Among the Atlantic mountains only a few summits attain a sufficient elevation to claim a wreath of alpine flowers. Something of the nature of the gorgeous fields of bloom about the great peaks of the Pacific mountains is suggested on the treeless summits of the White Mountains, but although classed by botanists among alpine floras, the plants growing there fail to give a true idea of the display characteristic of the mountains which make a nearer approach to the lower limit of perennial snow. In the southern Appalachians the absence of a luxuriant alpine flora is perhaps more than counterbalanced by the profusion of rhododendrons, azaleas, and laurels.
One instructive lesson suggested by this hasty glance at the plant life of North America is furnished by the quick response that vegetation gives to conditions of environment. Throughout the greater divisions of the forest, prairies, grass-covered plains and valleys, and flower-decked mountain heights there are constant variations from locality to locality in the plant life to meet seemingly obscure or but slight changes in the conditions of temperature, humidity, exposure to sunlight, soil composition, soil texture, etc.; and besides there is a never-ceasing struggle for existence among the plants themselves which leads to important modifications of a flora. These changes occur from locality to locality, frequently within a short radius, but more than this, the resultant of the various modifying conditions on which plant life depends are not constant even for a given locality. The study of extinct floras has shown that during the preceding ages in the earth's history marvellous changes in the plants of many regions, and, in fact, of the entire earth's surface, have taken place. The distinct impressions of palm-leaves, for example, are commonly found in the rocks of the Cascade Mountains, where spruces, firs, and cedars now dominate the landscape. Still more striking is the fact that even treeless Greenland and the largely ice-covered islands of the arctic archipelago were formerly clothed with forests as luxuriant and varied as those now growing in the southern Appalachian region. Although the migrations of existing forests during the few centuries of which we have historic records have been too slow to be appreciated by man, yet it is safe to conclude that changes similar to, and in fact a continuation of, those known by geologists to have taken place in the distribution of the vegetation of the continent since the Tertiary period are still in progress. With far-reaching and exceedingly slow changes in climatic conditions and in elevation above the sea due to upheaval and denudation, the plants of our forests, prairies, and mountainsides, are being moved here and there, in ever-changing combinations. Nature thus secures a rotation in the vegetation of a region, as the careful husbandman varies his crops from year to year. The suggestion in this connection furnished by geologists is that we are living in a spring-time following the great winter, known as the Glacial epoch, and that the tropical, temperate, and subarctic forests are migrating northward in an orderly march, and each in turn ascending higher and higher on the more lofty mountains.