DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Bark of a paper birch, Betula papyrifera. The picture shows the trunk of a mature paper birch tree. The bark is thin and smooth; red colours are new bark, covered by remaining strips of old bark (white): most has peeled off from the outer surface. Short horizontal lines visible throughout are transverse lenticels. Lenticels are porous openings in the bark that function in gaseous exchange between the interior of the trunk and the air. Those in this picture are between 2-5cm long; in Betula, they elongate as the trunk grows in girth. B. papyrifera is a native of the Northern USA, and is a pioneer species, the first tree to colonise an area following fire or avalanche. The bark has a high oil content, which protects the tree, but paradoxically makes strips of bark useful as firelighters. Birch "paper" was also traditionally used to make containers and as a waterproof layer for roofing.
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