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.

This paper is the first embryological report on the genus Cephalanthus L. and contributes to future studies in other genera of the tribe Naucleeae. The development of the anther wall in Cephalanthus glabratus (Spreng.) K. Schum. corresponds to the dicot type and microsporogenesis is simultaneous. The young wall of the anther has four layers; epidermis, endothecium, one middle layer and the secretory tapetum. The tissue of the septum has idioblasts with crystalline sand. In the wall of the mature anther, only the endothecium and remnants of epidermal cells were preserved. The occurrence and morphology of orbicules were observed in the mature anthers of all three species of Cephalanthus with scanning electron microscopy. The orbicules have taxonomic value at species level, since these structures allow us to distinguish between the species analysed. The presence of a protruding oncus was observed for the first time in Cephalanthus in non-acetolysed pollen grains. According to our observations, this structure is not affected by the state of anther dehiscence in C. glabratus because the oncus is visible and prominent in the mature pollen grains in both indehiscent and dehiscent anthers.

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Este trabajo estudia los domacios foliares en Rubiáceas argentinas, los cuales son estructuras morfogenéticas frecuentes en el envés de las hojas de plantas leñosas, que permiten establecer relaciones mutualistas con ácaros u hormigas. Se clasifican en tipos relativamente bien conservados comunes a numerosas familias: mechones de pelos, bolsillos, cavidades y criptas. Alrededor de 290 familias de “dicotiledóneas” tienen domacios, siendo Rubiaceae la mejor representada con todos los tipos conocidos y transicionales. La clasificación intrafamiliar según datos moleculares, divide a Rubiaceae en tres subfamilias: Rubioideae, Ixorideae y Cinchonoideae. En Argentina viven 41 géneros y 132 especies de Rubiáceas, de las cuales 34 son leñosas (27%). Como resultado se encontraron tres tipos de domacios: mechón de pelos, bolsillos y en cripta en 14 especies de Rubiáceas. Se menciona por primera vez domacios en una enredadera sufrútice de la familia, en el género Manettia. Los resultados obtenidos fueron contrastados con estudios moleculares recientes, lo que permitió confirmar la condición plesiomórfica de los mismos, ya que los diferentes tipos son comunes a las tres subfamilias y a casi todas las tribus de la familia. Basado en el presente muestreo, las tribus Anthospermeae, Rubieae, Paederieae y Pavetteae carecen completamente de domacios.