25,000 North America 18,000 Europe 12,500 Plants occur over most of the Earth's land surface, but the distribution of species is distinctly uneven.
Distribution is affected by environmental factors including climate, soils, and topography.
Knowing the distribution of species enables scientists to establish criteria and guidelines for their conservation and ecologically sustainable use. Botanists have collected the most data on plant species found in countries with relatively low diversity. Britain's flora has probably been studied more extensively and better inventoried than that of any other country, while in many tropical countries, only a handful of botanists are studying tens of thousands of plants. Often, once a plant has been named or described, it is not seen again for years. Almost nothing is known about the distribution of immense numbers of tropical plants.