Leaf Anatomical Characters of Rhabdadenia (rhabdadenieae, Apocynaceae), Their Taxonomic Implications, and Notes on the Presence of Articulated Laticifers in the Genus

Rhabdadenia (Apocynaceae) includes three species, Rhabdadenia biflora, Rhabdadenia madida, and Rhabdadenia ragonesei. These species are morphologically similar and phenotypically plastic in their vegetative organs and flower color, and despite recent taxonomic revisions of the genus, species identification and delimitation remains a challenge. In this study, we investigate leaf and stem anatomy of all Rhabdadenia species to establish unequivocal taxonomically diagnostic characters. We encountered distinct anatomical differences in (1) leaf characters, notably the stomata distribution, which is restricted to the main veins in R. ragonesei and spread across the entire lamina for the other two species; (2) the presence and form of papillae, which are dome type in R. biflora, cone type in R. madida, yet absent in R. ragonesei; (3) the number of layers of palisade parenchyma, which are two in R. biflora and one in both R. ragonesei and R. madida; (4) the shape of the vascular bundle in the midrib, which is concave-convex in R. ragonesei and convex-concave in R. madida and R. biflora; (5) the leaf blade margins, which are sharply arched downward in R. madida, slightly flexed in R. ragonesei, and straight in R. biflora. A dichotomous identification key is presented. This work enables unequivocal, rapid identification of Rhabdadenia species using leaf anatomy, even in the absence of reproductive parts. Our work also determined that Rhabdadenia is distinct within Apocynaceae by the presence of articulated anastomosed ramified laticifers, as opposed to the unarticulated laticifers that are typical of the family.