44 THE SCIENTIFIC MONTHLY
spicuous differences from year to year, but in the same year tracts, only a few hours' ride apart, vary greatly in the state of development of their vegetation.
The soil moisture of a region so variable in rainfall and so diverse in surface topography and in depth and texture (and consequently in water-absorbing power and retaining capacity) as the environs of Tucson, presents a problem of great complexity, even if one considers it from the physical side alone, and not in its more complicated relation to the plant organism.
One must remember that the water which is of service to the plant is not the amount recorded by the rain gauge. Only a portion of the water, falling upon a given spot, may become soil moisture for the plants which grow there. Besides the loss through superficial run off already indicated, there is unquestionably a sub-surface drainage which, when there is sufficient precipitation to bring it about, tends to irrigate some spots at the expense of others. In consequence, there is great variation in soil moisture in habitats otherwise apparently uniform.
By no means all of the water which sinks into the soil can be used. In the development of every plant organism there is a time factor. While physiological processes are carried through with great rapidity in desert plants, there is a minimum beyond which this time element can not be reduced. Duration of soil moisture, not merely absolute amount, is of great importance.
With regard to the permanence of soil moisture, the various habi- tats differ widely. Cannon has found that the upper levels of the soil of the bajadas were air dry at the end of three weeks after the rains, while those of Tumamoc Hill and of the Santa Cruz flood plain re- mained moist for a period exceeding six weeks. On Tumamoc Hill, the superficial soil layer may be so thoroughly baked during the dry fore-summer that the water content falls to about 2 per cent, of its vol- ume; but beneath, the water supply is probably adequate for the growth of the more hardy and deeper-rooted shrubs, throughout the periods when other conditions are favorable. Anomalous as it may seem, the great evaporating power of the air is the cause of the retention of the considerable quantities of moisture in the lower layers of the soil — at a depth available to many perennials and in amounts sufficient for life and even growth during dry seasons. From the surface layers, evapora- tion is so rapid after a rain that a dry mulch is formed, preventing more or less effectively the loss of water from beneath.
Temperature shows not merely a fluctuation of over 100 degrees Fahrenheit during the year, all falling above zero, but great diurnal variation as well. In the growing and even flowering season of the winter annuals, the days are warm — or hot, in the terminology of the more temperate regions, — while the nights have freezing temperature.
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