CORRELATION OF DISTRIBUTIONAL FEATURES. 397 



each of the 17 leading climatic features as read from the distributional 

 areas of various vegetations and species, as described above. In these 

 a different scale has been used in measuring the length of the range of 

 climate in each of the blocks, and therefore it has been impossible to 

 give on them the figures for the extreme values. The same scale was, 

 of course, used in laying out the range of the same climatic feature on 

 all of the graphs. 



The reading of these graphs or diagrams may be illustrated by a case 

 of each kind. In figure 21 are shown the extremes of the number of 

 days in the normal frostless season for the 9 types of generalized 

 vegetation of the United States, the extreme range, or amplitude, of 

 this feature being from 25 to 365 days. Reference to the tables will 

 show that the values for the Desert region show a minimum of 25 (or 

 in some anomalous stations in the Klamath Lake region) and a maxi- 

 mum of 305. The first block of the diagram shows, therefore, the por- 

 tion of the whole amphtude of this factor in the United States that is 

 to be encountered in the Desert region. It has no reference to the 

 variations in this factor from year to year in the Desert, and gives no 

 indication of the relative proportions of the Desert that are visited 

 by short or long frostless seasons. The second block of this graph 

 shows the minimum, 197, and the maximum, 319, for the Semidesert 

 region. A comparison of this block with the former one gives a means 

 of contrasting the amplitudes of the length of frostless season for these 

 two vegetations. In the seventh block, showing the amplitude for 

 the Southeastern Mesophytic Evergreen Forest, we have a minimum 

 of 131 and a maximum of 365, this being the vegetational area in which 

 is found the highest value for this climatic feature. 



As a rule, the maximum and minimum values are each found only 

 in a single vegetation. In the graph showing the amplitude in the 

 number of hot days (mean daily temperature of 68° or over), however, 

 it will be seen that the minimum (no hot days) is found in 4 vegeta- 

 tions, the maximum in only 1, while the Hygrophytic Evergreen 

 Forest shows no hot days at all in any part of its area. The position of 

 the shaded portion of each block is determined entirely by the values 

 for the absolute extremes in the United States. These graphs merely 

 depict, in other words, the comparative amplitudes of climatic condi- 

 tions in the 9 leading vegetational areas of the United States. 



The graphs shown in figures 27 to 74 are constructed in the same 

 manner as those just considered. Each figure gives a picture of the 

 range of each of the 17 leading climatic conditions for the area in 

 question. A slight familiarity with the extreme values of these con- 

 ditions, as given in table 152, will enable the reader to interpret the 

 graphs, and it will be found much easier to compare the graphs for two 

 species or areas than it is to compare the numerical values on which 

 they are based. 



