Bark anatomy of Polylepis (Rosaceae): a loose stratified phellem instead of the lenticels?

Bark structure of Polylepis incana (Sanguisorbeae, Rosaceae) is described and compared with that in related genera Cliffortia and Leucosidea. Tribe Sanguisorbeae shows an extraordinary diversity of bark abscission patterns. The outermost bark portions are peeling off along the non-conducting secondary phloem (Leucosidea, Cliffortia ruscifolia), or along the periderm (C. strobilifera). The protective function is performed by phellem (Leucosidea), or by sclerified secondary phloem (Cliffortia). In Polylepis, the separation layers occur in phellem and non-sclerified phloem, while a prominent protective layer is absent: its function is performed by multiple uniseriate layers of suberized phellem cells. Such pattern of peeling bark has not been reported yet elsewhere. Lenticels lack in Polylepis, but its phellem is similar in its structure (probably, also in some functions) to stratified filling lenticular tissue. Smooth surface of Polylepis bark is maintained by permanent abscission of thin layers representing an exterme case of the peeling type of bark architecture.