552 CORRELATION OF DISTRIBUTIONAL FEATURES. 



The palustrine and semipalustrine shrub Ilea virginica (see plate 23) 

 has a more restricted range than Decodon, very similar in its general 

 outlines to that of Ilex opaca, and apparently limited, like the latter, 

 by high values for temperature and moisture conditions. 



Artemisia tridentata (fig. 60).— This plant is the dominant element of 

 the vegetation of the Great Basin, and it extends in diminished abun- 

 dance eastward to the edge of the Great Plains, upward into the moun- 

 tains, southward to northern New Mexico and Arizona, and still more 

 sparingly into southern California. The map of its distribution 

 (plate 22) is not drawn to indicate the mountain areas from which it is 

 absent. When these breaks in the distribution are taken into account 

 the shrub is found to occupy an area which is much more homogenous 

 in its climatic conditions than its wide extent would seem to indicate. 

 The amplitude for Artemisia is wide with respect to the number of 

 days in the average frostless season, the number of cold days in the 

 year, the normal daily mean for the coldest 14 days, and the normal 

 daily mean for the year. With respect to the number of hot days and 

 the physiological summation of temperature the amplitudes are nar- 

 row, however. The precipitation conditions also exhibit narrow ampli- 

 tudes, with the exception of the number of days in the longest dry 

 period. The amplitude of evaporation and humidity conditions is 

 made to appear wide because of the extension of its area to the Pacific 

 coast, where the plant is extremely rare, its most westward abundant 

 occurrence being in the Cuyamaca Mountains, 40 miles from the coast. 

 The values for the moisture ratios exhibit narrow amplitudes, reaching, 

 in two cases, the lowest values for the country. 



It is manifest that the southern limitation of Artemisia is not solely 

 a matter of its inability to withstand extremely arid conditions, since 



TCMPCII»TUKC 



D«T* IN Normal Frostli** Scason (F. S.) 



Hot Oavs, F. S. 



CoLB Days. F. S. 



Phvsiolooical Summation, F. S. 



Normal Daily Mean, coldest 14 pats of Year C 



Normal Daily Mean, Year C 



Preciritation 

 Normal Daily Mean, F. S. ■ 



Days in lonoest Normal Rainy Period, F. S. ■ 

 Days in longest Normal Dry Period, F. S. C 

 Mean Total, Year 



Evaporation 

 Daily Mean, 1887-S, F. S. [ 



Moisture Ratios 

 Normal P/E, F. S. 

 Normal ir/E, F. S. 

 Normal P/E, Year 



Humidity 

 Normal Mean, F. S. 



Sunshine 

 Normal Daily Duration, F. S. 



Moisture-Temperature Indices 

 Normal P/fe X T, F. S., Physiological Metmoo 



Fig. 60. Climatic extremes for Artemjsia tridentata. 



