554 CORRELATION OF DISTRIBUTIONAL FEATURES. 



with much more favorable conditions, particularly with respect to the 

 longest dry periods, daily mean evaporation, and normal humidity. 

 The distributional area of this plant contains few climatological sta- 

 tions, and it is certain that the conditions actually met by it include 

 the lowest values of normal daily mean precipitation, the longest 

 normal dry periods, and the lowest mean total precipitation o the 

 year. In the vicinity of Death Valley and in the arm of the Mojave 

 Desert which stretches southeast toward the Colorado River are to be 

 encountered the most arid areas in the United States, and in them 

 Covillea is one of the most ubiquitous plants. 



The narrow amplitude of the moisture ratios indicates here, as in 

 the case of Artemisia, that conditions of greater general favorableness 

 with respect to this condition are either directly inimical to Covillea, 

 or else that they are accompanied by associated conditions which are 

 of importance in limiting its range. The extremely narrow amplitude 

 in number of days in the longest rainy period signifies that there is a 

 great importance in this factor, probably having to do with the effect 

 of prolonged wet periods in making the conditions of soil aeration 

 injurious to Covillea. The distributional boundary lies close to the 

 northern limit of the area in which there are no cold days, and this 

 condition, with its associated conditions of low winter temperatures, 

 is undoubtedly of great importance in controlling the northward 

 Umitation of the plant. Covillea appears, in brief, to be confined to a 

 region in which there are no prolonged periods of rain and no severe 

 periods of cold. The narrow amplitude of the physiological summation 

 of temperatures for the frostless season indicates importance for this 

 condition, but the fact that Covillea is an evergreen points to the low 

 temperatures of winter having a greater significance in its limitation 

 than do the summations for the growing season. 



TCMKRATUnC 



D«rs IN Normal FnoBTLKS Sc*«ON (F. S.) I 



Hot 0»vs. F. S. CIZ 



Colo Day*. F. S. EZ 



Phvsiolosical Summation, F. S. I 



Normal DAikV Mcan, coldmt 14 days or Viar I 



Normal Daily Mean, Yur I 



Prccipitation 



NoRMAi. Daily Mcan, F. S. I 1 



Days in lonocst Normal Rainy Pcriod, F. S. ■ 



Days in lonocst Normal Dry Pcrioo. F. S. I 



Mian Total. Year C 



Evaporation 



Daily Mean, 1887-0, F. S. L 



Moisture Ratio* 

 Normal P/E, F. S. ■ 



Normal ir/E, F. S. ^ 



Normal P/E, Year ^ 



Humidity 

 Normal Mean. F. S. Bi 



Sunshine 

 Normal Daily Duration. F. S. I— 



Moisture-Temperature Inoiccs 

 Normal P/E x T, F. S., Physiological Method QI 



Fig. 61. Climatic extremes for Covillea tridentata. 



