The Ozark region is a plateau and mountain area of more than 40,000 square miles at the western edge of the eastern deciduous forest, and host a rich assemblage of plant species. Overall, nearly 3,300 vascular plant species are known from the region, within approximately 1066 genera and 186 families. This diversity is due, in part, to a long presence uninterrupted by glaciation, varied site and soil conditions, and a midcontinental geographic position. This geographical location has resulted in many species with distributional centers outside of the Ozarks reaching their range limits here. For example, the range of certain plants from the southwestern United States extends to the Ozarks, typically in dry-soil environments such as the glades. On moist, north-facing slopes, plants from the northern and eastern United States may be found. Some typically southern species occur in special habitats, including many parts of the Boston Mountains.