Sclerenchyma: Structure, Classification and Functions

Sclerenchyma is a simple permanent tissue in plants. Sclerenchymatous cells are dead at their maturity. Cells do not have protoplast when they completely developed. They have thick secondary cell wall. The secondary cell wall is lignified and very hard. Most of the sclerenchymatous cells show intrusive growth. The present post discusses the characteristics, structurea, Function and Types of Sclerenchyma with suitable examples.

Types of Sclerenchyma

Ø  Based on size, two types of sclerenchyma are described.

(I).   Sclereids

(II).    Fibres

(I). Sclereids

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.

(1).      Brachysclereids

(2).      Macrosclereids

(3).      Osteosclereids

(4).      Asterosclereids

(5).      Trichosclereids

(1). Brachysclereids

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.

(2). Macrosclereids

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.

(3). Osteosclereids

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.

(4). Asterosclereids

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.

(5). Trichoscelereids

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).

(II). Fibres

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)

Function of Sclerenchyma

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.

Key Questions…

1. What are sclerenchymatous cells?
2. How the sclerenchymatous tissue is classified in plants?
3. What are sclereids?
4. What are fibres?
5. How sclereids are classified?
6. What are brachysclereids?
7. What are macrosclereids?
8. What are osteosclereids?
9. What are asterosclereids?
10. What are trichosclereids?
11. How fibres are classified?
12. What are xylary fibres?
13. What are extraxylary fibres?
14. What are libriform fibres?
15. What are gelatinous fibres?
16. What are phloeic or phloem fibres?
17. What are perivascular fibres?
18. What are hard fibres?
19. What are soft fibres?
20. What are the functions of sclerenchyma?

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