British botanists have embarked on an ambitious project to make a catalogue that will use DNA signatures to "barcode" every plant species of the world. Botanists know, for instance, that there are 85 different types of fern in the British Isles; but how many ferns are there in the tropical rainforest? The answer is unknown. The scheme is part of an international barcoding initiative that aims to make taxonomy more effective for science and society. The leading researchers join Quentin Cooper to discuss the progress thus far. Producer Colin Grant