Bark anatomy of chosenia (Salix arbutifolia, Salicaceae): origin of cuticular cracks and mechanism of cork abscission

The microscopic structure of the bark of chosenia (Salix arbutifolia) has been examined in detail. Its bark exhibits several anatomical traits characteristic of other Salix species, including the presence of phellem cells with thick lignified walls in the first periderm coupled with exclusively non-sclerified phellem cells in subsequent periderms. Detailed anatomical observations confirmed that these thick-walled cells belong to the periderm rather than to the multiple epidermis, as some authors have suggested. S. arbutifolia is similar to S. cardiophylla, its closest phylogenetic relative, in possessing an exceptionally thick cuticle on the epidermis of young twigs. This cuticle undergoes tangential expansion and thickening during the vertical contraction and horizontal stretching of epidermal cells. The openings observed on the cuticular surface of S. arbutifolia are expansion cracks (i.e., ruptures in the protective tissues subtended by newly formed periderm) rather than lenticels (i.e., transformed parts of the existing periderm). In the subsequent periderms of chosenia, the phellem is subdivided into an outer layer of thin-walled (phelloid) cells and an inner layer of densely packed cells with thicker walls. The outer phellem layers facilitate the separation and shedding of bark flakes, while the inner phellem layers protect the bark surface exposed after their abscission. Thus, the two-layered phellem enables the regular shedding of the outer regions of the rhytidome, contributing to the shaggy appearance of mature bark - a distinctive feature of S. arbutifolia compared to other Salix species.