The Sierra Nevada is the most continuous lofty range of mountains in North America. From the great valley at its western base to the crest of the range the distance is about sixty miles. Because of the great height of the mountains, there is found within these few miles every variety of climate between the sub-tropical atmosphere of the valley, where oranges ripen to perfection, and the arctic cold of the summits, where little or no vegetation can live. Thus, by climbing a single mountain range, we may experience all kinds of climate, and have an opportunity to observe the different forms of plant life such as we could not otherwise obtain without a journey of several thousand miles. [Illustration: FIG. 122.--FOREST BELT OF THE FOOT-HILLS, SIERRA NEVADA MOUNTAINS] Passing through the groves of valley oak, and beyond the orange orchards at the foot of the mountains, we reach the foot-hills and begin to ascend. Several species of oak are found upon the hillsides and in the valleys, while mingled with them in many places appear such shrubs as the California lilac, chamiso, and manzanita. Where the soil is too poor or the slopes too steep for the trees, these shrubs, commonly called "chaparral," are massed together in almost impenetrable thickets. The first of the coniferous trees which we meet is an odd-looking one known as the digger pine. Instead of having a single straight trunk it divides a short distance above the ground into many branches. The large cones are armed with long hooked spines, so that they must be handled rather carefully, but when opened they are found to be filled with nutritious nuts. These nuts were an important source of food for the Indians who once inhabited the foot-hills. Now the Indians are gone, but the nuts are not wasted, if one may judge by the fragments of the cones with which the squirrels strew the ground. [Illustration: FIG. 123.--THE DIGGER PINE] The road climbs the foot-hills by many turns and windings through canons and up and down ridges. At an elevation of about two thousand feet specimens of the yellow pine appear. The trees increase in size and grow more closely together as we ascend. We soon find ourselves in the edge of the forest belt which extends unbroken northward to the arctic zone, and upward to the line of almost perpetual snow. The yellow pine, so named from the color of the bark, sometimes attains a diameter of six feet, but does not form so dense forests as we shall find higher on the mountains. The rays of the warm sun, reaching down between the trees to the carpet of needles and "bear clover," draw out their spicy fragrance. The yellow pine, although it does not afford as good a quality of lumber as some of the other pines, is one of our most important trees because of its wide distribution through nearly all mountains of the West. It has a much wider range in elevation than most trees, one variety reaching upward nearly to the timber line.