Page:The Scientific Monthly vol. 3.djvu/52

46 lying plains towards the Gila Valley to the northwest. Above the village, the smoke curls high and irregular; below, it is drawn into the cold air current and borne down the valley in a stream whose upper limits seem marked off along the bajadas of the Santa Catalinas with almost the linearity of the draftsman’s T-square.

Shreve has shown that the temperatures in the valley may, in consequence of this factor, be many degrees below those of the hill.

When the temperature becomes sufficiently high—although absolutely it is very low—for the germination of the winter annuals the soil is apt to contain a moderate amount of moisture, at a little distance below the surface, but remains always at a relatively low temperature. Thus, although the soil surface and the sub-aereal parts of the plant (the stems and leaves) may be exposed to rather intense heat during the day, the more deeply penetrating roots are subjected continuously to the retarding influence of low temperature, while the shoots must carry on their physiological activities under the influence of alternating high and low temperatures.

The summer annuals, on the other hand, germinate after the rains have not only soaked but cooled the superficial layers of a substratum which has been both dried and heated to a great depth by the intense insolation of the fore-summer. Thus, their roots develop under conditions of favorable temperature at least, and generally of both temperature and moisture. The summer rains cool the air and change the atmosphere conditions from those of intense heat and enormous evaporating power to those of high relative humidity. Thus there is a brief period of optimum conditions for the luxuriant growth of plants with extensive leaf development. Relative humidity may range from 10 per cent. to saturation. In its relation to the evaporating power of the air and consequently to transpiration, variation in relative humidity is a factor of fundamental biological significance.

Such, briefly, are the salient physical features of the region. It is evident that the plants which inhabit it must derive their water from rainfall, not only meager in quantity, but irregular in local and temporal distribution, and which fails, to a great and highly variable extent, to penetrate into the substratum. This moisture they must draw from a soil irregular in depth and texture and in water-holding capacity and sometimes highly impregnated by mineral salts. Saturation of the soil for brief periods is followed by a condition of complete dryness in most localities. In others, deep-rooted species may obtain water throughout the year. All their physiological processes must be carried out under widely ranging temperatures. Their aereal shoots are exposed to intense insolation in an atmosphere which is generally dry, and often moving at a considerable velocity. Brief periods of high relative humidity may alternate with those of excessive evaporating power of the air.