CLIMATIC CONDITIONS OF THE UNITED STATES. 233 



age value of D/C is 9.44 and that this value ranges from 7.49 to 10.44. 

 Similarly, the average value of (?/D is 1.60, the range being from 0.65 

 to 2.20, and the average value of G/C is 15.0, the range being from 

 4.8 to 20.0. While the first ratio value D/C shows a geographic varia- 

 tion that is not apparently related to temperature conditions (Living- 

 ston and Livingston, 1913), both of the other ratio values, GjD and 

 GjC, exhibit a variation that is obviously related to temperature, and 

 the charts of these values (not here reproduced) appear very much 

 alike and also very similar to the charts of the summations themselves, 

 as far as the direction of zonation is concerned. 



The main differences between these three summation charts, that 

 require attention at the present time, have to do with the relative 

 magnitudes of the summation or seasonal efficiencies indicated for the 

 various stations. By the remainder method (table 6, plate 38) the 

 seasonal temperature efficiency of southern Florida is about four times 

 as great as that of middle New England. By the exponential method 

 this ratio appears to be about 3.7 (table 7, plate 39), and by the 

 physiological method it is 6.1 (table 7, plate 40). Which of these three 

 ratios most nearly expresses the actual relation between the seasonal 

 temperature efficiency for middle New England and that for southern 

 Florida can not be determined without much more knowledge than is at 

 present available. The ratios just given show clearly that the physio- 

 logical method indicates a much greater range of seasonal temperature 

 efficiency throughout the country than is indicated by either of the 

 "other methods, but whether this greater range is also represented by 

 corresponding differences in plant growth must be left an open question 

 for the present. As has been stated, we follow Livingston in deeming 

 it highly probable, for various theoretical reasons, some of which have 

 been expressed above, that the physiological method of obtaining 

 efficiency summations for temperature will prove of more ser\dce than 

 either of the others. 



Attention should finally be called to the fact that the chart of our 

 plate 40 brings out five zones or provinces of temperature efficiency 

 for plant growth. These zones are somewhat similar to those shown 

 on plate 34, but the present chart is of course much less detailed. 



(F) ABSOLUTE TEMPERATURE MAXIMA. 



The absolute maximum temperatures as given in the Summary by 

 Sections were placed upon a chart and isotherms were drawn for 100° 

 and 110° F. Most of the area of the United States was thus shown to 

 lie between these two lines. It is a remarkable fact, and one that 

 emphasizes the importance of the duration factor in climatology, that 

 there is, on the whole, so httle variation between the highest tempera- 

 tures on record throughout the country. The following list of stations 

 and their highest observed temperatures (according to the Summary) 

 are given here merely as an illustration of the fact just mentioned: 



