Question

Sclerenchyma fibres are absent in
A. Protoxylem
B. Protophloem
C. Metaxylem
D. Both A and B

Answer 1

Hint:The protoxylem and protophloem are the first formed vascular tissues from the procambium meristem. These are living cells and divide to form more tissues.

Complete answer:
Sclerenchyma is the tissue that hardens and stiffens the plant. The supporting tissue of plants is sclerenchyma. There are two types of sclerenchyma cells: sclereids and fiber cellular. Their cell walls are made up of lignin, cellulose and hemicellulose. The main support cells of plant tissues are sclerenchyma cells that have ceased elongation. The sclerenchyma fibres are the raw material for many textiles (e.g. flax, hemps, jute and ramies) and are of great economic significance. Mature sclerenchyma consists of dead cells with very thick cell walls that account for up to 90% of the entire volume of the cell. The thick, hard walls are necessary for strengthening the sclerenchyma cells and supporting elements in plant parts that have ceased elongation.
During the plant organ development, primary xylem is the first form of xylem. The protoxylem is first formed from the meristem procambium. The protoxylem matures fully during organ development, and later it disappears through mechanical compression powers, which emerge from organ growth. Initially the second wall of the protoxylem conducting cells, vessel elements and tracheids show ring thickenings which later become helicoidal. As protoxylem is produced in close proximity to apical meristems, it supplies the meristems with main water.
Protophloem and metaphloem are primary phloem. The first to appear is protophloem and late metaphloem. The procambium meristem consists of protophloem. Protophloem has sieve elements which are not well-developed in angiosperms, but it includes sieve cells which are poorly developed in gymnosperms and ferns.
Both protoxylem and protophloem are developed in the primary stage of plants that need living cells. Sclerenchyma fibers are dead cells. Therefore, the protoxylem and protophloem does not contain sclerenchyma fibers.

Thus, the correct answer is option D. i.e., Both A and B.

Note:Vascular tissues occur at various stages of growth. During primary plant development, xylem and phloem derive from the procambium meristem. Protoxylem and protophloem emerge first at both the embryonic and postembryonic stages, but they are gradually replaced by metaxylem and metaphloem, which are the mature vascular systems in organs that only display primary growth.