19211 PALMER, BOTANICAL RECONNAISSANCE OF SOUTHERN ILLINOIS 131 



swamp flora reaches its higliest development in southeastern Missouri 

 and northeastern Arkansas. 



To anyone familiar with the flora of the Middle States and accustomed 

 to regard Illinois and the adjacent region as well beyond the limits of the 

 South it is something of a revelation to see such trees as Cypress, Pecan, 

 Swamp Hickory {Carya aqualica). Water Elm {Planera aquatica), 

 Tupelo, Swamp Maple (Acer ruhrum), Spanish (Quercus rubra) and Bas- 

 ket Oak and other southern species growing here in great abundance 



and to a maximum size. 



To the influence of the Mississippi River and its great confluent, the 

 Ohio, which joins it here, is doubtless due the markedly southern char- 

 acter of the flora. WTiile the direct effect of the enormous volume of 

 water carried by these streams upon humidity and temperature, and of 

 the work they are doing and have done in the recent past in creating 

 wide alluvial bottoms and extensive swamps, offers an obvious and suf- 

 ficient explanation of the fact that so many species can maintain them- 

 selves here much beyond the limits of their general northward range, the 

 original conditions and causes that led to their introduction into this 

 salient can only be accounted for by an understanding of the past physi- 

 ographic history of the region. 



Geological evidence has established the fact that until the end of the 

 tertiary period, and probably continuing even somewhat later, a great 

 embayment of the Gulf of Mexico extended northward into this region. 

 At that time the Mississippi River discharged itself into the extended 

 Gulf probably above the present mouth of the Ohio. The presence of 

 this large body of water with its thermal currents pouring in from the 

 tropical seas must have given the region a much warmer and more equable 

 climate than prevails at present. Climatic and physiographic conditions 

 in the lower portion of the Coastal plain were probably quite similar 

 to those obtaining there at the present time; and the forest that came 

 up to the shores of the Gulf Embayment had much in common, both in 

 aspect and floral character, with its modern successor, which there 

 is every reason to believe was directly evolved from it. 



Since the last uplift of the Ozark plateau and the obliteration of the 

 Mississippi embayment, the flora of the Coastal plain has been subjected 

 to vast nuitations; and both the composition and boundaries of the great 

 forest have changed in many ways. There has doubtless been a gradual 

 retreat of the extremely southern species from the central Mississippi 

 Valley, and an extension of the more hardy species over the Ozark region. 

 The causes of the latter movement are not involved with the present 

 discussion and will be taken up in a later paper. 



Obviously the natural tendency of heavy -fruited species, such as the 

 Oaks, Ilickorys, Cypress, Tupelo and Sweet Gum, is to extend their range 

 down stream, as their seeds are taken up and transported by floods and 

 deposited in favorable situations when the waters subside; and while 

 such trees as the Willows, Maples, Ashes and others are more or less 



