CORRELATION OF DISTRIBUTIONAL FEATURES. 545 



of the cases already mentioned in which the southernmost Hmit of 

 plants in Florida is not well known, so that it is impossible to state 

 just how far Quercus falcata may fall short of extending into the region 

 of the temperature conditions represented by Key West. The one 

 temperature condition for which Quercus falcata shows a narrow ampli- 

 tude is the number of cold days, due to the fact that its range barely 

 extends into the region in which any cold days are experienced. 



The moisture conditions in the area of this oak are very similar to 

 those of the Southeastern Evergreen Forest, except in the case of the 

 number of days in the longest normal dry period. While the Evergreen 

 Forest withstands a maximum of 182 days (the reading for Key West), 

 Quercus falcata has its maximum at 63 days, at the southwestern edge 

 of its range. The normal dry period, the moisture ratios, and the 

 relative humidity of the frostless season appear to be important con- 

 ditions for the limitation of Quercus falcata. Its distributional area 

 may be defined as the region in which the frostless season is more than 

 180 days in length, the moisture ratio greater than 0.80, the humidity 

 greater than 70 per cent, the normal number of cold days in the year 

 less than 30, and the number of days in the longest normal dry period 

 not more than 63. 



Quercus macrocarpa (fig. 54) . — The area occupied by this oak covers 

 the Northeastern States, omitting the Coastal Plain, and extends as 

 far west as eastern Montana and central Oklahoma. Its southern 

 limit is roughly coincident with the northern limit of Quercus falcata, 

 and its northern limit is in Canada. Quercus macrocarpa is like Q. 

 alba in exhibiting broad amplitudes for nearly all of the climatic condi- 

 tions. It encounters approximately the lower half of the scale of 



TlMPcmtTunc 



Oays in NonM«k Fro*tl(«i Scason 'F. S.) LI 



Hot Davs, F. S. M 



Cold Days, F. S. H 



Pmvsiolocical Summation, F. S. [I 



Normal Oailt Mcan, colokt 14 dats or Ycak ■ 



Normal Dailt Mcan, Year ■ 



PRECtPITATtON 



Normal Daily Mcan, F, S. CI 



Days in lonscst Normal Rainy Pcriod, F. S. d 

 Days in longcst Normal Dry Pcriod, F. S, 



Mcan Total. Ycar (^ 



Evaporation 



Daily Mcan, 1887-6, F. S. [^ 



Moisturc Ratios 



Normal p/e, F. S. CI 



Normal it/E, F. S. I ^ I 



Normal P/E. Ycar rr^"li^^^——T 1 



Humidity 



Normal Mean. F, S. I I^^^^M^^^W— M— H I 



sunshinc 



Normal Daily Duration, F. S. EH^ MBB^ 1 



Moisturc-Tcmrcraturc Indices 



Normal P/E a T, F, S., Pmysiolocical Method I l^^^^^^^^—l i^^^^^T" "I 



Fig. 54. Climatic extremes for Quercus macrocarpa. 



