dryas, and not unlike it in general effect. Anybody who cultivates rock-gardening, indeed, must be thoroughly familiar with this curious mat-like habit of the northern mountain flora; for many of the saxifrages, alyssums, arenaries, and stone-crops which form his favorite masses of bloom are members of this truly Alpine and sub-Arctic vegetation. Often their very names betray their origin: lovers of rock-gardens will know what I mean when I mention such cases as Erinus Alpinus, Lychnis Lapponica, Sedum Kamschaticum, Alyssum montanum, Silene alpestris, Dianthus petrœus, Saxifrage nivalis, and Arenaria montana.
Nor is it only herbaceous species that undergo this curious dwarfing and acquire this strange matted tuftiness, in order to meet the needs of high Arctic and Alpine situations. Trees and bushes have similarly to accommodate themselves to the exceptional conditions of the snowline and the region just below it. Every tourist who goes up Mount Washington must have noticed how, near the limit of arboreal vegetation, the pines and spruces grow shorter and more stunted by slow degrees, till at last they disappear altogether from the scene. But even after they are gone, so far as the naked eye is concerned, they persist in part for the eye of the botanist. Three dwarf willows, for example, occupy the summits of the White Mountains, beyond the so-called limit of trees. All of them are prostrate, matted, and Alpine in type; none of them rises above the general level of the herbaceous vegetation in whose midst they are found. The first, known as Cutter's willow, may also be gathered among the other higher mountains of the extreme Northern States, such as the Adirondacks and the Maine ranges. The second, the silvery-pointed willow, a very pretty plant of glossy, satin-like sheen when young, is confined to the moist Alpine ravines of the White Mountains themselves. The third and most dwarfish species of all, the herbaceous willow, has lost all resemblance of its descent from what was once a forest tree, and has degenerated into a rare ordinary herb, seldom rising above an inch or two from the ground, but still producing from its terminal buds the tiny catkins which keep up the memory of its former high estate. This last degraded scion of the willow stock, which creeps and roots underground for considerable distances, is common to both sides of the Atlantic, being found also in the Alps and Pyrenees, as well as in Arctic and sub-Arctic Europe: but the White Mountains are its only known station in the United States.
It is interesting to note that just the same dwarfing of the trees and shrubs took place everywhere during the fiercest rigor of the Glacial Epoch. In the little bed of glacial clay, containing plant remains of the Great Ice Age, on the coast of Norfolk in England, we still find the leaves of a tiny, shrubby birch (Betula nana), which grows even now in the Highlands of Scotland and in Scandinavia, attaining there at times to tree-like size, but which dwindles near the
