52 THE DATE PALM. 



loam or a loam. The lands lying near the New River or Salton River 

 beds, or near Mesquite or Salton Lake, can be drained into these 

 lower levels, and in many other places wells may be put down to pro- 

 vide an outlet of drainage water into the great body, of ground water 

 which lie$ from 20 to 50 feet below the surface. Though required 

 for the best growth and successful fruiting of the date palm, drainage 

 is less necessary than for most other trees. Even if the ground water 

 of the Salton Basin rose to within reach of the roots it would not kill 

 the date palm, for, although this ground water is very brackish, con- 

 taining from 0.4 to 0.6 per cent of dissolved salts, and would kill most 

 ordinary plants, it is less alkaline than some of the artesian water used 

 to irrigate flourishing date plantations in the Oued Rirh country in 

 the Sahara (see pp. 86 and 121). 



EFFECTS OF ATMOSPHERIC HUMIDITY AND BAIN ON THE DATE 



PALM. 



An essential requirement of the date palm, in order that it may pro- 

 duce fruit of the best quality, is that the air be very dry during the 

 season when the fruit is developing. Regions having abundant sum- 

 mer rains, and even those having a heavy precipitation in autumn, are 

 unsuited to the profitable culture of this tree, but rains in winter ma}^ 

 be beneficial. It has usually been held that the presence of humidity 

 in the air is directly disadvantageous, but it is probable that the chief 

 action of water vapor in the atmosphere is indirect and results from 

 its peculiar action in screening out the heat from the sun's rays f/ and 

 thereby preventing the temperature from going to the excessively 

 high degree necessary to ripen the fruit properly. The same dry air 

 which allows excessive heating during the day permits an equally great 

 fall of temperature by radiation into a cloudless sky at night and 

 brings about the enormous daily range of temperature characteristic 

 of desert regions. The date palm, however, suffers no check from 

 cool nights, unless the temperature falls below a point somewhere 

 about 18 C. (64.4 F.), and is favored by excessively high temper- 

 atures, which are, indeed, necessary for the production of dates of the 

 highest quality. 



Table 1, on the following page, gives the mean relative humidity at 

 four points where the date palm is grown, for the months of April to 

 September, inclusive. 



Very, Frank W. Atmospheric Radiation. Bui. G, Weather Bureau, U. S. Dept. 

 of Agriculture, 1900. 



