Revealing leaf anatomical characters that impact the taxonomy of Calea sect. Calea (Asteraceae)

This is the first report of a comparative leaf anatomy study of Calea, a genus of Asteraceae with a markedly complicated taxonomic history. The purely structural analyses allowed us to distinguish anatomical patterns that have not yet been explored in the literature of the group. The different types of trichomes, both tector and glandular, are informative in the taxonomy of the genus. The other secretory structures and the different conformational patterns of the vascular system in the midrib and petiole deserve to be highlighted for contributing to the distinction of Calea species, especially C. serrata from C. pinnatifida, two morphologically similar species. The similarity analysis reinforces the structural similarity between the species of Calea sect., distinguishing them from the outgroup, whose leaves are distinct due to the absence of laticifers and isobilateral mesophyll. Histochemical studies are needed to investigate the metabolites secreted by the cavities, laticifers, nectaries and secretory trichomes. Many of these structures have chemical content that has not yet been explored in the genus. We present an identification key and a binary/multistate matrix that can support future proposals for infrageneric classification in Calea.