Question

if lateral meristem helps to increase the girth and they are also present in small plants, then why their stems are very thin?

Answer 1

Increase in the girth of the plant by the lateral meristem is considered as secondary growth. This growth is absent in monocots due to the different arrangement of vascular bundles. The arrangement present does not allow an increase in thickness. Hence, some plants only grow in height but not in thickness. Small plants gain thickness as they grow old but if it is a monocot, then there will be no increase in girth at any point in their lifetime.