Question

why secondary is absent in monocot and present in Dicot ?

Answer 1

Secondary growth is initiated by the activity of the
vascular cambium as far as the steler region is
concerned.This intrafascicular cambium is absent in the open
vascular bundles of the monocot stem, thus the process
cannot take place. As for the roots, dicots as well as
monocots have closed vascular bundles. however, dicot roots
favour development of the cambium for secondary growth while
in monocots this is altogether absent.
Secondary growth in the extrasteler region is initiated by
the production of secondary vascular tissue. Since, as
explained above, monocots do not undergo secondary growth in
the steler region, there are no secondary tissues to
initiate such a growth in the extrasteler region.

Exceptions exists to this rule. Anomalous secondary growth
may occur in monocots; though this does not follow the
general route of secondary growth. In Bougainvillea, a
series of cambia arise outside the oldest phloem. Palmm
trees undergo diffused secondary growth in which increase in
the trees diameter is due to enlargement of the
parenchymatous cells.