Three pieces of stem (20–100 mm long) were cut from (1) branch tips without a visible periderm layer;
(2) lower parts of the stem where periderm was starting to form;
(3) thicker stems with mature bark that had more or less thick periderm.
Transverse, radial, and tangential sections (20–40 μm thick) of
stems were cut with a freezingmicrotome (Ernst Leitz GMBH,Wetzlar,
Germany).