(Structure, Types and Functions of Sclerenchymatous Cells in Plants)
What are Sclerenchymatous Cells?
Ø Sclerenchyma is a simple permanent tissue in plants.
Ø Sclerenchymatous cells are dead at their maturity.
Ø Cells do not have protoplast when they completely developed.
Ø They have thick secondary cell wall.
Ø The secondary cell wall is lignified and very hard.
Ø Most of the sclerenchymatous cells show intrusive growth.
Different Types of Sclerenchymatous Cells in Plants:
Ø Based on size, two types of sclerenchyma are described.
Ø Sclereids are short sclerenchymatous cells.
Ø They are also called as stone cells.
Ø They possess very thick and hard cell wall with very small (narrow) lumen.
Ø The shape of sclereids varies in different plant parts.
Ø The pattern of secondary wall thickening is usually uneven.
Ø Lignified secondary wall possesses many pits.
Ø Sclereids are abundantly distributed in the plant body.
Classification of sclereids:
Ø Five different types of sclereids are described in plants based on the shape of cells.
Ø Brachysclereids are shaped like parenchyma cells.
Ø Cells are more or less isodiametic and polygonal in cross section.
Ø Brachysclereids are commonly found in the fleshy edible parts of some fruits.
Ø Macrosclereids are elongated and columnar sclerenchymatous cells.
Ø They are very thick walled cells with narrow lumen.
Ø Macrosclereids usually occurs in epidermal cells of seed coats.
Ø Osteosclereids, as the name suggests, are bone shaped sclereids.
Ø They are elongated cells with bulged ends like the bones.
Ø The bulged bone line portion of osteosclereids may be branched in some plants.
Ø Osteosclereids are found in the seed coat of Pisum.
Ø Asterosclereids are star shaped sclereids as the name suggests.
Ø They have lobes or arms radiating from the central body.
Ø The number of arms varies in different species.
Ø Asterosclereids are frequently found in the petiole of Nymphaea.
Ø Trichosclereids are hair like elongated sclereids.
Ø Sometimes the trichosclereids are branched.
Ø They occur in the leaves of Olea and Musa (banana leaf).
Ø Fibres are the second category of sclerenchymatous cells in plants.
Ø Unlike sclereids, the fibres are much elongated cells with pointed ends.
Ø They are present in almost all plant parts.
Ø Their cell lumen is very narrow.
Ø Fibres possess very thick and hard lignified secondary cell wall.
Ø They are the main mechanical support in plants.
Ø Fibres are associated with the vascular bundles as bundle cap or sheath.
Ø Fibres also forms elements of xylem and phloem as xylem fibres and phloem fibres
Classification of fibres:
Ø Plant fibres have been classified into two broad categories.
(1). Xylary fibres
(2). Extraxylary fibres
(1). Xylary Fibres:
Ø Xylary fibres are the fibres located in the xylem of plants.
Ø They are also called as wood fibres.
Ø Different types of xylary fibres are there in plants, as follows:
(a). Fibre tracheids:
Ø They are transitional forms between tracheids and extreme fibres.
Ø Fibre tracheids are longer than sclereids but they are shorter than other true xylem fibres.
(b). Libriform fibres:
Ø Libriform fibres are highly specialized xylary fibres.
Ø They are long cells with narrow lumen.
Ø Libriform fibres possess numerous slit like pits on their secondary cell wall.
(c). Gelatinous fibres:
Ø Gelatinous fibres possess cellulosic secondary cell wall
Ø They lack the lignin content in the secondary cell wall.
Ø Due to the presence of cellulose, the secondary cell wall in gelatinous fibres appears shining and gelatinous in cross section.
Ø Gelatinous fibres are highly hydroscopic and they absorb and hold large amounts of water.
Ø Gelatinous fibres are found in the tension wood of some trees.
(2). Extraxylary fibres
Ø They are fibres located external to the xylem.
Ø Most common extraxylary fibres are phloem fibres.
Ø Phloem fibres are also called as bast fibres.
Ø Different types of extraxylary fibres occur in plants as follows:
(a). Phloic fibres
Ø Also called as phloem fibres
Ø They are located in the primary and secondary phloem of vascular tissue.
Ø They are best known as bast fibres.
(b). Cortical fibres:
Ø Cortical fibres occurs in the cortex of plants
Ø They occur either singly or in groups.
Ø Provide mechanical support in young plant parts.
(c). Perivascular fibres:
Ø Perivascular fibres are present in the pericycle of plants.
Ø They form the vascular bundle cap of dicots and bundle sheath of monocots.
Commercial classification and Economic importance of plant fibres:
Ø Plant fibres have immense economic importance.
Ø They form textile fibres, brush fibres, paper fibres etc.
Ø Commercially plant fibres have been classified into two groups based on their physical appearance and toughness.
(A). Hard Fibres
(B). Soft Fibres
(A). Hard fibres:
Ø Hard fibres are obtained from the monocot plants.
Ø They have very thick lignified cell wall.
Ø They are very hard and stiff fibres.
Ø Example: Cocos (coir), Agave, Musa
(B). Soft fibres:
Ø Soft fibres are obtained from dicot plants.
Ø All soft fibres are bast fibres (phloem fibres).
Ø Soft fibres are soft, narrow and flexible.
Ø They are usually shining.
Ø Example: Corchorus capsularis (jute)
Functions of Sclerenchymatous Cells
Ø They are the main mechanical tissue in plants.
Ø They provide mechanical support in both primary and secondary plant parts.
Ø Sclerenchymatous cells also take part in conductive system of plants.
Ø They act as the components of vascular tissue system (xylem and phloem)
Ø The vessels and tracheids of xylem are sclerenchymatous cells.
Ø Hypodermis of xerophytic plants will be sclerenchymatous to prevent water loss.
Ø Sclerenchymatous cells in the fruit wall help in its dehiscence and seed dispersal.
Ø Sclerenchyma of seed coats protects the seeds from desiccation.
- What are sclerenchymatous cells?
- How the sclerenchymatous tissue is classified in plants?
- What are sclereids?
- What are fibres?
- How sclereids are classified?
- What are brachysclereids?
- What are macrosclereids?
- What are osteosclereids?
- What are asterosclereids?
- What are trichosclereids?
- How fibres are classified?
- What are xylary fibres?
- What are extraxylary fibres?
- What are libriform fibres?
- What are gelatinous fibres?
- What are phloeic or phloem fibres?
- What are perivascular fibres?
- What are hard fibres?
- What are soft fibres?
- What are the functions of sclerenchyma?