CORRELATION OF DISTRIBUTIONAL FEATURES. 551 



physiological summation is as low as 2,100, and where the normal 

 daily mean of the coldest 14 days of the year reaches 11°. Some of 

 these conditions may, indeed, be exceeded at the extreme limit of this 

 plant in Canada. 



Much more complete information regarding Cephalanthus will be 

 needed before it is possible even so much as to suggest some of the 

 conditions that may be keeping it from spreading into other parts of 

 the United States, if indeed it is not now making secular movements 

 to the west and north. A knowledge of its relative abundance in 

 different parts of its area and of the character of ^he habitats which it 

 occupies throughout the edge of its distribution might aid in solving 

 the problem which it presents. It seems to be a plant that would be 

 well worthy of a thorough ecological study. 



Decodon verticillatus (fig, 59). — This is an aquatic or palustrine shrub 

 found throughout the States east of Wisconsin, Missouri, and Louisi- 

 ana, with the exception of southern Florida. The temperature condi- 

 tions which it encounters are almost as wide in amplitude as those 

 encountered by Cephalanthus. The amplitudes of the moisture condi- 

 tions are somewhat narrower than in the case of that plant, but the 

 only condition that can be regarded as having a significantly narrow 

 ampUtude is the number of days in the longest dry period, which 

 reaches a maximum value of 78 days. The isoclimatic lines for the 

 latter condition indicate that the area of Decodon is roughly limited 

 by the Une for 25 days in the longest dry period. Another line approxi- 

 mating the limit of Decodon is that for a mean annual precipitation of 

 30 inches, the distribution of the plant extending over the region in 

 which the rainfall is greater than that amount. 



TCMPCRATURC 



D*rs IN NoKMAi FdOSTitss ScusoN (F. SJ m 



Hot Days, F. S. H 



Colo Days, F. S. B 



Physiological Smmmation. F. S. 9 



Normal Daily Mean, colocst 14 days of Year IZ 



Normal Daily Mean. Year fl 



Preciritation 



Normal Daily Mean, F. S. CI 



Days in longest Normal Rainy Period, F. S. CI 

 Days in longest Normal Dry Period, F. S. 



Mean Total, Year C 



Evaporation 



Daily Mean. 1887.8, F. S. C 



Moisture Ratios 



Normal P/E, F. S. C 



Normal rr/e. F. S. C 



Normal P/E, Year C 



HUMlOfTT 



Normal Mean, F. S. C 



Sunshine 

 Normal Daily Duration, F. S. C 



Moisture-Temperature Indices 



Normal P/E > T, F. S., Physiological Mcthoo C. 



Fig. 69. Climatic extremes for Decodon verticillatus. 



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