Question

why can lenticelsbe seen only on the bark of older trees and do not on young green stem

Answer 1

Bark of a tree contains dead cells and stomata which cannot open when needed. Lenticels are present in the bark to facilitate the gaseous exchange and transpiration as lenticels present are the ones that never close. Hence, there is a need of lenticels in older trees more than the young plants where exchange of gases is possible through stomata.