CORRELATION OF DISTRIBUTIONAL FEATURES. 399 



degrees Fahrenheit, the length of the period of the average frostless 

 season is expressed by fs. Other abbreviations are self-explanatory. 



In the majority of cases the highest and lowest climatic values for 

 each area have been read from maps bearing the detailed readings of 

 all stations, but in several cases we have used generaUzed maps based 

 on the work of others, for which the detailed data have not been pub- 

 lished. This is true of the normal daily mean temperature and of 

 the mean annual precipitation (lines 46 and 53 of the tables). In 

 these cases we have derived the extremes directly from the isoclimatic 

 lines, often being compelled to use + and - signs to indicate that 

 the extremities of an area lie well beyond or well short of a given 

 isoclimatic line. These signs have also been used in connection with 

 other conditions whenever a scarcity of stations made it appear more 

 accurate to do so. The use of an isoclimatic line rather than a station 

 for the derivation of a reading is indicated by a superscript a in each case. 



In the last table of this series (table 152) are to be found the lowest 

 and highest readings recorded in the United States for each of the 31 

 climatic conditions, together with the names of the stations to which 

 these extreme values correspond. This table affords a means of com- 

 paring the climatic extremes of a given botanical area with the extremes 

 of the entire country. In many cases this comparison has been facili- 

 tated by the preparation of graphs, in which blocks representing the 

 entire range of each condition for the United States have been shaded 

 throughout the portion of the range encountered by a given plant or 

 vegetation. Of the 62 national extremes, 11 are the lowest minima 

 or the highest maxima possible in any region, as the minimum of hot 

 days or the maximum of 100 per cent of dry days in the frostless 

 season. These absolute extremes are printed in bold-faced type in 

 table 152. Several other conditions approach their logically abso- 

 lute values, as the minimum readings of 0.03 and 0.04 for two deriva- 

 tions of the moisture ratio will exemplify. In the majority of the 

 cases, however, the extremes for the United States are exceeded 

 either in Mexico or in Canada as well as elsewhere, as has been said. 

 It is unfortunate that the incompleteness of the basic data on both 

 climate and vegetation have made it impossible to extend the present 

 investigation so as to include the whole land area from the Isthmus of 

 Tehuantepec to the Arctic Ocean. 



