16 



BULLETIN 271, U. S. DEPARTMENT OP AGRICULTURE. 



The early maturity of all the Sudan varieties is an indication of a 

 somewhat definite number of available heat units required to mature 

 the crop, the high temperatures of the early part of the growing 

 season hurrying them along more rapidly than, for example, those of 

 Khargeh and Dakhleh. The summation of 4,708 heat units above 

 64.4° F. shown at Merowe for the months of May to October, inclusive, 

 to be reduced to what the date crop has actually required should have 

 subtracted from it the summation above 64.4° F. of the month of 

 October and half of September, or 1,096 units, leaving 3,612. Some- 

 thing should be added, however, for the excessive heat of Dongola in 

 March and April over stations farther north. 



If we regard the Dongola season as beginning 6 weeks earlier, 

 through the more rapid push of the last half of March and the month 

 of April, we can get a 6-months' summation of heat units at the time 

 the trees actually use them, which will be a fairer index of the crop 

 requirements than that in Table I". For this purpose Table II has 

 been prepared. 



Table II. — Mean monthly temperatures and summation of heat units above 64-4° F. 

 for the growing period of the date at Merowe, Dongola Province, from March 15 to 

 September 15. 



Month and days. 



Mean. 



Above 

 64.4° F. 



Days. 



Summa- 

 tion. 



Mar. 15-31 - 



Apr. 1-30 



May 1-31 - 



June 1-30 



July 1-31 - - - - 



Aug. 1-31 



Sept. 1-15 - 



Mean, 7 months 



Total, 6 months (middle of March to middle of September) 



76.82 

 85.46 

 89.87 

 92.21 

 90.23 

 89.33 

 91.49 



87.77 



°F. 



12.42 

 21.06 

 27.47 

 27.81 

 25.83 

 24.93 

 27.09 



15.5 



30 



31 



30 



31 



31 



15 



192. 51 

 631.80 

 789. 57 

 834.30 

 800. 73 

 772.83 

 406.35 



This calculation of the summation reduces the heat units used by 

 about 300, but still shows that the Dongola dates are produced under 

 higher heat conditions, as well as lower humidity conditions, than 

 any date crop of which we have record. 



A review of Tables I and II shows clearly the necessity of estimating 

 the time-heat requirements of the dates of different varieties, not 

 arbitrarily from May to October, inclusive, as was the best that could 

 be done with the imperfect information at hand when such studies 

 were begun, but from some definite period in the history of the devel- 

 opment of each variety, as exemplified by individual trees in typical 

 localities. Atmospheric moisture, soil character, and soil moisture 

 will be important factors to be taken into account. Probably the 

 date of pollination is as definite a time as could be selected at which 

 to begin the estimate of heat requirements. 



