532 COKEELATION OF DISTRIBUTIONAL FEATURES. 



shown on the same map with the cumulative occurrence of the ever- 

 green broad-leaved trees (plate 3), in order to demonstrate the manner 

 in which the former group fills the break in the distribution of the 

 latter. The region of maximum occurrence is in extreme southern 

 Texas, while 5 or more species are found in the Texas Semidesert, 

 along the lower Rio Grande, and in southern Arizona. 



The maximum occurrence of microphyllous trees is in a region with 

 a frostless season of 300 days or more, and the areas with 5 or more 

 species are confined to regions with a season of from 240 to 300 days. 

 Nowhere does the occurrence of as many as 5 species encounter any- 

 cold days nor a mean temperature for the coldest fortnight that is 

 lower than 40° (or for the largest areas, 50°) . The physiological sum- 

 mation of temperature is above 15,000 for 5 or more species and above 

 20,000 for 10 or more. 



The moisture ratio for the region of maximum occurrence of micro- 

 phyllous trees falls rapidly from 0.80 on the Gulf coast to 0.40 in the 

 interior, and for the region of 5 or more species it falls from values 

 above 0.60 to values below 0.20. Dry periods of 75 days are experienced 

 on the Texas coast, of 100 days and more along the Rio Grande, and 

 of 250 days and more in southern Arizona. 



In spite of the occurrence of the maximum number of trees of this 

 type in the relatively moist climate of extreme southern Texas, as 

 many as 5 species of the group are able to withstand the extremely 

 arid conditions of the desert near the mouth of the Colorado River. 

 The encountering of rainy periods of 25 days (or of 50 days in Texas) 

 appears to limit the western and eastern occurrence of 5 or more species. 



While the microphyllous trees are confined longitudinally by mois- 

 ture conditions, their latitudinal range is restricted by temperature 

 conditions. The continuity of the region of 5 or more species from 

 California to Texas is broken only by the highlands of the Conti- 

 nental Divide near the Arizona-New Mexico boundary, where all 

 temperature conditions are relatively severe. 



Eastern Deciduous trees (fig. 43). — The 13 most common and wide- 

 spread deciduous trees of the eastern United States are all found in a 

 region stretching from Massachusetts and New York to Delaware and 

 Ohio, and southward to northern Alabama. The region with 8 or 

 more of the 13 embraces southern New England, southern Michigan, 

 eastern Iowa, the whole of Arkansas, and nearly the whole of South 

 Carolina. The region with from 1 to 7 species embraces all the remain- 

 ing States east of the one-hundredth meridian, barring southern 

 Florida (see plate 5). 



An effort has been made to show the climatic extremes of these 

 three areas graphically and in such a way as to make their direct com- 

 parison easy, and the result is shown in figure 43. In this figure we have 

 a rough means of determining the optimum conditions for deciduous 



