Question

In monocot stem,secondary growth doesnot occur because vascular bundles are :-

Answer 1

Combium is formed by growth of vascular combium in open bundles.
Due to closed bundles of monocot stem,no secondary growth takes place.

Answer 2

As in monocot stem vascular bundles r in ring arrangement. They have most of vascular bundles outside edge of stem . There is no pith region in monocot . So, this is a closed type vascular bundles which doesn't show secondary growth.
Hope so this will help u .