Developmental Morphology of the Controversial Ramulus Organ ofTristicha trifaria(Subfamily Tristichoideae, Podostemaceae): Implications for Evolution of a Unique Body Plan in Podostemaceae

Abstract

Premise of research. The Podostemaceae (riverweed family) show unique morphologies and are adapted to rheophytic habitats that experience torrential floods. The fact that the Podostemaceae are sister to the Hypericaceae family in molecular phylogenetic trees suggests that the unparalleled morphologies of the Podostemaceae could have been the result of saltational evolution from the normal body construction. However, their evolutionary course remains unresolved. To deduce the evolutionary course of the Podostemaceae, Tristichoideae is a key plant taxon because it belongs to the basalmost clade of the Podostemaceae and has a unique organ termed the “ramulus.” The ramulus is not so deviated compared with the aggregated leaves of Podostemoideae and was once interpreted to be the determinate shoot for Terniopsis (another genus of Tristichoideae).Methodology. A specimen of Tristicha trifaria belonging to the basalmost clade of the species and having been previously collected in Cameroon was observed. Additional...