CORRELATION OF DISTRIBUTIONAL FEATURES. 553 



it encounters the lowest normal daily mean precipitation (Reno, 

 Nevada), the highest evaporation (Winnemucca, Nevada), and the 

 lowest moisture ratio (Winnemucca, Nevada). The most trying con- 

 ditions with respect to these three important moisture conditions are, 

 therefore, not found near the southern edge of the distribution of 

 Artemisia, but well within the region of its greatest abundance. The 

 narrow amplitude of the number of hot days suggests that this may be 

 a condition of importance in limiting this plant at the south, and the 

 maximum of 118 hot days for its area is close to the value of the 

 isochmatic line of 120 days, which is seen to approximate the dis- 

 tributional limit in Arizona and Nevada. The amplitude of the 

 physiological summation of temperature is also narrow, and the maxi- 

 mum value encountered by Artemisia is 8,400. The evidence would 

 indicate that these and associated temperature conditions are respon- 

 sible for the southern limit, or else that they cooperate with the low 

 moisture conditions in rendering the deserts along the lower Colorado 

 and Gila Rivers untenable for this plant. 



The eastern limit of Artemisia appears to be set by some of the 

 several moisture conditions which present isoclimatic lines closely 

 paralleling its course. The indications of these correlations are that 

 the plant nowhere encounters a mean annual rainfall of more than 20 

 inches, reaches no areas in which the longest normal rainy period is 

 more than 25 days, nor the moisture ratio more than 0.40, and that it 

 is accustomed to normal longest dry periods of at least 75 days in 

 length. 



It is more than probable that the northern limit of Artemisia is set 

 by conditions similar to those that appear to be responsible for its 

 eastern boundary, with the possible cooperation of low temperature 

 conditions. Low temperatures accompanied by arid atmospheric 

 conditions appear to permit the northward extension of the plant into 

 Canada, but low temperatures accompanied by more humid conditions 

 appear to keep it from the northern Rockies and the Coast Range, as 

 well as from the higher slopes of the Sierra Nevada. 



Covillea tridentata (fig. 61). — The range of Covillea extends from 

 southern Nevada and interior California through southern Arizona 

 and New Mexico to the lower part of the valley of the Rio Grande. 

 The area which it occupies in the United States is less than half of its 

 total range in North America, which extends southward through the 

 deserts of central Mexico. 



No cold days, in our sense, are encountered by Covillea, and the 

 amplitude of the normal daily mean temperature of its area is rela- 

 tively wide. In other respects the temperature conditions present nar- 

 row amplitudes, especially when compared with those for Artemisia. 

 The data for moisture conditions reveal the extremely arid conditions 

 under which Covillea exists, showing that it also extends into regions 



