Female flowers and systematic position of Picrodendraceae (Euphorbiaceae s.l., Malpighiales)

This is the first comparative study of floral structure of the recently established new family Picrodendraceae (part of Euphorbiaceae s.l.) in Malpighiales. Nine species of eight (out of ca. 28) genera were studied. Female flowers are mainly completely trimerous, and in such flowers the perianth consists of one or two whorls of sepals. A floral disc (which probably functions as a nectary) is mostly present. The free parts of the carpels are simple (unbranched) in all ten species studied. Each carpel contains two crassinucellar, anatropous or hemitropous, epitropous (antitropous) ovules, which are covered by a large obturator. The inner integument is thicker than the outer (equally thick in two species studied), and commonly both integuments form the micropyle. In mature ovules the vascular bundle commonly branches in the chalaza, with the branches extending to the base of the inner integument but not entering it. A nucellar cap and, less often, a nucellar beak is formed. Floral structure supports the close relationship of Picrodendraceae with Phyllanthaceae and Euphorbiaceae s.str. within Malpighiales, as suggested (but not yet strongly supported) by some recent published molecular analyses. These three families share a unique combination of characters, including (1) unisexual, apetalous trimerous flowers, (2) crassinucellar ovules with a nucellar beak, (3) a large obturator, and (4) explosive fruits with carunculate seeds.