43 



river. Scattered species of Austroriparian affinity extend north in 

 ever decreasing" nnmbers, for some 300 miles (500 km.). One of 

 the most conspicuous plants of this nature is Carya illinocnsis, the 

 pecan, which follows the alluvial bottom-land of the Mississippi river 

 as far as southeastern Minnesota. Few species of this group occur 

 on the sand deposits of northern and central Illinois, although a num- 

 ber occupy the sand-bars of the lower Mississippi. 



The southern boundary of the great Northeastern Conifer Prov- 

 ince of the north and northeast passes southeastward across Wis- 

 consin and Michig-an, and numerous species persist south of this 

 line. Definite but isolated associations of Piiius Strohiis and of 

 Larix laricina, each with its usual attendant species, are found in 

 various places in northern Illinois, and many scattered species of 

 northern range, such as Populus treniuloides and Betiila aWa, var. 

 papyrifcra, live in associations of other provinces. Some of them 

 are concerned in the vegetation of the sand deposits. 



Between the Austroriparian Province on the south and the 

 Northeastern Conifer Province on the north there are extensive 

 plains, reaching from the base of the Appalachian mountain system 

 on the east to Nebraska on the west. This area is known as the 

 Deciduous Forest Province, and is occupied, as its name indicates, 

 by deciduous forests, with Qucrciis, Acer, Fraxinus, Tilia, Fagus, 

 Nyssa, Liriodcndron, Aescuhis, and Carya as some of the leading 

 genera. In the eastern part of the province the forest is almost con- 

 tinuous, broken only by minor associations of an edaphic nature. At 

 the west, from Indiana and Illinois to Nebraska, it becomes discon- 

 tinuous, and a portion of the area, becoming proportionately larger 

 westward, is occupied by the prairies. 



The Prairie Province, last on the list, extends in a long strip north 

 and south through the Great Plains at the eastern base of the Rocky 

 Mountains from Texas to Saskatchewan, and an eastward extension 

 passes across Iowa and Illinois into Indiana, sharing the area with 

 the deciduous forests (Pound and Clements, i8p8). Throughout 

 its whole area the dominant vegetation is prairie. 



Each of these four provinces is composed of many plant asso- 

 ciations, wdiich occupy usually definite habitats, and which are related 

 to each other by certain successional trends. In each there are asso- 

 ciations occupying limited areas of extreme environment, and these 

 tend to converge, through the effect of various physiographic and 

 biotic agencies, toward the dominant or climax vegetation of the 

 region. In each of the provinces the successional events in the es- 

 tablishment of the dominant vegetation are relatively simple. There 



