XVI International Botanical Congess

The APG-system comprises 40 orders and 462 families of flowering plants. In order to avoid future arbitrary lumping or splitting of the orders phylogenetic definitions are proposed according to the format Athe order X is the most inclusive clade comprising its type but none of the types of the 39 other orders@. A general problem in systematics is the selection of clades to name and study. Criteria for selecting clades need to be considered, e.g. stability, size, support for monophyly, and ease of identification. There is also a growing need for knowing clades of the same age and hence phylogenetic dating will become increasingly important. This may be done by counting inferred changes along the branches of molecular phylogenies and calculating change rates using fossils. In research on flowering plants a first goal could be to identify the possibly 100 clades that date back to the Early Cretaceous 100 Myr ago.