Fig. 1

Cross-section of a 5 year old birch twig (LM, 10x), stained with an aqueous solution of 0.5% toluidine blue O (w/v). a shows the inner and the outer bark, the vascular cambium and the xylem. b Detail of the outer bark (LM, 40x) on the right. The phellogen or cork cambium is responsible for the production of the outer bark cells and of the phelloderm. It produces phelloderm towards the inside, a layer of living non-suberized parenchyma cells and phellem towards the outside, a protective tissue made of dead cells of the same size as the phellogen. In this sample only two layers of thin-walled cells are present (arrows)