Taxonomic and Ecological Implications of Foliar Morphoanatomy in Cephalanthus (Naucleeae, Rubiaceae)

Abstract

Cephalanthus belongs to Rubiaceae tribe Naucleeae, and is characterized as shrubs to a small trees, mainly from the lowlands, with verticillate leaves. We hypothesized that the foliar morphoanatomy of Cephalanthus species provides characters informative to our taxonomic study, and that their leaves present adaptations to waterlogged habitats. To test these hypotheses our aims were to describe the macro- and micromorphology and anatomy of leaves in all Cephalanthus species. We describe and characterize leaf disposition, shape, size of blade and petiole, characteristics of the epidermis and indumenta, including the micromorphology of the cuticle and stomata, and the types of leaf domatia, if present. The anatomy of the petiole and leaf blade was also analyzed, including the mesophyll, vascular systems, ontogeny of the stomata, presence and typification of oxalate of calcium crystals, and their distribution in the foliar tissues. Uniformity in the morphoanatomical characters allows us to define the genus Cephalanthus, but inside this uniformity we can also differentiate between the species by means of a dichotomous key. The presence of domatia in pockets in C. occidentalis, in contrast to the tuft of hairs in C. salicifolius and C. tetrandrus, shed light on the taxonomic delimitation of the former because historically they were considered a single species. Cephalanthus species show variability in micromorphological characters, but have a strong uniformity in anatomical organization. The leaves of Cephalanthus show adaptations to seasonal environments; some are typical hydromorphic traits while other reflect adaptations to their heliophilic habitat and tolerance to dry periods.