They are called lenticels.
"Outlines of Lessons in Botany, Part I; From Seed to Leaf" by Jane H. Newell
These lenticels are characteristic of all birch and cherry trees.
"Studies of Trees" by Jacob Joshua Levison
Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves.
"Disease in Plants" by H. Marshall Ward