Question

Anatomically, the root of the fairly old dicotyledonous plant is distinguished from its stem by

• A. position of protoxylem
• B. absence of secondary xylem
• C. absence of secondary phloem
• D. presence of cortex

Answer 1

The correct option is A position of protoxylem

Procambium, which is a meristematic tissue, gives rise to the primary xylem and phloem.
In the case of primary xylem, there are two types of xylems that are formed - protoxylem and metaxylem. Protoxylem is the first formed primary xylem. Metaxylem is the primary xylem that is formed after the protoxylem.


The protoxylem and metaxylem are arranged in two different ways in stem and roots.
In stem, the protoxylem is placed towards the centre. This is known as an endarch condition.
In roots, the protoxylem is arranged towards the periphery. This is known as the exarch condition.
Hence, a fairly old dicotyledonous root is distinguished from a dicotyledonous stem by the position of protoxylem.