Wheeler: flora of southeastern Minnesota. 365 



generally wooded to some extent whatever the direction of the 

 slope (Plate XXII., A and B). 



Near the heads of the creeks at the bases of the northern 

 slopes are many moist limestone cliffs with their characteristic 

 abundance of liverworts, mosses and ferns, sometimes almost to 

 the exclusion of the higher seed plants. The moist cliffs bear 

 more of the northern types of plants rare to this region than 

 any one other special area. 



The zones of forest vegetation on the bluffs are often very 

 distinctly marked out by a few species. The oaks, ^. rubra, 

 J^. macrocarfa and ^ coccinea extend from the valley to the 

 ridge of the bluff in varying degrees of abundance and thus 

 do not determine the zone. With these, however, are a few 

 species which are limited either to the base or ridge. The 

 aspen and a large-tooth poplar as primary and the ironwood, 

 juneberry and wild crab as secondary species mark out the basal 

 zone of forest and the white oak, white birch and shagbark 

 hickory in varying proportions mark out the ridge forest. Be- 

 tween the zone containing white birch on the ridge and that con- 

 taining the aspen at the base is a zone which is almost entirely 

 covered with dark-barked trees. Thus the zones are clearly 

 shown by the white bark of the white birch on the ridge and 

 that of the aspen below with the dark-barked trees between. 



The vegetation areas of the bluffs may be considered as moist 

 cliffs, wooded slopes and ravines, ridge forests, bare slopes and 

 open ridges, and dry rocks. The plants of the moist cliffs are 

 hydrophytic ; those of the wooded slopes and ravines which in- 

 clude a large part of the bluff area are mesophytic ; the ridge 

 forests are xerophytic and the bare slope, open ridge and dry 

 rock plants which grow on the most exposed and dry areas in 

 this region are distinctly xerophytic. 



Moist cliff vegetation. 



This group of plants is one of the most interesting of this re- 

 gion. One is always on the lookout for rare plants to this part 

 of the state from the secluded and often almost inaccessible 

 moist cliff. The short list of plants given here might be said 

 to be almost peculiar to moist cliffs as they are rarely found 

 elsewhere. Others might be named that grow upon moist cliffs, 

 but which are more characteristic of moist woods. 



Some of the plants characteristic of moist cliffs are : 



