheids are shorter with bordered pits.
Figure: Quercus maceration showing fibre-tracheids and libriform fibres
Fibre cells generally loses their protoplasm and become dead at maturity but in many woody plants fibre retain their protoplasm and act as storage cell for carbohydrate and convert them into sugars when plant requires. Otherwise, their prime function is to provide mechanical support. Septa or cross wall formation takes place in phloem or xylem fibre of dicot species that undergoes regular mitotic division after secondary wall is deposited which leads to partitioning of fibre into two or more compartment. Such fibres are known as septate fibre. These fibres not only occur in dicot but also in some monocots such as Palmae and Bambusoideae. These fibres are non-vascular in origin. The septa include a middle lamellae and two primary wall that may or may not be lignified and remain in contact. The septate fibres of bamboos are characterized by thick polylamellate secondary walls with additional secondary wall lamellae. Septa of the fibre containing protoplast are interconnected by plasmodesmata, thus indicating role of septa in support and in addition to it they perform the storage function because they contain starch grains and sometimes crystals of calcium oxalate.
Sometime fibre also contains gelatinous layer (G layer) which makes the innermost secondary wall layers, it contains high cellulose content but lignin is absent that distinguishes it from the outer secondary wall layers. Presence of cellulose makes the G layer hygroscopic and thus it swells up by absorbing large amount of water, it may occlude the lumen of the cell and upon drying it pulls away from the rest of the wall. They are also called as reaction fibre or mucilage fibre. These fibres are neither strictly xylary nor extarxylary as they have been found in xylem and phloem of roots, stem and leaves of dicots and in nonvascular tissue of monocot leaves. The role of G layers in leaves may be to assist in the maintaining the orientation of leaves with respects to gravity and display of the leaflet to the sun.
Extarxylary fibres are located outside the xylem and these are of three types:
a) Phloem fibres
b) Pericyclic or perivascular fibre
c) Cortical fibre
Phloem fibres
Phloem fibre also known as bast fibre which originates in early part of primary phloem but functions as fibres after their primary function i.e. conduction ceases and thus called as primary phloem fibre or protophloem fibre. Other examples of primary phloem fibre are stem of Sambucus(elderberry), Tillia (Basswood), Liriodendron (Tulip tree), Vitis (Grape vine), and Robinia pseudoacacia (Black locust). Flax fibres (Linum usitatissimum) exist as single band with several layers in depth and are located on the outer periphery of vascular bundle. When fibres are located within the secondary phloem called as secondary phloem fibres. Soft fibres are obtained from phloem fibre of eudicots and represent the bast fibre of commerce. They are soft, flexible and may or may not be lignified. The example of bast fibres are hemp (Cannabis sativa) used in cordage, jute (Linum usitatissimum) and remie (Boehmeria nivea) used in textile.
Pericyclic or Perivascular fibre
Perivascular fibres are extraxylary fibre found in stems of dicots, located in the periphery of vascular bundles inside the innermost cortical layer as in Aristolochia and Cucurbita. Extraxylary fibres also include the fibre of the monocot whether or not associated with the vascular bundles. They often have thick cell wall and variability seen in lignin deposition on cell walls.
Cortical fibres are extraxylary fibre found in stem and originate in cortex e.g. Barley. Cortical fibre gives mechanical strengths to the plant body.
The fibres obtained from monocots are basically obtained from leaves and are hard and stiff in nature, and thus they are called as hard or leaf fibre. In contrast to soft fibre, hard fibres are rich in lignin present on walls, for e.g. abaca or manila hemp (Musa textilis), bowstring hemp (Sansevieria sp.) and newzeland hemp (Phormium tenax). All of them are used in cordage making. Henequen and Sissal (Agave sp.) is used in cordage and coarse textile. Pineapple fibre (Ananas comosus) is used in textile. The plants like maize (Zea mays), sugarcane (Saccharum officinarum) and esparto grass (Stipa tenacissima) are used for making paper whereas leaf fibres in the xylem as raw material.
Figure: The broken ring of perivascular (extraxylary) fibres in Aristolochia.
Some cells in plants are not fibre but they act as fibre and thus are included in fibre such as cotton fibre which are obtained from the epidermal hairs of the seed of Gossypium. Some plant structure can also be used as fibre such as stem of rattan (Calamus palm) and Raffia is composed of leaf segments of raffia palm.
