HEAT REQUIRED TO MATURE FRUIT. 65 



although the zero point for this plant is passed by the mean tempera- 

 ture only three weeks earlier. 



The mean temperature at Biskra usually remains above 18 C. 

 (61.4 F.) from about April 5 until about November 3, some 212 days, 

 or nearly 7 months. At Salton the period having a mean temperature 

 above 18 C. extends from about March 12 until about November 20, 

 or some 253 days, nearly six weeks longer than at Biskra. As has 

 been already stated, the flowering season will probably begin a month 

 earlier at Salton than at Biskra because of the abrupt rise of temper- 

 ature in spring, and as the mean temperature remains above 18 C. 

 (6i. 4 F.) nearly a month and a half longer in autumn, the season is 

 nearly two months and a-half longer, and, moreover, is decidedly 

 hotter throughout. It is evident that if the Deglet Noor date can 

 mature at Biskra it can certainly ripen perfectly at Salton. 



AMOUNT OF HEAT REQUIRED TO MATURE THE DATE. 



The comparison of the sums of the daily mean temperatures gener- 

 ally employed in determining the heat requirements of plants can be 

 made only between regions having a somewhat similar climate, and 

 some botanists have gone so far as to deny entirely that any trust- 

 worthy conclusions as to the development of a plant can be drawn 

 from estimates of its heat requirements. To say that the sum of heat 

 decides when a plant flowers or when its fruits ripen has been held to 

 be equivalent to asserting that the other factors of equally vital 

 importance to the plant, such as the amount and nature of its food 

 supply, the supply of water, the amount of light, etc., have no appre- 

 ciable influence on its development. This criticism doubtless has 

 much force in the case of humid regions, where a variable and capri- 

 cious rainfall greatly influences the growth of vegetation. In rainless 

 deserts, however, where all cultivated plants are watered artificially 

 and where the sunshine is almost uninterrupted by clouds, the sum of 

 heat becomes a factor of predominant importance in the life history of 

 plants, and consequently comparisons between similar desert regions 

 in respect to their adaptability for any given cultures may very prop- 

 erly be made by determining the sum of heat for the critical periods 

 of the plants in question. 



The amount of heat necessary to ripen the fruits of the date palm 

 has generally been calculated by adding together the daily mean tem- 

 peratures during the months when the dates are developing, generally 

 from about May 1 until October 31, six months in all. In this bulletin 

 the sum above 18 C. is counted for greater convenience in making 

 comparisons, though generally the sum is reckoned from C. The 

 table on the following page gives the summation of effective tempera- 

 tures during the fruiting season of the date palm for a number of points 

 in North Africa and in the Southwestern States. 

 13529 No. 5304: 5 



