HEAT REQUIRED TO MATURE FRUIT. 67 



The records taken at Ayata, rt Algeria, in the Oued Rirh country, 

 are of particular interest. The Deglet Noor date is there grown 

 largely for export and the meteorological observations are taken in an 

 oasis largely planted to this variety. Even here in the interior of the 

 Sahara Desert (see map, PI. II, p. 76) the summers are frequently too 

 cool to permit this choice date to ripen properly. From three years' 

 observations it is considered that about 2,000C. are required to ripen 

 the Deglet Noor date satisfactorily. At Biskra the Deglet Noor is 

 grown, but does not attain the superatively good quality which has 

 made the dates of the Oued Rirh famous. 6 It will be noticed that 

 Phoenix is somewhat cooler than Biskra, making it doubtful whether 

 this date will ripen there in ordinary seasons. On the other hand, 

 there can be no doubt about the Salton Basin stations being hot enough 

 to bring Deglet Noor dates to maturity, even at Indio, in the northern 

 edge of the basin, and at Imperial, while at Salton the sum of heat 

 during the coolest summer recorded there was greater than the average 

 sum for Tougourt, and almost the same as the maximum sum for an 

 exceptionally hot summer at Ayata, when the Deglet Noor matured- 

 perfectly. There can then be no doubt that the Deglet Noor date will 

 ripen fully in the Salton Basin, even when the season is exceptionally 

 cool. The importance of this demonstration can scarcely be overesti- 

 mated, since it renders it possible to establish in America the culture 

 of this choice date, the most expensive of dried fruits, with certainty 

 of success. 



The date palm requires very high temperatures, very much higher 

 than those recorded by thermometers exposed in the shade, and to 

 measure accurately its heat requirements it will probably be necessary 

 to devise a thermometer which can be exposed to the sun and which 

 will indicate the temperature reached by the leaves. Accordingly a 

 summation of the maximum temperature was made for the days from 

 May 1 to October 31, which it is thought will give a better idea of the 

 adaptability of a climate for date culture than does the sum of the 

 daily mean temperatures. In making this summation 18 C. (64.4 F.) 

 was taken as the zero point, as in the preceding table, and when the 

 daily minimum fell below that point a deduction was made for the 

 temperatures below the zero point, where they were considered as 

 being a setback c and as preventing the observed maximum tempera- 

 ture from causing the growth or development it would otherwise have 

 done. 



Holland, Georges. Hydrologie du Sahara algerien, p. 416. 



& In the oasis of Chetma, near Biskra, the Deglet Noor date is said to ripen per- 

 fectly, thanks to the warm spring water with which the oasis is irrigated (see p. 49). 



c For example, the mean maximum for October at Biskra is 27.4 C., or 9.4 

 above 18 C.; the mean minimum is 16.2 C., 1.8 below 18 C. Now 9.4 is 83.93 

 per cent of the total daily range of 11.2, and so instead of counting 31 X9.4=291.4 as 

 the sum for the month, only 83.93 per cent of this sum is counted, or 244.57 C. 



