Traditional ways of classifying plants have been based on the way plants, particularly their flowers, look, so that plants which look similar have been grouped together. As scientists have discovered more about the make-up of individual cells and have been able to examine the DNA of living things, they have found that there are other characteristics that plants share that make it more appropriate to group them together in different combinations. Previously, the main division of flowering plants was between Monocots (with one seedleaf) and Dicots (with two seedleaves). It has now been established that some monocots did have two seedleaves at a very early stage in their development, so that it is not appropriate to classify them on this basis as it is not their original form. The scientists have also found that some plants evolved earlier or later than had previously been thought, so the systems traditionally used did not reflect the order in which they evolved.

The idea of changing the way plants are classified was first suggested around ten years ago, and scientists have continued working on the best way of grouping them to reflect their molecular similarities, relationships and evolution. They have now worked out a new system of plant classification known as APG III, because it is the third revision of the plan by the Angiosperm Phylogeny Group.

Fortunately for gardeners, the new system does not change the parts of the classification system we use most of the time, leaving Species, Genus and Family more or less as they are, with a few alterations where the genetic evidence shows that the relationship between plants is not as we had previously thought. The new system affects the higher levels of classification, downgrading the importance of flowering plants within the plant kingdom as a whole, and grouping plants into a number of 'clades', distinct groups of related plants. The main groupings in Angiosperms are magnoliids, monocots (which includes commelinids), eudicots and core eudicots, with rosids, fabids (or eurosids 1), malvids (or eurosids II), asterids, lamiids (or euasterids 1), and campanulids (or euasterids II) being divisions of these groups. The new system classifies flowering plants on the basis of their genetic makeup, producing a 'Family Tree' of plants. There's a chart showing which Family or Order is in each clade here.

The scientists have moved some plants into different Families or different Orders as they find out more about them, and they are still trying to work out where a few plants fit in, but the new system is now beginning to be used by scientists and botanic gardens throughout the world.

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Other entries with information on Plant Names and Classification

Classification of Plants - an introduction to Plant Classification
Classification of Flowering Plant Families - a table of Plant Families grouped into their traditional Orders and Superorders
Classification of Plants - Table showing Families and Orders under APG III
Latin Names - the meanings of some common Latin botanical names
Plant Families - an introduction to Plant Families

Useful websites

Kew Gardens - Introduction to APG III
Phlogenic Classification - Article in the Botanical Journal of the Linnaean Society, 2009 (this is in PDF Format)
Plant Finder - Explanation of the effects of APG III (this is in PDF Format)
Wikipedia - Information on APG III
Cladistics - An explanation of how a system based on clades works, on the BBC website.
Angiosperm Phylogeny Group - (quite technical!)