Jennings : A Botanical Survey of Presque Isle. 399 



Plateau, (3) the Southeastern Conifer Forest, centering in the south 

 Atlantic and Gulf coastal plain, and (4) the Insular Tropical Forest 

 of the southern part of the Florida peninsula, centering in the West 

 Indies." As far as Presque Isle is concerned the first two centers 

 alone need to be considered. 



The Northeastern Conifer Forest center, at least as far as the woody 

 species are concerned, practically includes the eastern part of Mer- 

 riam's Canadian Area together with the northern part of his Allegha- 

 nian Area, and in a general way this forest center may be regarded 

 as the center for our first group of Presque Isle species (A), including 

 altogether one hundred and fifteen species. 



Our second group (B) of forty-two Presque Isle species is very closely 

 co-extensive with Transeau's Deciduous Forest center, including 

 thus the Carolinian Area and the southern extension of Merriam's 

 Alleghanian Area, thinning out in all directions from this region. 



It is thus to be seen that the region in which Presque Isle is located 

 is more or less intermediate in geographical position between these 

 two centers, but the true relations of the different elements as enter- 

 ing into the vegetation of Presque Isle are most evident when the for- 

 mations and successions are individually considered. The intermedi- 

 ate position of Presque Isle is such that variations in the local ecolog- 

 ical conditions of the soil, wind-exposure, etc., may swing the habitat 

 into either the one or the other forest center (climatic or geographic) 

 while in the more permanently intermediate habitats there may be a 

 vegetation composed in part of derivatives from both centers; or, 

 from historical causes the one or the other forest center may even 

 there predominate. 



In general it may be stated that on Presque Isle, proceeding from 

 the physiographically youngest habitat to the oldest, the relationship of 

 the corresponding vegetation swings gradually from the Northeastern 

 Conifer Forest to the Deciduous Forest center, and on the northern side 

 of the peninsula, with greater exposure and coarser soil particles, the 

 northern element persists longer than on the southern side, where these 

 conditions are less pronounced. 



Transeau ^® has shown that the great forest centers are correlated very 

 closely with certain "rainfall-evaporation ratios, " the ratio of total 

 rainfall being largely dependent upon the same conditions of tem- 

 perature, wind velocity, relative humidity, etc., which most largely in- 



86 Transeau, E. N. /. r., 883-8S6. 



