Comparative carpological study of Poikilospermum (Urticaceae) in relation to taxonomy

Fruit morphology and the anatomical structure of the pericarp, fruiting perianth, and seed coat were studied in 15 species of Poikilospermum, a genus whose position within the Urticaceae has long been controversial. Possible evolutionary trends of their transformation are suggested for both subgenera; plesiomorphies were found in P. oblongifolium and P. scabrinervium. Structural peculiarities of the fruit connected with its ejection out of the tubular perianth are discussed. The archaism of the fruit in Poikilospermum is revealed, indicated, as in Boehmeria, by the presence of the rudiment of an aborted carpel in the form of a large two-lobed rib. Using carpological anatomical characters, the species studied are classified into informal groups, such characters being able to pull the species within the subgenera into rough groupings where gross morphology has been unable to do so. It is shown that heterobathmy may be strongly associated with the genus Poikilospermum. Each subgenus has its own set of primitive carpological characters: in subgenus Poikilospermum the absence of a fruiting perianth which encases the fruit, and also of capitate inflorescences with swollen receptacles; in subgenus Ligulistigma remnant rudiment of the second carpel and ovary loculus, as well as a primitive, less simplified seed coat. Though the position of Poikilospermum as indicated by molecular data is within Urera, our results suggest that Dendrocnide (the only genus of the Urticeae that has a pyrenarium fruit type) may be closest to Poikilospermum, although the pericarp structure and dissemination behaviour in Poikilospermum are more specialised than those exhibited by Dendrocnide. Seed coat structure is also shown to exhibit traits seen in Moraceae.