As easy as APG III - Scientists revise the system of classifying flowering plants

Scientists from Kew have led a global revision of the system botanists use to classify flowering plants. This work will have a fundamental impact on how botanic gardens organise their collections and the future use of plant information to improve quality of life.

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20 Jan 2010

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White water lily and Trithuria submerse, the new waterlily relative

Left - Nymphaea candida, waterlily (photo: M. Christenhusz) | Right  -  Trithuria submerse, the new waterlily relative (Photo: P. Rudall & R. Bateman)

In 1998 the orders and families of flowering plants (angiosperms) were re-classified based largely on genetic (DNA) information. The new classification was published by a consortium of botanists called the Angiosperm Phylogeny Group (APG), in which scientists from Kew were influential.

This APG I publication was the first attempt to formally classify flowering plants from genetic data. It therefore reflected evolutionary relationships better than previous classifications in which scientists had to rely on comparing plants through their appearance and physical characteristics.

In the APG I classification, it was unclear how many orders (the rank above family) would be needed. And many families were not yet allocated to an order. Now, in the second update slightly more than 10 years on, APG III has relatively few families unplaced to order, and 14 new orders have been recognised.

Among the groups now placed to orders, nearly all are families of parasitic angiosperms, most of which were unplaced in APG I. This major change has been due to the enormous effort that has gone into working on the classification of these problematic groups of plants.

Some unusual findings

This new work has thrown up some unusual relationships, for example the corpse flower (Rafflesia) found in the Indonesian rainforest, noted for producing the largest individual flower in plants, is actually related to the poinsettia (Euphorbia pulcherrima), which has one of the world's smallest flowers. 

In APG III there are only five taxa (two small families of parasitic plants and three genera) unplaced in the angiosperm tree, which makes this version of the classification much more complete than the previous ones.

In APG II, there were a number of families for which two alternate versions were permitted, wider (one larger family) and narrower (2-10 more narrowly defined families). APG III eliminates these alternative versions because the user community did not like this concept - they preferred that the APG authors decide which was better.

As a result, in almost all cases, APG III uses the broader concepts, which makes the classification simpler.

APG III was published in the October 2009 issue of the Botanical Journal of the Linnean Society

Re-organising herbarium collections

Herbaria and botanic gardens have started to take up the APG system for organising their collections.

Herbaria in particular need a way to take a phylogenetic classification, based on a hierarchical system, and turn it into an arrangement that suits the physical herbarium structure.

A linear version of APG III has been accepted by the APG III consortium as the basis for re-organising the herbarium collections at Kew, RBG Edinburgh, the Natural History Museum in London, the Musée national d’Histoire naturelle  in Paris, Conservatoire et Jardin Botaniques in Geneva and the Nationaal Herbarium Nederland in Leiden, Utrecht and Wageningen.

The Natural History Museum in London has recently moved its entire flowering plant herbarium into the new Darwin Centre, and in so doing re-organised it completely according to the new system. This will improve the use of their collections in teaching and research.

Plant and fungi classification

The hierarchy of taxonomic ranks in botany.

Rafflesia, the 'corpse flower', is related to poinsettia

  • Class
  • Subclass
  • Superorder
  • Order
  • Family
  • Genus
  • Species

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