Fig. 2
Betula phellem from a macroscopic to a microscopic scale. a Location of outer bark (phellem) on a birch trunk with typical horizontal lenticels. b Phellem layer consisting of thin-walled and thick-walled cells. During the growth of the trunk the outer layers are stretched in tangential direction and the thin-walled cells are compressed in radial direction (arrows). c Shape of the two cell-types with main chemical components. Note that they lie perpendicular to the long axis of the tree