During the past decade many groves have been planted on the prairies, and have flourished, especially when the adjacent fields are cultivated so as to allow the earth to store a larger share of the winter rain; the success of this tree planting, it has been claimed, is evidence that the nature of the soil is not a determining factor in the problem, because trees will grow if protected from fire. The success of arboriculture on the formerly treeless plains and plateaus, however, decreases as one travels westward. On the western border of the prairies and on the great plateaus, remote from streams, trees can be made to grow only by the aid of irrigation. If this region had never been swept by fire it is safe to say it would still be treeless. Each of the explanations referred to above to account for the treeless condition of the prairies--one referring it to soil conditions, and the other to the former prevalence of fires--certainly has much in its favour, and for certain localities seems satisfactory, but each point of view should include a broader range and recognise the fact that the requisite critical conditions have been furnished by wide-reaching climatic causes. The Prairie plains furnish but one phase of the gradual change that occurs in the natural mantle of vegetation when traced from the dense, well-watered forests of the Appalachians and the Alleghany plateau westward to the semiarid and truly arid lands of the great plateaus and Rocky Mountain region, where only such plants as are able to withstand long-continued drought can grow. This same broad conclusion is sustained also at the north, where the prairie dovetails, as it were, with the subarctic forest. The general or underlying reason for the treeless condition of the vast central portion of the continent is doubtless a lack of sufficient rain. The precipitation that does occur comes mainly during the winter season, when the land is colder than the ocean; in summer the land becomes highly heated and imparts its temperature to the air, which thus has its capacity for moisture increased, and prolonged droughts occur. At the south, in Mexico and the adjacent portion of the United States, the trade-winds blow over a region which is more highly heated than the ocean from which they come, and are hence drying winds. To the west of the Great plateaus rise the Rocky Mountains, where climatic conditions are different on account of elevation, and, as we have seen, forests occur at considerable elevations, but not in the broader valleys. The conditions unfavourable for tree growth are continued and even intensified in the valleys of the central portion of the Pacific mountain region, and culminate in the deserts of the Great Basin and western Mexico. Throughout all of this vast treeless region the controlling condition is deficiency of moisture, particularly during the summer or growing season.