CORRELATION OF DISTRIBUTIONAL FEATURES. 579 



this group may be very satisfactorily described in terms of the tem- 

 perature provinces alone, but neither temperature nor moisture 

 conditions alone are adequate to bring out any climatic correlations 

 that may suggest an explanation as to why the broad-leaved trees 

 occur only west and east, while the microphyllous ones occur in an 

 intermediate region. 



Comparing plate 3 with figure 19, it becomes at once apparent, 

 however, that the broad-leaved trees occur mainly in the very warm 

 humid province (Florida and Louisiana) and that they occur in smaller 

 number of species in the warm and medium humid and in the warm 

 and medium semihumid provinces. Especially in California they occur 

 in the warm and medium semihumid provinces. The microphyllous 

 trees occur mainly in the warm and medium arid, and they occur as 

 fewer species, especially in Texas, in the warm and medium semiarid. 

 The two-dimensional provinces are thus fairly satisfactory in corre- 

 lating the distribution of these two groups of trees with the two 

 primary groups of climatic conditions. 



The distribution of the eastern deciduous trees (plate 5) must be 

 described, first, in geographic terms. They occur east of the Une 

 joining Hudson Bay with the Gulf of Mexico, and they are absent 

 from all climatic provinces west of this line. In the area thus demarked 

 their area of greatest density lies within the cool and medium semi- 

 humid provinces. This area does not correspond to all of the area of 

 these provinces, but it does not significantly overlap any of the humid 

 provinces. It occupies about the eastern half of the cool semihumid prov- 

 ince (Kentucky to Massachusetts), and the northern marginal portion of 

 the eastern half of the medium semihumid province. For the most part, 

 these trees may be said to occur in greatest number of species with the 

 more humid and warmer conditions of the cool semihumid province. 



The distributional area of Liriodendron tulipifera, one of the eastern 

 deciduous trees, is also not possible of description in terms of our 

 climatic provinces alone. It must first be stated that this tree occurs 

 only in the East. Its area occupies most of the eastern half of the 

 cool semihumid province, not reaching the boundary of the cool humid 

 on the north and extreme northeast. It occupies about the eastern 

 two-thirds of the medium semihumid and all of the medium humid 

 provinces. It also occupies the eastern lobe of the warm semihumid 

 and a portion of the warm humid (Georgia, etc.). It does not extend 

 into the very warm temperature province. 



The area of greatest frequency for this tree, which may be considered 

 as its geographic and climatic center of distribution, lies practically 

 in the center of the distributional area just described. This smaller 

 area may be defined as occupying the southeastern triangular lobe of 

 the cool semihumid province and the northern half of the central por- 

 tion of the medium semihumid, which adjoins that triangular lobe at 

 the southwest. Thus, this center of distribution occurs with cool- 



