Planta in Relation to Geography.— Most of the land surface of the earth is covered by a Freen mantle of vegetation, which varies in its makeup at different points in accordance with several factors, the most general of which in its variation is temperature. Certain areas of the north polar and south polar re gions, permanently or almost permanently covered with snow or ice, and various simi larly cold areas on mountains of higher and higher elevation in lower latitudes, are devoid of vegetation. Next to this is an area of suf ficient warmth in summer to produce a vege tation of herbaceous plants and shrubs but devoid of trees — the Arctic, Antarctic and Al pine vegetation. Then come the temperate areas of the earth, characterized by a vegeta tive covering able to withstand freezing dur ing a portion of the year, yet sufficiently warm to permit an abundant growth of trees. Next follows the tropical area, with a vegetation not subjected to frost and characterized es pecially by forests made up in part of palms. A factor of probably even greater importance, but more broken and restricted in its distri bution, is moisture. The four great tempera ture categories outlined above are cross hatched by moisture lines parallel with the lines of equal precipitation. With too little moisture forests cannot exist and we have plains and deserts of grass or brush. Neither do forests exist in a soil too persistently moist and poorly drained and thus we have moors, bog, natural meadows and savannas. The ex treme of moisture is reached in the plants called aquatics, growing either in fresh water or in the ocean, often wholly submerged.
Each of the other factors in plant growth, light, air, food and the complex mechanical relations of the plant, varies greatly from one locality to another and in their various com binations with different degrees of heat and moisture they furnish an almost endless va riety of environments. Each of these com binations of conditions has its characteristic association of plants, which, adapted to the conditions, and to each other, form a com munity. The study of plants in their detailed relation to these local surroundings forms a branch of geographic botany known as plant ecology. North America furnishes a good series of geographic areas with sufficient climatic differences to necessitate different floras. The vegetation of the continent is divided by Merriam into the following zones: Arctic, Boreal, Transition, Upper Austral, Lower Austral, Tropical. The Arctic zone extends from northern Labrador northwest ward across the northern edge of the con tinent to Bering Strait, dipping southward along the shores of Bering Sea to Bristol Bay, Alaska. The vegetation of this zone consists of herbaceous or of depressed woody plants, trees being absent. Over large areas, known as tundras, the ground is permanently frozen underneath, a few inches of the sur face thawing each summer and permitting the growth, in a cold, wet soil, of an often luxuri ant but low vegetation. The Arctic zone is represented southward as far as southern Cal ifornia and northern Arizona by certain Alpine plants on the summits of mountains high enough to have a timber line, approximately , 12,000 feet in that latitude. The Boreal zone,
sometimes subdivided into a northern, or Hudsonian, belt and a more southerly, or Canadian, belt, extends from the Arctic zone southward to a line traversing the northern part of New England, Ontario, Michigan and Minnesota, jumping to the higher elevations of the Adirondack and Appalachian Mountain systems, then continuing westward across North Dakota and Assiniboia to British Columbia, dipping south in the higher eleva tions of the Rocky Mountains nearly to Mex ico, in the Cascades and Sierra Nevada to southern California and along the shores of the Pacific' to northern California. The most characteristic feature of this zone is forests of spruce or balsam fir. The Transition zone covers most of New England, New York, Michigan, Minnesota, North Dakota, about half of South Dakota and the southern part of Assiniboia, thence extending southward through the Plateau and Great Basin to Ari zona, New Mexico and California, in the southern parts of those States reaching down to an elevation of about 6,000 feet. The most characteristic tree of the eastern, humid part of this zone is the white pine; of the west ern, arid part, the yellow pine. The Upper Austral zone, as represented in the eastern United States by the so-called Carolinian flora, covers the lower Hudson Valley, south ern New Jersey, Delaware, eastern Maryland, the Piedmont section of the South Atlantic States, middle Tennessee and Kentucky and most of Ohio, Indiana, Illinois, Iowa and Mis souri, northwestern Arkansas, southeastern South Dakota and eastern Nebraska and Kansas. It is especially characterized by its forests of certain species of oak and hickory. The flora of the western part of this zone, known as the Upper Sonoran, covers the prin cipal part of the arid western plains, from Washington and Montana southward through the Mexican plateau. The flora is devoid of trees and is commonly characterized by sage brush or bunchgrass. The Lower Austral zone is divided, like the last, into an eastern humid and a western arid part. The eastern, containing the Austroriparian flora, covers the coastal plain from Chesapeake Bay to middle Texas, extending northward in the Mississippi Valley to extreme southern Illi nois and Indiana. One of the most charac teristic wild plants is the cane, while cotton is the conspicuous cultivated plant. In the and region of western Texas, the great valleys of New Mexico and the deserts of southwestern Arizona, southern Nevada and southeastern California, lies the western part of the Lower Austral zone, containing the flora known as the Lower Sonoran, charac terized especially by the creosote bush and the mesquite. This flora has large extensions into northern Mexico. The Tropical zone covers the lower third of the Florida Penin sula, enters the extreme southern point of Texas and on the Pacific coast reaches north on the east side of the Gulf of California to the lower Colorado and Gila rivers. From these northern extremes the tropical flora ex tends southward through Mexico, Central America and the West Indies. Various gen era and species of palms form the most con spicuous hnd characteristic features of this flora.