Sclereids
The term sclereid was coined by Tschierch in 1885. They are short sclerenchyma cells having thick and strongly lignified secondary walls with many simple pits. The secondary walls are multilayered. Some sclereids have thin secondary wall and contain living protoplast at maturity. Sclereid protects soft plant tissue from herbivores or mechanical damage.
Type of Sclereids:
Depending upon the basis of size and shape of sclereids, they have been classified into five main groups,
a) Brachysclereids or stone cells
b) Macrosclereids
c) Osteosclereids
d) Astrosclereids
e) Trichosclereids
Brachysclereids
Brachysclereids are also known as stone cells. These are isodiametric or elongated cells and are distributed widely in cells of cortex, phloem and pith of stem and are also found in the flesh of food. They are present in fruits of Prunus, quinace (Cydonia); elongated sclereids are present in the endocarp region of apple seeds and stone fruits (Drupe); exocarp region of fruits of Manilkara achras; pulp portion of Mimusops elangi andPsidum guajava, cortex region of Cinnamomum zeylanicum and exocarp region of Moringa olefera.
Macrosclereids
Macrosclereids are elongated and rod like sclerenchyma cells which form palisade like epidermal layer in seed coats of legumes. They are well developed in exocarp region of Malus sylvestris.
Osteosclereids
Osteosclereids are columnar in shape but their ends become enlarge in such a way that it appears bone like structure. They are well distributed in the sub-epidermal layer of seed coat of some plants e.g. adaxial leaf surface of Nymphaea nouchali, Phillyrea latifolia, Hakia and Osmenthuscontain osteosclereids.
Figure: A) Fruit of Prunus showing Brachysclereids B) magnified stone cell (sclereid) from the juicy mesocarp of a 'Bartlett' pear C) Maceration of bean seed coat showing macrosclereids D) W.M. of Osteosclereids E) T.S. leaf of Nymphaea showing astrosclereids F) Banana leaf clearing showing trichosclereids.
E) http://www.biologie.uni-hamburg.de/b-online/library/webb/BOT410/anatweb/images/ParColSclr/NymphLfXSLive2Scler.jpgF)http://botweb.uwsp.edu/anatomy/images/schlerenchyma/pages/Anat0040.htm
Astrosclereids
When the central body of cell develops arms or lobes like extension giving appearance of star are known as astrosclereids. They are found in leaf of eudicot, adaxial surface of leaf of N. cristata.
Trichosclereids
Tricosclereids are thin walled sclereids resembling hairs with branches. They are found on the adaxial surface of leaves of Olea europea and Banana leaf.
Besides this two additional types of sclereids are also reported in plants:
Filiform sclereids are long cylindrical cells similar to fibres and are found in palisade and spongy parenchyma of olive (Olea europaea) leaf.
Figure: Filiform sclereid in leaf of Olea europaea.
Fibre sclereids are the fibre that differentiates in phloem and have characteristic of both fibre and sclereids and thus named as fibre sclereids. They have been reported in secondary phloem of root and shoot of the rosette leaves of Arabidopsis thaliana. Sclereids are distributed in almost every organ of plant body ranging from epidermis, ground tissue to vascular tissue and occur singly or in cluster. When they occur as singly they are known as idioblast.
Dicot leaves are rich in variety of sclereids but are absent in monocots. Two pattern of distribution of sclereids are mainly observed in dicots; terminal pattern and diffuse pattern. In terminal pattern they are confined to ends of small veins as seen in Hakea, Mouriria, Boronia andArthrocnemum whereas, in diffuse pattern either they occur either solitary or in groups dispersed throughout the tissue without any spatial relationship to the vein endings. The examples of diffuse pattern are seen in Olea, Osmanthus, Pseudotsuga and Trochodendron.
Foliar structure as found in clove scale of garlic (Allium sativum) the sclereids forms part of the entire epidermis. The plant species having well developed intercellular spaces or air chambers possess trichosclereids such as Monstera deliciosa, Nymphaea (water lily) and Nymphaea (Yellow pond lily).
Seeds contain seed coat which is hard and this hardness is due to development of secondary wall in the epidermis and in the layers or layers beneath the epidermis e.g. seeds of bean (Phaseolus), pea (Pisum), soybean (Glycine max) contain columnar macrosclereids in epidermis and osteosclereides beneath the epidermis. The seed coat of coconut (Cocos nucifera) contains ramiform pitted sclereids.
Figure: Stone cells or sclereids
Functions
Sclereids provide mechanical strengths and plays important role in guiding light within mesophyll. They are responsible for gritty texture found in some fruits like pear.
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