ATMOSPHERE AND CLIMATE, IVK. \:> 



zero, plants are subject to an attack of frost. With most 

 plants, the colds occurring in Egypt are insufficient to d( > 

 more than put a temporary stop to growth ; but even here 

 such crops as potatoes and tomatoes are often frosted. 

 If a frosted plant is very slowly heated, the cell is 

 able to take up water, and quickly recovers from its 

 frosting. When quickly heated by the direct rays of the 

 sun. the water is evaporated very rapidly and the frosted 

 plant soon dies. Hence plants which are protected from 

 the early rising sun or plants which have just been 

 watered, seldom suffer from the cold. Cold weather in 

 autumn often prevents the full ripening of the cotton 

 crop, and frost causes damage to the sugar-cane by changing 

 the crystallisable cane sugar into non-crystallisable sugars. 

 While the growth of a crop depends upon certain limits 

 of temperature, the full ripening of the crop further 

 depends upon the crop growing a certain number of days 

 at a certain temperature. If the number of days between 

 the sowing and ripening of a crop is multiplied by the 

 average temperature prevailing during that time, the result 

 is the day -degree units of heat required for the ripening 

 of the crop. The number of day-degree units required 

 i\ any particular crop is fairly constant. Thus berseem 

 sown on October 13th gave a crop of 10*5 tons in 48 days 

 with an average temperature of 23*7 C., and a crop sown 

 on November 12th gave 10'G tons alter 74 days of an 

 average temperature of 14' 3 G. The first crop took a much 

 shorter time to mature, but received just about the same 

 day-degree units of heat, namely 1037 against 1065 units.* 



* Tit IK method of measuring the ltet received only applies 'if the nuueiffium and 



tn'i n 'nun in tfiiificriitn 1'i^ft. of irlt'u-li an r/w/v/f/r 9 /.v fallen. <irc nui tuo I'nr t/pfirt. "KDS. 



