Over the last 40 years, modern techniqu. Many of the "classic" guides were written in a time when the Linnean classification system still reigned supreme, all the way up to "Kingdom Plantae."
Modern phylogenetic techniques have substantially altered the classification of plants above the level of Order. The Angiosperm Phylogeny Group of the Linnean Society of London is responsible for reviewing the most recent research and publishing updates every few years. The relationships at right are a subset of the APG III classification system, from 2009.
The basic taxonomic unit is a "clade", which is defined to be a monophyletic group - in other words, a group whose members all descended from a common ancestor. In the past, taxonomic grouping was made on the basis of physical characteristics, a gross division at first, but carried out with increasing sophistication as time went on. However, modern molecular techniques have allowed a more precise determination of the actual evolutionary relationships between extant species, bypassing the confounding effects of convergent evolution. In many cases, it has confirmed the close relationship of traditional taxa. In other cases, it has confirmed that some taxa are merely "junk drawers" for otherwise unrelatable species. And most importantly, it has sometimes shown that uncontroversial taxa are actually a combination of two or more unrelated groups, and that a reclassification is necessary.
Taken together with the ICBN's normalization of nomenclature, a 2010 view of flowering plant classification differs from the "classic" guides in the following essential points: