The taxonomist works not only with the external appearance of an organism, but also with its genetic make-up. This basic division was made clear by Johannsen in his selection experiments with the kidney bean. In nature, however, individuals tend to be grouped together in large numbers. Such groupings are termed populations. On the popular level this term refers to all the organisms, both plant and animal, which occupy a given geographical area or habitat. For the general taxonomist, on the other hand, a population is any group of individuals considered together at any one time because they have features in common. The phytogeographical implications of one aspect of genetics, namely the phenomenon of polyploidy, have been recognised for a long time however. Polyploidy is the occurrence of multiple chromosome sets in an organism and it is widespread in the flowering plants, especially among perennial herbs.