Author: Jan Scholten
Backlund and Bremer have formulated principles to recognise clades, taxa, formally:
1. the main principle is that clades should be monophyletic. This does not indicate which clades should be named as families, orders etc.
2. It is helpful if clades are easily recognizable.
3. It is helpful if clades that are well established in the literature are preserved
4. The size of groups is taken into account; small ones should be combined whenever possible.
5. It is helpful if nomenclatural changes are minimized.
Changes form Apg2
1. Alternative circumscriptions for some families were permitted in Apg2. In APg3 one of the alternatives is chosen, based in part on the circumscriptions in textbooks (e.g. Judd et al., 2007), dictionaries (e.g. Mabberley, 2008) and the general literature. In general, the broader circumscriptions have been favoured and adopted.
2. Positions of isolated families like Ceratophyllaceae, Chloranthaceae and Picramniaceae have been ascertained and this has necessitated addition of orders not previously recognized by Apg1 and Apg2.
3. A few families were wrongly placed. Guamatela used to be placed in Rosaceae, but molecular data places it in Crossosomatales. Hydatellaceae have been moved from Poales to Nymphaeales.
4. There are a few cases where the general pattern of relationships has not changed much since Apg2, but our appreciation of the pattern of variation has. For example, this helps justify inclusion of Ixerbaceae in Strasburgeriaceae.
5. Finally, a few family circumscriptions suggested by Apg2 did not reflect general usage, so we have modified these, an example being the broadly circumscribed Brassicaceae, which are here split into three families.
Amborellales, Nymphaeales, Austrobaileyales
Magnoliids: Laurales, Magnoliales, Piperales, Canellales, Chloranthales.
Monocots: Acorales, Alismatales, Petrosaviales, Dioscoreales, Pandanales, Liliales,Asparagales
Commelinoids: Dasypogonaceae, Arecales, Commelinales, Poales, Zingiberales
Eudicots basal: Ceratophyllales, Ranunculales, Proteales, Buxales, Sabiales, Trochodendrales, Gunnerales.
Rosids basal: Saxifragales, Vitales
Eurosids I: Zygophyllales, Celastrales, Oxalidales, Malpighiales, Fagales, Fabales, Rosales, Cucurbitales
Eurosids II basal: Myrtales, Geraniales.
Eurosids II: Crossosomatales, Malvales, Sapindales, Brassicales, Huerteales, Picramaniales
Asterids basal: Berberidopsidales, Dilleniales, Caryophyllales, Santalales, Cornales, Ericales
Euasterids I: Garryales, Gentianales, Lamiales, Solanales, Boraginales.
Euasterids I basal: Boraginaceae, Icacinaceae, Oncothecaceae, Vahliaceae
Euasterids II: Aquifoliales, Campanulales, DIpsacales, Apiales, Escalloniales, Bruniales, Paracryphiales.
Angiosperm Phylogeny Group; An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III; Botanical Journal of the Linnean Society, 161, 105–121; 2009.
Backlund A, Bremer K; To be or not to be principles of classification and monotypic plant families; Taxon 47: 391–401; 1998.