Lenticel

A lenticel is porous tissue in the bark of trees. Its cells make large intercellular spaces in the periderm of the bark. They are found in the woody stems and roots of dicotyledonous flowering plants. 
Lenticels are also found in other woody plants, starting in the Carboniferous period. The development and increase in these primitive lenticels gave a system for aeration and gas exchange in these plants.
Lenticels work as pores for the direct exchange of gases. Otherwise the bark is impermeable to gases. The name 'lenticel' comes from its lenticular (lens-like) shape. The shape of lenticels is one of the characteristics used for tree identification.