22 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



The peculiarly verdant arboreal character of the desert of southern Ai-izona and Sonora 

 appears to be due primarily to the double rainy period. In the majority of deserts rain 

 falls only during a single season, which is often the winter, when the temperature is un- 

 favorable to growth. Inasmuch as the winters of the less elevated portions of Arizona do 

 not last long, those portions are favored with a relatively good growth of herbaceous 

 annuals in winter, and also in summer, as is fully described in the paper of MacDougal 

 previously referred to. Trees, as is well known, require a prolonged season of growth. The 

 rains of the brief moist season in most deserts do not store the ground with sufficient 

 moisture to enable the trees to mature their various organs and produce seed. In the 

 region under discussion, however, the winter rains start the growth of trees, and supply 

 sufficient moisture to enable the plants to subsist matil the arrival of the summer rains. 

 These lengthen the growing season to a period equal to that in many regions which are much 

 better watered. Of course, moisture is scarce for a long interval during the rainless fore- 

 summer, and the ground is too dry for ordinary trees. Nevertheless, many desert species 

 have become adapted to the double rainy season. Hence, although Arizona is a genuine 

 desert from an agricultural point of view, the scenery of the southern part by no means 

 suggests this. The country is far more verdant than many regions whose agricultural 

 possibilities are much greater. 



In the elevated plateau-regions of northern Arizona and New Mexico the vegetation 

 is of the usual type, chiefly grasses, small quick-growing herbs, and low stunted bushes 

 of the sage type. This is due to the relatively small amount of rain in summer as com- 

 pared with winter, and to the length of the winter, which prevents growth in February, 

 March, and even April. Thus the twofold rainy season is largely robbed of its effect. In 

 the higher mountains, at an altitude of 7,000 feet or more, pine forests, fringed with oaks 

 on the lower border, cover large areas. Here the moisture of the winter remains in the 

 ground so late that there is practically only one growing season. 



