HOT SUMMERS NECESSARY. 63 



HOT SUMMERS NECESSARY FOR THE DATE PALM. 



Date palms require a definite sum of heat in order to mature their 

 fruit properly, but the amount varies greatly for different sorts. In 

 general the very early ripening kinds are watery and unfit for drying, 

 being more like table grapes than like ordinary dates. They can le 

 grown far to the north where the summers are not warm enough to 

 ripen later varieties. The Wolfskill is such a date (see fig. 3, p. 31). 

 The sorts ripening in inidseason can often be dried, but lack the sweet- 

 ness and exquisite flavor of the late sorts, such as the Deglet Noor 

 (see p. 33). The late sorts, and especially the one just named, require 

 enormous amounts of heat in order to ripen properly. The Deglet 

 Noor date is produced in the oases of southern Algeria and southern 

 Tunis, where fortunately there are well equipped meteorological sta- 

 tions whose records furnish a basis for a comparison of the climate 

 there with that of American deserts, so far as records are available for 

 the latter. 



It has been calculated by De Candolle 05 that temperatures down to 

 18 C. or 6-1.4 F. have no effect on the flowering or fruiting of the 

 date palm, and a study of the record sheets of a self-recording ther- 

 mometer kept at Biskra in the midst of a date orchard confirmed 

 the correctness of this assumption. In other words, this relatively 

 high temperature is the zero point for this plant, so far as flowering 

 and fruiting are concerned, though it is able to grow at somewhat lower 

 temperatures. The curves shown in the accompanying diagram (fig. 7) 

 represent in a manner plain to the eye the heat conditions of Biskra, 

 Algeria, in the northern part of the Sahara Desert, in comparison with 

 those of Sal ton, in the lowest part of the Salton Basin.* 



The curves highest up in the diagram represent the mean maximum 

 temperatures, the curves in the middle show the mean temperatures 



a Geographic botanique raisonnee, I, p. 371. 



& The curves for Biskra for maximum and minimum temperatures are based on 

 averages of twelve one-half years' observations by M. Colombo, summari/ed by 

 Snpan (Petermann's Mitth., Vol. 32, 1886, Lit. ber., p. 32); forthe mean temperature, 

 on ten years' observations by Colombo, published by Mareassan (Ann. de 1'inst. riat. 

 agronom., Paris, 1895). The curve for the maximum temperature for Salton is based 

 on the record for two selected years, 1893 and 1899, each having nearly the same sum 

 of heat for the fruiting season, from May to October, inclusive, as the average of the 

 t \\ fl ve years recorded. Prof. Alexander G. McAdie, director of the Pacific coast divi- 

 sion of the Weather Bureau at San Francisco, kindly furnished the records, as yet 

 unpublished, for these years. This curve is smoothed a little and is somewhat lower 

 than the true mean maximum, as it is based on observations taken at 2 o'clock p. in., 

 which do not always give the highest temperature which occurs during the day. 

 The curve for the mean temperature at Salton is based on twelve years' observations 

 published by Professor McAdie. (California Section, Climate and Crop Service, 

 Weather Bureau, February, 1901, p. 4.) The curve forthe minimum temperatures 

 for Salton is not based on any observations, as the minimum temperatures are not 

 available; the mean minimum temperatures are estimated to be as far below the mean 

 temperatures as the mean maximum temperatures are above. 



