Rachis of the genus Paspalum L. (Poaceae: Panicoideae: Paniceae): Anatomy and taxonomic significance of the primary branches of the inflorescences

Infrageneric classification of Paspalum is mainly based on characters of the inflorescence and spikelets. Some of the inflorescence characters usually used are the width and external morphology of the rachises of the racemes. Nevertheless, morphological terminology used in the literature is frequently imprecise. We examine the internal structure of the rachises of the racemes in Paspalum to: (a) compare it with the anatomy of a vegetative culm, (b) precisely describe its variation within the genus to search for features that could be used in taxonomy or phylogeny. Species representative of all subgenera and informal groups were studied. Serial transversal sections were made and analyzed using light microscopy.Many distinctive characters from the central keel and lateral wings allow to distinguish six anatomical types of rachises: Type I is the most frequent in the genus, being present in species of all subgenera; Type II characterizes Conjugata, Dissecta and Disticha groups; Type III is exclusive of subgenus Harpostachys; Type IV is common in Ceresia subgenus and it is also present in two species of Gardneriana group and in Paspalum guttatum of Erinatha group; Type V is present in Bonplandiana and Racemosa groups, and Type VI is exclusive of Paspalum falcatum of Falcata group. Rachises of the racemes are undoubtedly caulinar structures although many species present flat rachises, with vascular bundles arranged in one only row and with abundant photosynthetic tissues. This study permits to differentiate, by means of anatomical characters, rachises that are morphologically similar or undistinguishable. Some anatomical types of rachises are related to taxonomic groups or subgenera, stressing their taxonomic value. Using anatomical characters of the rachis of the Paspalum raceme can improve phylogenetic analyses.