Morphological Characteristics
Bark: The bark of Polylepis consists of numerous layers of thin, dark red exfoliating sheets. In some cases, the layered bark can be more than an inch thick. A majority of the larger branches have similar shredding bark. It would seem that the bark serves as insulation from both the nightly frosts and the intense daytime irradiation. The thick bark of Polylepis also serves an important function as protection against fire and isolation against nocturnal frosts. It is thought to originally have been a protection against epiphytic mosses, whose thick masses may damage trees by adding weight to the branches and providing a suitable environment for fungi which attack the trees.
Branching Pattern and Leaf Arrangement: Polylepis trees tend to have twisted, crooked stems and branches with repeated sympodial branching. Contorted growth is often associated with windy, cold, or arid habitats. The leaves are generally congested along the branch tips often at the end of long, naked branch segments.
Stipule Sheath: Each leaf has a pair of stipules fused around the branch forming a sheath. The crowding of the leaves results in a pattern of stacked, inverted cones due to the overlapping of the stipule sheaths. On the top of the sheaths on either side of the petiole there are often projections, or spurs. The presence or absence of these spurs and their size are important taxonomic characteristics.
Leaves and Leaflets: All species of Polylepis have compound, imparipinnate leaves, but the number of pairs of leaflets varies within and between species. The arrangement of the leaflets and the position from the terminal leaflet of the largest pair of leaflets determine the shape of the leaf. The outline of the leaf is usually rhombic in species with one pair of leaflets. Depending on the position of the largest pair, the leaf can be trullate to obtrullate in taxa with more than one leaflet pair.
Leaf Anatomy: The leaves of all species are built on a dorsiventral arrangement of cells, with the epidermis and palisade layer on the adaxial surface and the spongy tissue on the abaxial surface.
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