XVI International Botanical Congess
Within the Asteridae and related groups two blocks of orders can be recognized as regards corolla tube formation: The Asteridae A-block (Gentianales, Polemoniales, Lamiales, Scrophulariales) is characterized predominantely by late sympetaly (petals are initiated separately and become connected by extension of their bases after stamen initiation). The Asteridae B-block (Dipsacales, Campanulales, Asterales, Apiales, Alangiaceae) is uniform throughout in its early sympetaly (petals arise on an early ring primordium). Transitions between both modes also occur (Asclepiadaceae, Gentianaceae p.p., Acanthaceae p.p., Orobanchaceae). Some interesting items will be discussed exemplarily (relation Asterales-Apiales, position of Menyanthaceae, Acanthaceae exhibiting all three modes).