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Anatomically fairly old dicotyledonous root is distinguished from the dicotyledonous stem by
A
Position of protoxylem
B
Absence of secondary xylem
C
Absence of secondary phloem
D
Presence of cortex
Detailed Solution
An old dicot root and an old dicot stem both undergo secondary growth, so both have secondary xylem and secondary phloem; both also have a cortex. These features cannot distinguish them.
The primary xylem, however, remains recognisable even after secondary growth.
In the root the xylem is exarch: the protoxylem lies towards the periphery and the metaxylem towards the centre.
In the stem the xylem is endarch: the protoxylem lies towards the centre (pith) and the metaxylem towards the periphery.
Hence the old dicot root is distinguished from the stem by the position of protoxylem.
The primary xylem, however, remains recognisable even after secondary growth.
In the root the xylem is exarch: the protoxylem lies towards the periphery and the metaxylem towards the centre.
In the stem the xylem is endarch: the protoxylem lies towards the centre (pith) and the metaxylem towards the periphery.
Hence the old dicot root is distinguished from the stem by the position of protoxylem.