Structure and formation of phellem of Betula maximowicziana

Betula species have phellems with distinctive features such as stratification into thin paper-like layers, which are easily split in the tangential direction, and linear lenticels. We aimed to clarify the structure and development of the characteristic phellems of B. maximowicziana. In a normal periderm, phellem, phellogen, and phelloderm consist of tangentially elongated cells that are arranged in radial files. The phellem consists of layers of 1.4 ± 0.5 cells thick of very thin-walled phellem cells alternating with layers of 7.1 ± 1.5 thick-walled phellem cells. Seasonal sampling showed that the former and the latter were formed in the early and middle-to-late stages of the growing period, respectively. In lenticels, filling tissues alternated with closing layers. Most cells were collapsed and loosely packed in the filling tissue while all cells were intact and arranged in radial files in the closing layers. The filling tissue cells had unique walls that appeared to be easily deformed. Each annual increment of phellem in Betula is composed of a thin-walled cell layer (early phellem) and a thicker layer of thick-walled cells (late phellem). It is likely that the combination of filling tissue and closing layer in lenticels helps to perform the dual functions of gas exchange and protection, and that the collapse of the cells in filling tissue effectively contributes to gas permeability.