= 6 BULLETIN 271, U. 8. DEPARTMENT OF AGRICULTURR. 
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 Dongols 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 IT has 
been prepared. 
TasLe I1.—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. 
= Above | Summa- 
Month and days. Mean. 64.4° I Days. | cane 
O78. DS 
Mae seo (atc ienren Cee ete nn Wee c: or tel etait 76. 82 12. 42 1525, | 192.51 
ADE a BOE Re 3-2 eee Ea a ee cee ee 85. 46 21.06 30 - | 631. 80 
MPa yO ete Seah = sees ae ey oie En See Sale nea rT 89.87 27.47 ol | 789.57 
ifota(a eS oie tees aioe Weaeeseg te a eren ee Ee ee emt ws 2 ae se ODO 27.81 30 | 834.30 
sitbyml= siya ee are Se eas Nope See eee Sete eee 90. 23 25.83 Suk 800. 73 
ING ee ale Senn BO Sine esac eek he ie SP aes ee RE 89. 33 24.93 BI | Oh aeRDERS 
SG Olin Taos oh neces age sso esceoecss cesses sobsestcsansnesegase 91.49 27.09 Tig. | 406.35 
IMiGani= FeMMONbHS 2ke yo kee eee ee Be ane et ie ee ee A Ae es pes eee, Sots ieee ee 
Total, 6 months (middle of March to middleof September) .|...-...... jefe. SOc he 4, 428.09 
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. | 
