DEVELOPMENTAL MORPHOLOGY OF THE CONTROVERSIAL RAMULUS ORGAN OF TRISTICHA TRIFARIA (SUBFAMILY TRISTICHOIDEAE, PODOSTEMACEAE): IMPLICATIONS FOR EVOLUTION OF A UNIQUE BODY PLAN IN PODOSTEMACEAE

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 Afro-Madagascan specimens with apparently different branching patterns were also observed. Observation was conducted under both LM-using a resin sectioning method-and SEM. Pivotal results. The ramulus first develops endogenously in the root tissue. It has an apical meristem and forms scaly leaves similar to a shoot apical meristem and then undergoes repeated sympodial branching. New branches arise from extra-axillary buds of the basal scaly leaves from the previous branch and replaces the latter. In Tristicha, two new branches usually arise per existing branch, resulting in a more complicated gross morphology than Terniopsis, which has only one branch. Tristicha shoots are also characterized by a well-developed common base, which is a product of the basal intercalary growth of ramuli. Conclusions. The ramulus of Tristicha is interpreted to be the determinate shoot portion of a sympodially branched shoot system, as in Terniopsis, not as a leaf or a leaf-stem fuzzy organ. The Tristicha shoot that bears two new branches can be compared in construction with the double sheathed (bithecous) leaf of Podostemoideae.