The Tropical zones (those north and south of the equator) are characterized by a more abundant and diversified underwood, and, while retaining some of the equatorial forms, present fewer parasites and less rapid and luxuriant growths. They contain many plants and trees which are peculiar to their own limits, and these are generally the hardiest and most abundant. All equatorial forms disappear in these zones, that is do not pass into the sub-tropical zones. And these characteristics obtain in both the northern and southern tropical zones, as well as in the mountain systems within the equatorial regions. The Sub-tropical zones, while retaining some of the more marked forms and general features of the tropical zones, such as palms, bananas, etc., exhibit the most striking characteristics of their own, consisting of a greater abundance of forest trees, especially those having broad, leathery and shining leaves, like the magnolias, the different species of laurels, and plants of the myrtle family. The tropical forms all disappear in these zones, as the equatorial do in the tropical zones. The Warm Temperate zones exhibit the same disposition to retain some of the hardier and more abundant sub-tropical forms that characterize the other zones, in respect to their adjoining isotherms. But the trees and plants peculiar to this zone north, (and the same is no doubt true of the corresponding zone south), are more numerous, and embrace a wider range of deciduous, as well as evergreen growths. Evergreen shrubs, heaths, cistusses, and leguminous plants are everywhere more abundant. The marked characteristic of these zones is that the trees, plants, and arborescent grasses differ more widely in their general character, as well as run more extensively into varieties. The Cold Temperate zones retain many of the deciduous trees of the warm temperate, but with less conspicuous blossoms, while a stronger tendency is shown toward social conifers, and the trunks of the deciduous trees are more profusely overrun with mosses, lichens, etc. These zones are also abundant in grasses. The Sub-arctic zone north largely retains its hold upon the social conifers, giving place, northward, on this continent, as well as in Europe and Asia, to birch and alder, alternating with willows where the soil is sufficiently moist. Green pastures are still abundant, and showy flowering herbs abound during the brief spring, summer, and autumn months. The Arctic zone retains few of the sub-arctic forms and its vegetation generally corresponds to what we call alpine shrubs, grasses, etc. The North Polar zone shows few signs of vegetation and is thought to be entirely devoid of shrubs. A few small herbacious perennials of the most extreme dwarf habit, with a few lichens and mosses, constitute its entire vegetation.