Question

Bordered pits occur in
(a) Secondary phloem
(b) Protoxylem
(c) Metaxylem
(d) Bark

Answer 1

Hint: Vascular tissue in plants are organized into discrete strands called vascular bundles, each of them contains xylem and phloem.
Bordered pits occur in Metaxylem in their lignified walls. The vascular system of stems consists of discrete vascular bundles while in the roots, it is organized within a single central vascular cylinder. The xylem is responsible for conducting water and minerals within the primary plant body and the phloem conducts the food.

Complete answer: Protoxylem is the first part of the primary xylem which later matures and these cells are usually smaller in size than the metaxylem. While metaxylem is the part of the primary xylem that differentiates after protoxylem and these cells are usually larger than the protoxylem.
The functioning of the xylem network depends to a large degree on bordered pits. They connect adjacent conduits and the finely porous pit membranes that prevent the movement of gas and pathogens between conduits. Thus it is said that the bordered pits act as safety valves in the hydraulic system of plants.
The center of the bordered pit is occupied by 'torus' which is made up of the primary cell wall material. The structure of pits varies greatly across species, with large differences evident in their porosity and thickness of pit membranes.

So, the correct answer is ‘Metaxylem’.

Note: Primary xylem consists of lignified tracheary elements (tracheids and vessel elements), which die at maturity (as they lose their protoplasts). The end walls of adjacent tracheids contain paired, small, rimmed, non-perforated pores; these are the bordered pits.
The maximum number of bordered pits is present in the gymnosperms followed by the angiosperms.