small opening in the bark that permits the exchange of gases.

Merriam-Webster Online Dictionary
lenticel (noun)
a loose aggregation of cells which penetrates the surface (as of a stem) of a woody plant and through which gases are exchanged between the atmosphere and the underlying tissues
lenticel (Wikipedia)
The dark horizontal lines on Silver birch bark are the lenticels.

A lenticel is a porous tissue consisting of cells with large intercellular spaces in the periderm of the secondarily thickened organs and the bark of woody stems and roots of dicotyledonous flowering plants. It functions as a pore, providing a pathway for the direct exchange of gases between the internal tissues and atmosphere through the bark, which is otherwise impermeable to gases. The name lenticel, pronounced with an [s], derives from its lenticular (lens-like) shape. The shape of lenticels is one of the characteristics used for tree identification.

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