Answer
Stomata must be able to close since they are the point of exit for water from the leaf, which has lots of surface area allowing for rapid and large-scale water loss which could damage the plant without the ability to regulate it. The lenticel only connects to the round (low surface area) stem, much of which is not directly exposed to the sun and, overall, cannot lose as much water as the leaf.
Work Step by Step
Look at pictures of leaves and woody stems in the book, and keep in mind the role of surface area for transport of water out of the plant as well as the role of the sun in driving rapid water loss from the leaf but not the stem.
