It was a lenticel, a characteristic preparation from the elder-tree.
One of the small areas on the surface of the stems and roots of woody plants that allow the interchange of gases between the metabolically active interior tissue and the surrounding air or pockets of air in the soil. Lenticels are portions of the periderm that have numerous pores or intercellular spaces. They appear as raised circular or elongated areas. The dark lines in birch bark and the tiny dots sometimes seen on skin of apples and pears are lenticels.