Question

In the following pairs, contain lignin as a common structural component?
A. Tracheid and collenchymas
B. Sclerenchyma and sieve tube
C. Sclerenchyma and tracheid
D. Parenchyma and endodermis

Answer 1

Hint: Lignin plays a role in supporting the tissues of vascular plants and few of the algae. These are particularly important in the formation of cell walls, especially in wood and bark, because they provide rigidity and do not easily break down. Lignins are a class of complex organic polymers that form essential structural materials for support to plants. It performs several biological functions such as, water transportation, mechanical support and resistance to various stresses.

Complete answer:
Sclerenchyma and tracheid contain lignin as a common structural component. The spaces in the cell wall between cellulose, hemicelluloses and pectin are filled by lignin. It plays a very essential role in providing support to tissues like xylem tracheids, vessel elements and sclereid cells. Lignin plays a very crucial role in conducting water in plant stems. Lignin is present in all vascular plants, except bryophytes. Also it is there in red algae too.

Sclerenchyma is a Greek word meaning hard tissue.
• They do not have protoplasts and are dead. During the course of maturation, the cells lose their protoplasm and become dead.
• Cells have highly thickened cell walls and they are elongated and narrow. Their walls consist of cellulose, hemicelluloses and lignin, here it functions in providing mechanical strength to the plant and its various parts. On maturation the cell walls become strong, rigid and impermeable to water due to the thick deposition of lignin.

Xylem being a complex tissue is composed of four different kinds of elements-
• Tracheids
• Vessels
• Xylem fibres
• Xylem parenchyma

Tracheids- These are elongated cells with tapering ends. Their walls are thick and lignified. These are the dead cells and are without protoplasm.

Hence, the correct answer is option (C).

Note: Xylem vessels have xylem members where each of them has a lignified cell wall and thus they are also devoid of protoplasm and are dead. Lignin is also found in middle lamella, as well as epidermal and hypodermal cell walls of some plants. Xylem fibres are also dead because of the deposition lignin. Fungi are the only organisms that can break down lignin.