Flower-like terminal structures in racemose inflorescences: a tool in morphogenetic and evolutionary research

Terminal flower-like structures (TFLS) occur in many angiosperms that possess indeterminate inflorescences such as spikes, racemes, or spadices. We describe and review TFLS in early-divergent angiosperms, especially the magnoliid order Piperales and the monocot order Alismatales, in which floral interpretation is controversial. Essentially similar TFLS occur in a wide range of taxa. Among magnoliids, they occur in some Piperales (Saururaceae and a few Piperaceae), but are absent from Chloranthaceae. Among monocots, they occur in some early-divergent families such as Acoraceae, Aponogetonaceae, Juncaginaceae, Potamogetonaceae, and Ruppiaceae. Similar TFLS with obscure organ identity are recorded in mutants of Arabidopsis. TFLS can often be interpreted as pseudanthia (close aggregations of reduced flowers), but in some cases the entire terminal pseudanthium is very similar to a true flower. In some cases, elaborated TFLS could therefore have given rise to what are normally termed 'true' (i.e. euanthial) flowers. Data presented here on terminal pseudanthia in Potamogeton and Ruppia support a pseudanthial evolutionary origin of reproductive units in the alismatid families Zannichelliaceae and Cymodoceaceae. Furthermore, in some alismatid species, either the entire inflorescence apex or an individual primordium at or near the inflorescence tip can be transformed into a filamentous or tubular (or intermediate) structure. A tubular structure enclosing stamens and carpels is described in Piper. This indicates that pseudanthium formation can provoke morphological novelties, perhaps due to new patterns of overlap between expression zones of regulatory genes and/or new spatial constraints.