Pollen development and orbicule and pollen grain morphology in species of Cephalanthus (Rubiaceae-Naucleeae) from the Americas

Abstract

This paper is the first embryological report on the genus Cephalanthus L. and contributes to future studies in other genera in 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 analyzed. The presence of a protruding oncus was observed for the first time in Cephalanthus in non-acetolyzed pollen grains. According to our observations, this structure is not affected by the state of anther dehiscence in Cephalanthus glabratus, because the oncus is visible and prominent in the mature pollen grains in both indehiscent and dehiscent anthers.