160 MISSOURI BOTANICAL GARDEN. 



Regional Associations. 



THE MISSOURI PLATEAU. 



The principal feature of the plateau on the Missouri side 

 of the river is the oak forest formation. Though a large 

 portion of this plateau, for agricultural purposes, has been 

 denuded of its tree growth, there still remain comparatively 

 extensive tracts where the flora is hardly, if at all, influenced 

 by human agency. The woods occupy a belt, in places 

 several miles in width, bordering the cliff's on the western 

 shore of the Mississippi (plate 10). The area covered by them 

 is mainly of a rolling character, with here and there flat sur- 

 faces varying in extent. These are ordinarily free from tree 

 growth and are covered by grasses and herbs. Where these 

 plains are of a comparatively large extent, as occasionally is 

 the case, there is a corresponding change in the ground 

 cover, resulting in a distinct prairie formation, not differing 



materially from that found on the Illinois plateau. 



Intermediate between the forest formation and the prairie 

 formation is the shrub formation. This, as is characteristic 



of temperate regions as contrasted with conditions prevail- 

 ing in the tropics, always forms the transition between the 

 forest and the prairie formation. Naturally, numerous 

 modifications, principally due to physiographical condi- 

 tions, can be noted. So in the forest formation the flora 

 of the dry hilltops varies from that of the lower slopes. Lime- 

 stone glades offer a flora distinctly their owm, shari)ly contrast- 

 ing with that of the well drained, shaded hillsides. Owing 

 to the sheltered conditions, the wanner air currents, the de- 

 position of seeds from the plateau through the agency of 

 wind and water, the uphill movement of the talus flora, and 

 the practically continuous supply of water through under- 

 ground drainage and various other conditions, those of the 

 ravines which are topographically older are rich in species 

 and combine many of the features of the upland and lowland 

 flora. Hill crests further serve to modify conditions, as do 

 the sinkholes, which may be either dry or wet, dependent 

 upon changes in the earth-crust indicated previously. 



