CORRELATION OF DISTRIBUTIONAL FEATURES. 499 



are high or low as contrasted with conditions in other parts of the 

 United States, and to observe whether the ampHtude of each condition 

 is wide or narrow. The last has already been mentioned as a valuable 

 means of discovering the climatic conditions which are most critical 

 in controlling the distribution of a given vegetation or plant. 



It will be fruitful to discuss these diagrams in connection with a 

 comparison between the vegetational boundaries and the isoclimatic 

 lines of the corresponding plates. In this manner it will be possible 

 to test out the indications given by narrow amplitudes of the condi- 

 tions (short blocks in the diagrams), and not only to discover which 

 of the various conditions appear to be the most potent in controlling 

 distribution, but also to find the particular intensity of the condition 

 which seems to be critical in each case. 



Desert (fig. 27) . — Wide amplitudes are exhibited by the Desert with 

 respect to all of the temperature conditions, the number of days in the 

 longest normal dry period of the frostless season, and also the daily 

 mean evaporation for the frostless season. Narrow amplitudes are 

 exhibited by the number of days in the longest normal rainy period 

 of the frostless season and by the moisture ratios and sunshine dura- 

 tion (plate 69). 



An examination of the 6 plates showing the isoclimatic lines for the 

 temperature conditions used on this diagram (plates 34, 35, 36, 40, 

 43, 45) will discover that numerous lines cross the Desert region in a 

 northwest-southeast direction, indicating, as we have already been 

 prepared to find, from the wide amplitudes in figures 21 and 22, that 

 the Desert possesses a wide range of temperature conditions, which 

 may also be found to the eastward in three or four other vegetations. 



0*y« IN NOflMAL FnOSTLCtt SCABON ff. S.i 



Hot Days, F. S. 



Colo Days. F. S. 



Physiological Summation, F. 5. 



Normal Daily Mean, coldest 14 days or Year 



Normal Daily Mean, Yean 



Precipitation 

 Normal Daily Mean, F. S. 

 Days in longest Normal Rainy Period. F. S. 

 Days in longest Normal Dry Period, F. S. 

 Mean Total, Yeah 



Evaporation 

 Daily Mean. 1887-8, F. S. 



Moisture Ratios 

 Normal P/Z, F. S. 

 Normal rr/E, F. S. 

 Normal P/E, Year 



Humidity 

 Normal Mean, R S. 



Sunshine 

 Normal Daily Duration, F. S. 



Moisture-Temperature Indices 

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



I — ^^ 



Fig. 27. Climatic extremes for the Desert. 



