Endogenous leaf initiation in the apical-meristemless shoot of Cladopus queenslandicus (Podostemaceae) and implications for evolution of shoot morphology

Podostemaceae are an ecologically and morphologically unusual aquatic group of angiosperms; e. g., Cladopus queenslandicus lives in rapids and waterfalls and has adventitious shoots on the subcylindrical roots prostrate on rocks. In this light and transmission electron microscopic study, we describe a peculiar organogenetic pattern of endogenous leaf initiation at the shoot tip in the absence of an apical meristem in C. queenslandicus. This pattern, like the one described previously in some other species, is in marked contrast to the general pattern seen in most angiosperms that the shoot apical meristem is essential to leaf formation on the flank. Leaf formation also involves two unique processes: (i) cell separation that occurs on the outside of the incipient leaf primordium and (ii) vertical tissue splitting in a region where a shoot apical meristem exists in other angiosperms. Comparison in the presence or absence of a shoot apical meristem, cell separation, and tissue splitting in Podostemaceae shows that this organogenetic pattern may be a defining evolutionary novelty of the Asian-Australian lineage of the subfamily Podostemoideae.