64 



THE DATE PALM. 



of the whole day, and the curves lowest in the scale show the mean 

 minimum temperature. In every case the dotted lines represent the 

 record for Biskra and the unbroken lines that for Salton. The months 

 of the year are marked off horizontally and the degrees of heat are 

 shown by the height of the curve from the base. The temperatures 

 can be read off in Fahrenheit at the left and in Centigrade at the right. 

 The heavy black horizontal line represents the zero point for the flow- 

 ering and fruiting of the date, 18 C. (64.4 F.). 



It is evident from the first glance at the curves that Salton is much 

 hotter than Biskra and that the daily range of temperature is much 

 greater, and as a consequence that the mininum is lower in winter, 

 at the same time that the mean temperature is higher. 



FIG. 



7. Curves representing the average maximum, mean, and minimum temperatures at Salton, 

 Cal., and at Biskra, Algeria. 



The zero point for the date palm, 18 C. (64.4 F.), is reached by the 

 mean temperature about April 5 at Biskra, while it is passed fully 

 three weeks earlier at Salton, or about March 12. The mean daily tem- 

 perature rises more rapidly at Salton than at Biskra, which brings about 

 the result that the period when the date palm flowers, which accord- 

 ing to Fischer's calculations^ is when the mean daity temperature is 

 between 20 and 25 C. (68-77 F.), extends from about April 20 to 

 May 20 at Biskra, whereas at Salton it extends from about March 23 

 to April 20; that is to say, if Fischer's calculations are correct, the 

 date palm will flower about a month earlier at Salton than at Biskra, 



Fischer, Th. Die Dattelpalme, ihre geographische Verbreitung und culturhistor- 

 ische Bedeutung. In Petermann's Mitth., Ergiinzungsheft No. 64, Gotha, 1881, p. 51. 



