Press Release

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


  • Flowering plants are less significant than scientists thought: downgraded from a class to a subclass
  • 'Oddball' parasitic plants better understood
  • Standard adoption of APG system by Herbaria and botanic gardens will allow research and conservation efforts to move forward rapidly

Scientists from the Royal Botanic Gardens, Kew (RBG Kew) have led a significant global revision of the system which botanists use to classify flowering plants. This work, published in the Botanical Journal of the Linnean Society (8 October 2009), will have a fundamental impact, not only on scientists, but on the way that botanic gardens organise their collections and future use of plant information to improve human quality of life. 

The APG classification of the orders and families of the flowering plants (angiosperms) was first published in 1998 by a global consortium of botanists, and it represented the first major group of organisms to be re-classified based largely on genetic (DNA) information. When first published, 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 APG III, slightly more than ten years later, there are relatively few families unplaced to order, and 14 new orders have been recognised. Among the groups now placed to orders are nearly all families of parasitic angiosperms, most of which were unplaced in APG I (published in 1998). This major change has been due to the enormous effort that has gone into working on these problematic groups of plants.

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

The other important change in APG III is that 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. The matter was debated at five international workshops held by Professor David Mabberley (now Keeper of the Herbarium, Library, Art and Archives at RBG Kew), when preparing the third edition (2008) of his The Plant-book , which uses the broader concepts favoured by those workshops. This was used as a basis for discussions by an international grouping of herbarium specialists who came together in 2008 under the leadership of RBG Kew's Professor David Mabberley and Professor Mark Chase, Keeper of the Jodrell, to agree a consensus to be used in their institutions - for preserved collections as well as in their associated living collections in their gardens.

As a result, in almost all cases, they and now APG III use the broader concepts, which makes the classification simpler. The use of the broader family limits does present some problems for some specialists who work on subsets of the genera within some of these broader concepts. For example, Chase, Reveal and Fay (also 8 October 2009, Botanical Journal of the Linnean Society ) developed a system of subfamilies within the three large families in monocot order Asparagales, namely Amaryllidaceae, Asparagaceae and Xanthorrhoeaceae, so that researchers working within these large families can refer to a formal set of subfamily names, which will improve communication about these groups.

Previous versions of APG focused strictly on defining orders and families, but some researchers requested a formal higher-level classification, that is above the level of order. Now that higher-level relationships are more clearly defined it is possible to provide such a classification. Chase and Reveal have therefore provided a set of 16 formally recognized superorders for the angiosperms, and the group as a whole is recognized as the subclass Magnoliidae. Previous classifications of the flowering plants have generally recognized several subclasses and placed the angiosperms in one or more classes. These sorts of treatments inflate the status of the angiosperms, which are, after all, the most recently evolved group in the land plant flora. If age of groups is taken into consideration, this type of treatment of the angiosperms makes it necessary to recognize all other plants at yet higher categories, and this results in the classification running out of enough higher categories. The Chase and Reveal ( 8 October 2009, Botanical Journal of the Linnean Society ) classification rectifies this situation and reduces the flowering plants to a single subclass composed of the 16 superorders.

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 a linear arrangement that can be implemented within a physical herbarium structure. Arranging order beds in a botanical garden is a similar sort of problem - how do you take a tree-like classification and turn it into something that can be handled in two dimensions? Haston et al . ( 8 October 2009, Botanical Journal of the Linnean Society ) provided such a linear version of APG III to the Kew-led consortium mentioned above. In consequence, this sequence has been accepted by the consortium as the basis for re-organising the herbarium collections at RBG Kew, RBG Edinburgh, the Natural History Museum (London), the Musée National d'Histoire naturelle (Paris), Conservatoire et Jardin Botaniques (Geneva) and the Nationaal Herbarium Nederland (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.

Such convergence on a single system of ordering collections with agreed circumscriptions of the families within it has never occurred previously, and this should provide the impetus for other herbaria also to accept this method of organising their collections.

Professor Stephen Hopper, Director, RBG Kew, says: "The work of the Angiosperm Phylogeny Group has been vital in reinvigorating taxonomic research. Effective conservation programmes depend fundamentally on systematics and taxonomy and it is important that these fields are transformed to meet the needs of 21 st Century science.

"You cannot conserve what you don't understand and in a time of increasing environmental pressure, loss of biodiversity and climate change, having the knowledge to manage these issues is now more important than ever.

"As the world's largest centre for plant diversity science and conservation, Kew 's staff see first hand the effects of climate change and habitat loss on a daily basis. We cannot emphasise enough the need for investment in systematics and taxonomy and collections management to enable increased efforts in conservation around the world.

"Kew's response to these environmental challenges is the focus of our Breathing Plant Programme, which includes a commitment to accelerate discovery and classification of plant diversity and make plant information rapidly accessible through presenting data and images online."

Vaughan Southgate, President of the Linnean Society of London, says, "I am delighted to see this paper and its cutting edge science published in the Botanical Journal of the Linnean Society of London. It is important that such significant papers are brought to the public's attention."

Sandra Knapp, botanist at the Natural History Museum and Botanical Secretary of the Linnean Society says, "This new APG system comes at a perfect time for my own institution as we move into the Darwin Centre, but it also provides the consensus on plant families that will allow both research and conservation efforts to move forward rapidly. Such publications are key to bring a field together."

RBG Kew's The Shirley Sherwood Gallery is currently hosting an exhibition of botanical paintings, arranged in the latest evolutionary sequence in plants, revealed by recent DNA analysis. The Art of Plant Evolution , running until 3 January 2010, aims to give visitors a sense of the contemporary scientific discoveries that are changing our understanding of plant relationships. More information here


For more information please contact Bronwyn Friedlander, Bryony Phillips and Tarryn Barrowman in the Kew Press office on 020 8332 5607 or

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Notes for Editors

Hierarchy of taxonomic ranks in botany:


Division or phylum









The Royal Botanic Gardens, Kew is a world famous scientific organisation, internationally respected for its outstanding living collection of plants and world-class Herbarium as well as its scientific expertise in plant diversity, conservation and sustainable development in the UK and around the world. Kew Gardens is a major international visitor attraction. Its landscaped 132 hectares and RBG Kew's country estate, Wakehurst Place, attract nearly 2 million visitors every year. Kew was made a UNESCO World Heritage Site in July 2003 and celebrates its 250th anniversary in 2009. Wakehurst Place is home to Kew's Millennium Seed Bank, the largest wild plant seed bank in the world. By 2010, RBG Kew and its partners will have collected and conserved seed from 10 per cent of the world's wild flowering plant species (c.30, 000 species). The aim is to conserve 25% by 2020 and funds are being actively sought in order to continue this vital work.

The Linnean Society of London is the world's oldest active biological society. Founded in 1788, the Society takes its name from the great Swedish naturalist, Carl Linnaeus (1707-1778) who developed the system of binominal nomenclature. This system today provides the fundamental framework for knowledge of the biota of the Earth, supporting effective conservation measures and the sustainable use of biodiversity. The Society is the custodian of Linnaeus' original library and collections and is creating a digital archive, enabling full global access. It encourages and communicates scientific advances through its three world-class journals, open meetings and website. The Society's Fellowship is international and its Fellows are drawn from all walks of life including professional scientists and amateur naturalists. The Society welcomes anyone interested in natural history, in all its forms.

The Botanical Journal of the Linnean Society is published on behalf of the Linnean Society of London and publishes original papers of relevance to, and reviews of, the taxonomy of all plant groups and fungi, including anatomy, biosystematics, cytology, ecology, ethnobotany, electron microscopy, morphogenesis, palaeobotany, palynology and phytochemistry.

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