United States, accompanying Humidity. See Meteorology; Climate; Storm.
Flora. The flora of the United States, when compared with that of Europe, is characterized by a far greater variety and wealth of species. This is especially noticeable in the trees, of which there are about 400 species in the United States, representing nearly all the European genera besides a considerable number of genera unknown in Europe, such as the hickories, magnolias, tulip tree, sassafras, liquidambar, and sequoia. Within the United States one finds also great differences between the various sections of the country, not only the natural differences due to climate as one passes from south to north, but also still more remarkable differences between the eastern and western sections. The latter are of course due in part to climatic variations, but in a large measure they are also due to the geological and topographical isolation of the East and West. Many of the most common trees, shrubs, and herbs of the Eastern States are unknown on the Pacific Coast, some of them being not even represented by cognate species. On the other hand, most of the characteristic species in the West are absent in the East. It is in the East that one finds the greatest local variety in vegetation, hundreds of widely different forms growing in luxuriant profusion side by side within narrow areas, while in the West there is much greater uniformity, immense areas being almost monopolized by a single species or at least a single type. The Eastern section is also the great forested area of the country. West of the Mississippi the forests rapidly disappear as one approaches the Great Plains, and reappear only on the mountains. Indeed, in the entire Western half of the country, with the exception of the northern part of the Pacific Coast, the forests are confined to the mountain slopes above a certain altitude, all the great valleys and lowlands being treeless except where irrigated by natural or artificial means.
Considering the country somewhat more in detail, one finds that it has been variously divided into floral regions. Under one of the most convenient and widely accepted systems the country is treated as belonging to eight such regions. Two of these are tropical, southern Florida and southern Texas, the latter being regarded botanically as a part of Mexico; two are subtropical or austral, the Southeastern Coast Region including the coast States from Texas to Virginia, and the Californian Region; the remaining four are temperate or boreal in character, namely, the Appalachian Region, including all the northeastern States north of the Southeastern Coast Region and east of the Mississippi; the Prairie or Great Plains Region, stretching from central Texas northward between the Mississippi and Rocky Mountains; the Rocky Mountain Region, included between the Great Plains and the Sierra Nevada-Cascade Range; and the Cascade Mountain Region, including the northern half of the Pacific Slope.
The flora of southern Florida is closely related to that of the West Indies and more remotely to that of South America. It consists largely of mangrove and cypress swamps and palm woods, with a profusion of vines and epiphytes, and including about 400 tropical species not found in any other part of the country. Southern Texas is a semi-desert area in which the mesquite and cacti predominate. The Southeastern Coast Region is characterized by large belts of pine forests in which the chief species are the long-leaf, short-leaf, and loblolly pine, as well as junipers, cedars, arbor-vitæ, and swamp cypress. The palmetto, the northernmost American palm, ranges as far as North Carolina. Other characteristic plants are the magnolia, live oak, and a number of evergreen angiosperms, as well as hickories and other trees common to the North.
The Appalachian Region has probably the most varied and luxuriant vegetation of all regions within the strictly temperate zones. This is due partly to the abundant rainfall coupled with the great and prolonged summer heat, partly to the fact that in this region a large number of Miocene plants survive side by side with the more recently evolved species. This is the great region of deciduous forests, and the one which contains nearly all the representatives of the European genera of trees, the oak, maple, ash, chestnut, elm, walnut, linden, poplar, beech, birch, etc., besides the strictly American trees, among which the tulip tree (Liriodendron tulipifera) is perhaps the finest and most characteristic example. Coniferous forests in this region are chiefly confined to higher ground, and the higher Appalachian Mountains have almost a typical Canadian flora, with their forests of hemlock, white pine, fir, mountain ash, and yellow birch. The undergrowth is also richly developed in this region, both in the mountains and lowland, and displays a wealth of vines and exquisite flowers, such as rhododendrons, honeysuckles, mountain-laurels, viburnums, and dogwoods. In the Great Plains Region the prevailing type of vegetation is the herbaceous, with grass predominating, but variegated with asters, sunflowers, and a number of other similar plants, while cottonwood and willows grow along the streams. In the Western plains the grasses give place to the sage-brush, and in the South, on the Llano Estacado, and in southern New Mexico and Arizona, to yuccas and cacti. In the Rocky Mountain Region the flora of the drier plains is extended into the valleys and lower plateaus, while the mountains are covered with coniferous forests. See paragraph Flora, under Rocky Mountains.
The Californian Region, being protected by the lofty Sierras from the immigration of Eastern plants, has a flora unique in its numerous endemic genera and species. The forests, which, except in the north, are confined to the mountain slopes or the great central river valley, are almost wholly coniferous, and are especially remarkable for their gigantic trees. The giant sequoia, which grows nowhere else, ranks with the Australian eucalyptus as the largest forest tree on the globe. Enormous size is attained also by the redwood (Sequoia sempervirens), the sugar pine (Pinus Lambertiana), the Douglas spruce, and the giant cedar. The non-coniferous trees in California are not numerous, and are largely evergreen. There are several oaks, but few of the other Eastern trees. The Cascade Mountain Region is a northward continuation of the Californian Region, but differs from it in the absence of the sequoia and of the evergreen angiosperms, while deciduous trees are still more
