270 MINNESOTA BOTANICAL STUDIES 



Muhlenbergia racemosa Sanicula canadensis 



Nepeta cataria^ Solidago canadensis 



Oryzopsis micrantha Stipa viridula 



Parietaria pennsylvanica Thalictrum purpurascens 



Phryma leptosepala Urtica gracilis^ 



Polygonum sagittatiim Vagnera stellatd^ 



Polygonum scandens Washingtonia longistylis 



SPECIES MORE COMMON NORTHWARD 



Aralia nudicaulis Heracleum lanatum^ 



Arisaema triphyllum Heuchera hispida 



Boehmeria cylindrica Impatiens biflord^ 



Campanula rotundifolia Lobelia syphilitica 



Elymus striatus Salamonia comutata 



Erysimum cheiranthoides Silphium perfoliatum 



Falcata pitcheri Thermopsis rhombifolia 



1 Forming patches or communities. 



It will be appreciated at once after an inspection of these lists 

 how closely the above species agree with common woodland species 

 farther east. The absence of many of the species that are noted 

 in the Missouri woodlands is also conspicuous. 



THE PAPER BIRCH ASSOCIATION 



Following in the wake of the retreating ice cap that covered 

 eastern Nebraska during Pleistocene time, there rolled northward 

 over the state a great boreal floral wave. As the ice receded farther 

 and farther northward nearly all of the boreal species, left thus 

 for a time in a less polar climate, eventually migrated far beyond 

 the northern limits of the state. One of the most notable examples 

 of a plant that was left within our borders when this retreating 

 floral wave carried so many species northward is the paper birch, 

 Betula papyrifera. This species has become perfectly established 

 in the cool, box-canyon-like ravines which lead from the northern 

 portion of the sandhills to the Niobrara River in Cherry, Brown, 

 and Rock counties. 



The most common type of woody vegetation in these spring- 

 branch canyons has already been noted in the preceding association. 

 However, in a number of such canyons and at many places the 

 paper birch exerts a decidedly controlling influence in association 

 surrounded by other trees of such woodlands. The usual Missouri 



