CORRELATION OF DISTRIBUTIONAL FEATURES. 531 



States, where no evergreen broad-leaved species occur. Although the 

 length of the frostless season is evidently a condition of great impor- 

 tance for the rich representation of trees of this type, it is apparent 

 that the temperature conditions of the frostless season itself are not 

 so important as are the conditions insuring a mild winter. In parts of 

 the California coast with a physiological summation of 5,000 to 7,500 

 there are over 10 species of evergreen broad-leaved trees, while in 

 Georgia and Florida the same number of species are to be found in a 

 region with summations of 15,000 to 17,500. In each of these cases 

 the frostless season is between 240 and 300 days in length. In spite 

 of such great differences in temperature summation between regions 

 with frostless seasons of so nearly the same length, we have, on the 

 other hand, an absence of cold days in both regions and daily mean 

 temperatures in both places that are above 45°, or even above 50°, for 

 the coldest 14 days of the year. 



The rapid increase in the total number of evergreen broad-leaved 

 trees encountered in passing from the central Eastern States into 

 peninsular Florida is paralleled by a rapid increase in the number of 

 hot days, by an increase in the physiological temperature summation, 

 by an elevation in the mean temperature of the coldest fortnight to 

 60° and above, and by increasing values for the moisture-temperature 

 index. In none of these conditions does the coast of California 

 approach the high values of southern Florida, except in the case of the 

 mean temperature of the coldest 14 days. 



The long frostless season and the mild winter, which favor the 

 abundance of broad-leaved evergreens, do so only in regions of high 

 moisture conditions. In the Southeastern States the region with 5 or 

 more species exhibits moisture ratios of 1 .00 or above, except in extreme 

 southern Florida. On the Pacific coast the greatest abundance of 

 evergreen broad-leaved trees is in a region with moisture ratios of 

 0.40 to 0.60. This marked difference must be interpreted in connection 

 with the much lower summations of temperature for the frostless 

 season which characterize the Pacific coast. Between coastal Cali- 

 fornia and eastern Texas the number of evergreens rises above 5 only 

 in the mountain ranges of southern Arizona and western Texas, where 

 the local conditions are not elucidated by our climatic data. 



In the correlation of moisture conditions with the cumulative dis- 

 tribution of the evergreen broad-leaved trees, it should be borne in 

 mind that our moisture data are chiefly elaborated for the frostless 

 season, and that the moisture conditions of the winter (even where it is 

 reduced to a length of less than 9 weeks) are surely of great importance 

 to these trees. The low moisture ratios of the frostless season on the 

 coast of California must be interpreted in the light of the fact that 

 the short frost season is there the time of the principal rainfall. 



Microphyllous trees. — The cumulative occurrence of this small 

 group of trees characteristic of the subtropical desert regions has been 



