CORRELATION OF DISTRIBUTIONAL FEATURES. 555 



Silphium ladniatum (fig. 62).— The range of this plant extends 

 from Texas to South Dakota and from Alabama to Pennsylvania, 

 but it is most abundant in the Grassland Deciduous-Forest Transition 

 and in the Grassland. The distributional area is such that the plant 

 encounters only 140 cold days in the frostless season. The amplitudes 

 are relatively wide in all of the temperature conditions and are also 

 wide in all of the moisture conditions except the number of days in the 

 longest normal dry period. The moisture ratios are also relatively 

 narrow in amphtude, the extremes ioTir/E in the frostless season being, 

 minimum 0.47, maximum 1.32. 



The eastern Hmit of Silphium is very roughly approximated by the 

 isoclimatic line of 0.80 for the moisture ratio ir/E, isotherms of 45° 

 and 50° for the annual daily mean temperature, and is closely followed 

 by the isotherm for a physiological simimation of 7,500°. The western 

 limit is closely approximated, at least in part, by the isoclimatic lines 

 of 20 inches annual mean total precipitation, 75 days in the longest 

 normal dry period, 25 days in the longest normal rainy period, and a 

 moisture ratio of 0.40. Although the extreme values of the moisture 

 ratio for Silphium are 0.47 and 1.32, nevertheless the location of its 

 entire range with respect to the isochmatic lines for this condition 

 indicates that the plant is found mainly where the moisture ratio is 

 between 0.40 and 0.80. The northward extension in the area pre- 

 senting this range of conditions is apparently controlled by teni- 

 perature conditions, among which the physiological summation is 

 most important. 



The central location of the range of Silphium ladniatum gives it a 

 distributional edge about 3,300 miles long, with only its southern 

 limitation formed by the ocean. These circumstances make it a par- 



TCHKIHTURC 



Days in Normal rnosTicas St«(ON (F. S.) 



Hot D»»s. F. S. 



Colo Days, F. S. 



Physiolosical Summation. F. 8. 



Normal Daily Mian, eoiocsT 14 oats of Yun 



Normal Daily Mcan, Ycar 



PRCCmTATION 



Normal Daily Mcan, F. S. 

 Day* in lonocst Normal Rainy Period, F. S. 

 Days in lonocst Normal Dry Piriod, F. S. 

 Mcan Totau Yttm 



Evaporation 

 Daily Mcan. 1887 -e, F. 8. 



Moisturc Ratios 

 Normal P/E, F. S. 

 Normal t/E, F. S. 

 Normal P/E, Ycar 



Humidity 

 Normal Mcan, F. S. 



sunshinc 

 Normal Daily Duration, F. S. 



moisturc-tcmpcraturc inoiccs 

 Normal P/E ( T, F. S., Physiolosical Method 



c 



Fig. 62. Climatic extremes for Silphium laciniatum. 



