[Illustration: Fig. 57. Typical Mixed Forest,--Red Spruce, Hemlock, White Ash, Yellow Birch, Balsam Fir, and Red Maple. Raquette Lake, New York. _U. S. Forest Service._] The soil in the forest is largely determined by the forest itself. In addition to the earth, it is composed of the fallen and decayed leaves and twigs and tree trunks, altogether called the _forest floor_. It is spongy and hence has the ability to retain moisture, a fact of great importance to the forest. MOISTURE. Some trees, as black ash and cypress, Fig. 58, and cotton gum, Fig. 59, grow naturally only in moist places; some, as the piñon and mesquite, a kind of locust, grow only in dry places; while others, as the juniper and Douglas fir, adapt themselves to either. Both excessively wet and dry soils tend to diminish the number of kinds of trees. In many instances the demand for water controls the distribution altogether. In the Puget Sound region, where there is a heavy rain-fall, the densest forests in the world are found, whereas on the eastern slopes of the same mountains, altho the soil is not essentially different, there are very few trees, because of the constant drouth. [Illustration: Fig. 58. Cypress and Cypress "Knees." Jasper Co., Texas. _U. S. Forest Service._] [Illustration: Fig. 59. Cotton Gums, Showing Buttresses. St. Francis River, Arkansas. _U. S. Forest Service._] TEMPERATURE. The fact that some trees, as paper birch and white spruce, grow only in cold regions, and some, as rubber trees and cypress, only in the tropics, is commonplace; but a fact not so well known is that it is not the average temperature, but the extremes which largely determine the habitat of trees of different kinds. Trees which would not live at all where there is frost, might flourish well in a region where the average temperature was considerably lower. On the other hand, provided the growing season is long enough for the species, there is no place on earth too cold for trees to live. Fig. 60. [Illustration: Fig. 60. Northern Forest,--Young Spruce Growing Under Yellow Birch. Santa Clara, New York. _U. S. Forest Service._] In general, cold affects the forest just as poor soil and drought do, simplifying its composition and stunting its growth. In Canada there are only a few kinds of trees, of which the hardwoods are stunted; south of the Great Lakes, there is a great variety of large trees; farther south in the southern Appalachian region, there is a still greater variety, and the trees are just as large; and still farther south in tropical Florida, there is the greatest variety of all. The slopes of a high mountain furnish an illustration of the effect of temperature. In ascending it, one may pass from a tropical forest at the base, thru a belt of evergreen, broad-leaved trees, then thru a belt of deciduous broad-leaved trees, then thru a belt of conifers and up to the timber line where tree life ceases. Figs. 61, and 62.