550 COERELATION OF DISTRIBUTIONAL FEATURES. 



suggestion of their importance as limiting the plant. Its distributional 

 edge, extending from central Michigan to the mouth of the Colorado 

 River, is 2,300 miles in length, which in itself suggests that very dis- 

 similar constellations of conditions are involved in its limitation in 

 differ.ent sections of this line. In a plant of palustrine habitat it is not 

 surprising to find that the normal moisture conditions of upland 

 habitats have no apparent importance. We find Cephalanthus occur- 

 ring in localities where the moisture ratios approach their minimum 

 values for the United States, and extending from there halfway through 

 the gamut of values for this compound condition. It also encounters 

 extremely low values for the normal daily precipitation and high 

 values for the number of days in the longest dry periods. These con- 

 ditions, however, have no apparent influence on the plant in the 

 habitats where it occurs, although they are probably responsible for 

 the fact that there are very few favorable habitats for it in the localities 

 where these extremes are registered. In the San Joaquin Valley the 

 atmospheric conditions are extremely arid, but there are numerous 

 areas of moist soil, and Cephalanthus is there abundant. It is by no 

 means true that all palustrine or swamp plants are able to withstand 

 extremely arid atmospheric conditions if they are supplied with an 

 abundance of soil-moisture, and only a relatively small number of the 

 plants associated with Cephalanthus in the southeastern United States 

 are found growing with it in southern Arizona and the San Joaquin 

 Valley. 



It is in the eastern half of its range that Cephalanthus encounters 

 the greatest amplitude of temperature conditions. It is there found 

 in localities with no hot days, as many as 137 cold days, where the 



TlnmiMTVKC 



Oat* ih Normal Frostlcs* Season iF. S.) 

 Hot Oat*. F. S. 

 Colo Days, F. S. 



Phvbiolocical Summation, F. S. H 



Normal Daily Mcan, coldut 14 days or Ycar I 



Normal Oah.y Mean, Year ' i 



Precipitation 



Normal Daily Mean, F. S. I ■ 



Days in longest Normal Rainy Period. F. S. ■■ 



Days in lonqcst Normal Dry Period, F. S. C^K 



Mean Total. Year I 



Evaporation 



Daily Mean. ISBT-e, F. S. I 



Moisture Ratios 



Normal P/E, F. S. I^H 



Normal ir/E, F. S. [IB 



Normal P/t, Year ^H 



Humidity 



Normal Mean, F. S. 1 



Sunshine 



Normal Daily Duration. F. S. I 

 i Moisture-Temperature Indices 



Normal P/E > T, F. S., Physiological Method I 



Fig. 68. Climatic extremes for Cephalanthus occidentalis. 



