Question

Bordered pits occur in

• A. Secondary phloem
• B. Protoxylem
• C. Metaxylem
• D. Bark

Answer 1

The correct option is C Metaxylem

Tracheid is the fundamental single elongated dead cell of xylem. During cell wall thickening, deposition of lignin covers the whole primary cell wall of tracheid, except at gaps termed as 'bordered pit-pairs'. The pit pairs represent the small regions of the unlignified primary wall from two adjacent cells and are concentrated at end walls between cells, forming a pitted region called as pitted end walls. A pit pair shares a common membrane of the primary wall and middle lamella and the raised wall around the pit aperture serve as a border. The xylem which is formed first is present is termed as protoxylem, while metaxylem is the xylem that develops afterwards. Protoxylem is characterised by the presence of annual, spiral, scalariform vessels and no tracheids, while metaxylem has reticulated and pitted vessels and some tracheids.

So, the correct answer is option C.