THE PKAIRIE-PLAINS CLIMAX. 217 



"Still another succession should be mentioned. Where the rocks which 

 disintegrate are sandstone, the vegetation which establishes itself after the 

 lichen formation is not the Ghitierrezia-Artemisia association, but a wire-grass 

 or a bunch-grass vegetation. If the soil formed is exceedingly sandy, the 

 bunch-grass vegetation may become permanently established. If, however, 

 the soil is less sandy, wire-grass vegetation will first become established, and 

 this will be followed gradually by pure short-grass cover. 



"We see, then, that short-grass vegetation represents the final stage of 

 three different successions: First, that beginning with the lichen formation 

 and passing through the Gutienezia-Artemisia association; second, that 

 beginning with the Uchen formation and passing through the bimch-grass 

 and the wire-grass associations; third, that beginning with the blow-out and 

 passing through the sand-hills mixed association, and, with the aid of fires or 

 of grazing, through the bunch-grass and the wire-grass associations." 



Shimek (1911 : 169) has discussed in detail the composition of the prairies of 

 Iowa, and the physical and biotic factors which are thought to cause or control 

 them. The paper is also valuable for the comprehensive bibliographical list. 

 His conclusions are summed up as follows : 



"1. Exposure to evaporation as determined by temperature, wind, and 

 topography is the primary cause of the treelessness of the prairies. 



"2. The prairie flora persists on the exposed areas because it is xerophytic. 



"3. Rainfall and drainage, while of importance because determining the 

 available supply of water in both soil and air, are not a general, determining 

 cause, both frequently being equal on contiguous forested and prairie areas. 



"4. Soils and geological formations are of value only in so far as they affect 

 conservation of water; the porosity of the former determining its power of 

 holding moisture, and the latter often determining topography. 



"5. Prairie fires were an effect rather than a cause, and where acting as a 

 cause were local. 



"6. Seed-dispersal probably largely accounts for the grouping of plant 

 societies on the prairies, but does not account for the presence of the prairie 

 flora as a whole. 



"7. Other assumed causes, such as the bison, the sea, etc., are of remote 

 interest and not to be taken into account in any attempt at the explanation of 

 the prairie as a whole." 



Gates (1912 : 6) has summed up the development of the xerosere of the beach 

 area of lUinois and Wisconsin in the diagram (flg. 8) on page 218. 



Gleason (1912 : 49) has reached the following conclusions as to the origin of 

 prairie and its relation to forest from the study of an isolated prairie grove in 

 central Illinois: 



"The origin of the prairie as a type of vegetation can not, however, be 

 referred to prairie fires as a cause, as was frequently supposed by early authors 

 and occasionally even in recent years. A prairie fire presupposes a prairie, and 

 in prairie fires we have merely one factor which has been of assistance in the 

 maintenance or extension of the prairie in its struggle against forest invasion. 

 In the last half century, since the cessation of prairie fires, the forests have 

 again begun an advance into the prairie, but, as is weU known, their route is 

 chiefly up the streams, and the migration is hmited to a comparatively small 

 number of mobUe species. Because of increasing cultivation, this migration 

 is very irregular and can never lead to any serious modification in the vegeta- 

 tion of the region. 



