ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 257 



pap(3rs of Gray*, Lesquereuxf and Henry Engelmann.t 



of 



y 



United States, which find their origin in climatic conditions, 

 in being due to local conditions, more particularly to the 

 water relations. An examination of the limited prairies of 

 the ^lissouri plateau shows them to have been swamp areas. 

 A study of the flora renders apparent the existence of 

 three sharply defined physiographic regions, viz., the upland, 

 the lowland and the river, in each of which an apparent 

 equilibrium has been reached. Modifications of these regions, 

 either the result of the influence of man or the action of 

 natural forces, forms connecting hnks between them. Such 

 are the swamps, the ravines, the sandbanks. In some, such 

 as in the older ravines, changes in the apparent equilibrium 

 are so slow as to be almost imperceptible. Here the sequence 

 of events must be arrived at through analogy. In others, 

 and this is true especially for the swampy regions, the changes 



may 



most im 



part. Under the influence of man, the components of the 

 original flora have undergone certain changes. Species have 

 disappeared, others not only have taken their places but are 

 attempting to take the place of others. The plants so active 



mai 



more 



a small percentage of the original flora will disappear through 

 their influence. The immigrants are limited chiefly to terri- 

 tory now under the influence of man. Under this influence 

 the entire flora will not be destroyed, for in portions set 

 aside for sjtecial purposes the original flora will persist at 

 least until such a time as the entire region shall have 

 reached base level. 



* Gray, A. Forest Geography and Archeology. Am. Journ. Sc. and 

 Arts. 3 ser. 16 : 85. 1878. — Geographic et Arch^ologie Foresti^res. 

 Ann. Sc. nat. 6 s6r. Bot. 7 : 126. 1878. 



t Lesquereux, L. On the Origin and Formation of the Prairies. 



Gcol. Surv. 111. 1 : 238. 1866. 



X Engclmann, H. Remarks Upon the Causes Producing the Different 

 Ciiaracters of Vegetation Known as Prairies, Flats and Barrens in South- 

 ern Illinois, with Special Reference to Observations made in Perry and 

 Jackson Counties. Am. Jour. Sci. and Arts. 2 ser. 36 : 384. 1863. 



17 



