CORRELATION OF DISTRIBUTIONAL FEATURES. 537 



The graphs showing the cUmatic extremes for Bulbilis (fig. 45) have 

 been drawn in such a way as to show the maximum and minimum 

 values for the center, only such maximum and minimum values for 

 the subcenter as lie outside those for the center itself, and only such 

 extreme values for the fringe as lie outside those for the subcenter. 



The ecological center exhibits several climatic conditions with 

 narrow amplitudes, notably the number of hot days, the physiological 

 summation, mean annual precipitation, evaporation, humidity, and 

 the moisture ratios. The position of the isoclimatic lines for the 

 physiological summation would indicate a somewhat wider amplitude 

 (5,000 to 15,000) for this condition than that based upon the readings 

 of the minimum and maximum stations and given in table 71. The 

 widest amplitudes are those of the number of cold days, the daily 

 mean temperature, and the daily duration of sunshine. For the 

 ecological center, then, we may state that the amplitudes are narrow 

 for all of the moisture conditions and are partially narrow and partially 

 wide for the temperature conditions. 



An examination of the graphs for the climatic extremes of the sub- 

 center and fringe shows that the amplitudes are wide for all of the 

 temperature conditions, even for those that show narrow amplitudes 

 for the center. With respect to the moisture conditions, however, the 

 amplitudes of the subcenter and fringe are not so greatly in excess of 

 those for the center, except in the cases of evaporation and humidity. 

 The moisture ratios show narrow amplitudes even for the fringe. 

 These facts indicate that the location of the Bulbilis areas as a whole is 

 chiefly determined by moisture conditions, and this is particularly 

 true of the center. The position of the zones of abundance is deter- 

 mined on the east and west by small but significant differences in the 

 moisture conditions, and on the north and south by temperature 

 conditions. 



2. SPECIES. 



The climatic extremes for all of the individual species treated in the 

 following pages will be found in tables 77 to 151. Some 31 species 

 have been selected from a total of 75 for the presentation of the 

 climatic extremes in graphic form (figs. 46 to 74). These have been 

 chosen so as to represent all types of plants and all types of distribution 

 represented among the larger number. The relation of climatic con- 

 ditions to the distribution of these species will now be discussed. 



Tsuga heterophylla (fig. 46). — This tree occupies an area in which the 

 conditions are similar to those of the Northwestern Evergreen Hygro- 

 phytic Forest, with differences due to the extension of the limits of 

 Tsuga into northern Idaho and Montana, well to the east of the Hj^gro- 

 phytic Forest. The number of cold days in the frostless season endured 

 by the easternmost individuals of this species reaches a maximum of 



