We have already seen that there is probably no such thing in nature as an absolutely continuous distribution of a species, even over small geographical areas. All distributions are therefore to some extent broken or disjunct. However, in many cases the disjunction reflects no more than the normal dispersal capacity of the species concerned, or an easily explained ecological barrier. Such barriers may be topographic, climatic, edaphic or biological in character. If the disjunctions so caused are not great, the distribution pattern may still be regarded as continuous at the wider scale. For example, many aquatic species, such as the reed (Phragmites communis) and the homwort (Ceratophyllum demersum) (Fig. 7.1), are considered cosmopolitan or sub-cosmopolitan at the world scale, even though they are absent from vast areas of dry land. P. communis is thought to be the most widely distributed of all flowering plant species.