254 MINNESOTA BOTANICAL STUDIES 



Eriogonum annuum Lacinaria punctata 



Euphorbia petaloidea Malvastrum coccineum 



Euthamia graminifolia Meriolix serrulata 



Festuca octoflora Opuntia fragilis 



Froelichia floridana Opuntia polycantha 



Gaura coccinea Parosela aurea 



Helianthus petiolaris Psoralea lanceolata 



Hymenopappus filifolius Sideranthus spinulosus 



Ipomoea leptophylla Silphium laciniatum 



Koeleria cristata Talinum teretifolium 



The wire-grass transition association is best developed about 

 the margins of the sandhill region upon soil less sandy than that of 

 the bunch-grass association but not so firm or clayey as that of 

 short-grass land. The widest stretches of the association are to be 

 found toward the southwestern limits of the region and again in 

 certain places along the Niobrara' River north of the region proper. 

 Shantz reports (60) that when the wire-grass occurs in eastern 

 Colorado it is also found bordering sandhill areas. 



As would be expected from its intermediate character this asso- 

 ciation partakes of the floral characters of the two closely related 

 plant formations. These similarities are especially noticeable as 

 we compare the secondary species of this association with those of 

 the bunch-grass association and of the short-grass formation. The 

 dominant species of this association are adapted to a much firmer 

 substratum than that of the bunch-grass association, and the second- 

 ary species that are common in the two associations are those that 

 prefer relatively quiet or clayey soils. This is a well-known char- 

 acteristic of the principal species of this association as well. The 

 tendency toward bunch-grass conditions is seen in the presence of 

 Andropogon scoparius, Psoralea lanceolata, Bouteloua hirsuta, and 

 Eragrostis trie bodes. The tendency toward short-grass is shown 

 by the other Boutelouas and Bulbilis as well as by a number of the 

 secondary species. A notable relation to the more eastern condi- 

 tions is shown by the presence of Koeleria, Stipa spartea, and Sil- 

 phium laciniatum. 



Typically the ground tone of the association is given by a 

 carpet of Bouteloua which is, however, more open than that so 

 characteristic of the short-grass formation. The wire-grass occurs 

 in low tufts or close bunches of slender, light buff-colored or silvery 

 stems scattered over this cover. The interval between the bunches 



