Question

Which one is not correct about sclerenchyma?
A. Sclerenchyma fibers associated with xylem
     tissues are called wood fibers
B. Sclerenchyma fibers associated with phloem
   tissue are called bast fibers
C. Endocarp of coconut is rich in sclerenchyma
    fibers
D. Brachysclereids are isodiametric and polygonal

Answer 1

Hint: The sclerenchyma cells are generally found in the pericarp layer of the fruit. Sclerenchyma consists of two types of elements, namely sclereids and fibers. They give strength and support to the plant parts.

Complete answer:
The pericarp is the protective layer of fruit, which has 3 layers: epicarp, mesocarp, and endocarp. In coconut, epicarp forms the outermost layer, mesocarp forms the fleshy middle layer and endocarp is the hard, woody layer that surrounds the seed. Sclerenchyma acts as mechanical tissue, it can withstand stress and strain caused by the stretching, twisting, bending of plant organs. So it supports and protects the soft tissues of the plant. The two elements of sclerenchyma are sclereids and fibers. Fibers provide maximum rigidity and elasticity to the plant organs. Sclereids provide stiffness to the part they occur. They form hard protective covering around the soft tissues. They form the hard endocarp of drupe fruits like coconut, mango, peach, etc. So the endocarp of coconut is formed of sclereids and not by sclerenchyma fibers.
- Sclerenchyma is non- living, supporting, or mechanical tissue, commonly found in the cortex and vascular region of stem, leaves, fruit wall, etc.
- They are hard,thick- walled, elastic, dead, and devoid of protoplast at maturity.
- They are compactly arranged without intercellular space.
- It consists of two types of elements, namely sclereids and fibers.
- Sclerenchyma fibers are thick- walled, elongated, unbranched cells having a narrow lumen and pointed tips. Three kinds of fibers are there: surface fibers, xylary fibers or wood fibers, and extraxylary fibers or bast fibers.
- Wood fibers are seen associated with xylem, bast fibers are seen associated with cortex, pericycle, and phloem.
- Sclereids are short and variously shaped sclerenchymatous cells, with thickened and lignified walls and narrow lumen. They are abundant in soft tissues such as the cortex, pith, the pulp of fruits, fruit wall, etc.
- There are 5types of sclereids: brachysclereids, macrosclereids, osteosclereids, trichosclereids, and astrosclereids.
- Brachysclereids are isodiametric and polygonal and they are commonly known as stone cells
So, the correct answer is 'Endocarp of coconut is rich in sclerenchyma fibers’.

Note: Fibres are relatively long cells and sclereids are short cells. Fibers provide rigidity and elasticity to the part they occur and sclereids provide stiffness. The main function of fibers is to provide mechanical strength to plant parts. Surface fibers, present on the surface of seeds and fruits, help in the dispersal of seeds and fruits.