Question

A transitional form between tracheid and libriform fibre is termed as
A. Bast fibre
B. Septate fibre
C. Fibre-sclereid
D. Fibre - tracheid

Answer 1

Hint:-The fibre is a narrow, elongated cell with a thick lignified wall and a narrow lumen tapering at both ends to a wedge-shape. Fibre develops in combination with other tissues in the ground and vascular tissues of all plant organs, such as leaves, stems, roots, fruits, etc.

Complete Answer:-
Fibre appears to occur as bands or as idioblasts (e.g. Cycas leaflet) in an uninterrupted cylinder (e.g. Linum usitatissimum) or alone. Fibres are classified into two groups according to the tissues in which they are associated:
(i) Xylem or xylary fibre, and
(ii) Extraxylary fibre.
- Xylary fibre is associated with the xylem. The xylary fibres are classified into: Libriform fibre, and Fibre-tracheid based on pit and wall thickness.
- Libriform fibre: In comparison to tracheids of the plant in which they exist, these fibres are longer, have a thicker cell wall with a simple pit. There is a longer pit channel than that of tracheids. Due to wall thickening, the inner aperture of a pit pair looks slit-like and is typically shaped at right angles to the outside aperture.
- Fibre-tracheid: This fibre is longer, has a thicker cell wall with a bordered pit than tracheids, but has a thinner cell wall than the plant's libriform fibre in which they exist. The borders of the boundary pit are much narrower than those of the Tracheids. As a consequence, the pit chamber becomes smaller than tracheids a. Due to thickening, the inner aperture behaves as a slit and is normally shaped at right angles to the outer aperture. The fibre-tracheids are thus transitional between tracheids and libriform fibres in all respects.
Thus, the right option is D.

Note:- Xylem and phloem are most often associated with them. They are found as a bundle cap (ex. sunflower stem) in groups above the vascular bundle and as a bundle sheath (ex. maize stem) around the vascular bundle.