CORRELATION OF DISTRIBUTIONAL FEATURES. 549 



Serenoa serrulata (fig. 57). — Only the extreme edge of the southern 

 Atlantic Coastal Plain is occupied by this palm, from southern South 

 Carolina to the eastern border of Texas. It occupies the warmest and 

 moistest portion of the area which has just been stated to be favorable 

 for the development of evergreen broad-leaved trees, with which it 

 may be classed. Considering the small area occupied by Serenoa, it 

 encounters a wide range of conditions in both temperature and pre- 

 cipitation, together with narrow ranges of evaporation, humidity, and 

 the moisture ratios. It encounters a frostless season of 231 days or 

 more and no cold days, in our sense. Its limit coincides closely with 

 the line of 50° for the normal mean temperature of the coldest 14 days 

 of the year, although the palm does not follow the region of these tem- 

 perature conditions into southern Texas. The encountering of the 

 conditions expressed by a moisture ratio of 1.00 appears to be respon- 

 sible for the westward limitation of a plant which is elsewhere con- 

 trolled by temperature conditions. 



TtMniuTuiic 



DAT* IN NORHAl FkOITUM SUaOX if. S.) 



Hot DAf*. F. S. 

 Cold Dats, F. S. 

 PHTsioioaiCAt^ Summation, F. 8. 



NollHAt. OAlkT MCAN, COLBCST 14 OATI Or VCAR 



Normal Oailt Mian. Vcar 



PRKCinTATiaN 



Normal Dailt Mcan, F. 8. 



DATS IN LONOIST NORMAL RAINT PC*IO0, F. S. 



DATS IN LONOtST NORMAL DRT PutlOO, F. S. I I 



Mian Total, Yiar 



cvaroration 

 Oailt Mcan. 1007.8, F, •, 



MoiRTURC Ratio* 

 Normal p/e, F. S. 

 Normal ir/E, F. S. 

 Normal P/E, Vcar 



Humidity 

 Normal Mcan, F. S. 



sunshinc 

 Normal Oailt Duration, F. S. 



Moisturc-Tcmpcraturc Indiccs 

 Normal P/E < T, F. S., Pmtsiolosical Mctmod 



Fig. 57. Climatic extremes for Serenoa serrulata. 



Cephalanthus occidentalis (fig. 58). — The distribution of Cepha- 

 lanthus is remarkable from the fact that it is one of the very few woody 

 perennials of the southern United States which has a nearly trans- 

 continental distribution. It is very infrequent west of the one-hun- 

 dredth meridian, apparently being absent from New Mexico, but 

 appearing again in southern Arizona and in the San Joaquin Valley of 

 Cahfornia. A' distribution which is so extensive both latitudinally and 

 longitudinally naturally encounters a wide amplitude of conditions, 

 both \vith respect to temperature and moisture. None of the ampU- 

 tudes of conditions for Cephalanthus are sufficiently narrow to give any 



