300 MINNESOTA BOTANICAL STUDIES 



the few more xerophytic grasses and herbaceous interstitials become 

 more prominent. 



The development of the controlling or secondary species to a 

 condition of local dominance is seen in a few instances. Thus 

 Spartina cynosuroides exercises an almost exclusively dominant in- 

 fluence over small areas in swales and other slight depressions. 

 Eatonia obtusata, Calamagrostis neglecta, and Elymus canadensis 

 often control small stretches of higher land at certain seasons. 



The species that are most frequent inclusions of the association 

 as members of a secondary layer are the copious Rudheckia hirta 

 and Ratibida columnaris which, with their myriads of yellow heads, 

 often transform the drier stretches of the hay meadows into perfect 

 fields of golden bloom that are visible for miles. Again the ubiqui- 

 tous lowland socials such as Mentha canadensis, Naumbergia thyrsi- 

 flora, and Steironema ciliatum wander from the moister associations 

 and develop low, strictly bounded aggregates in lower, moister 

 situations that occur in many places throughout the hay meadows. 

 The colonial Psoralea argophylla invades in a similar manner the 

 more open and drier portions of the association. The polydemic, 

 Lotus americana, forms here, as well as in other associations, very 

 dense communities that add considerable value to the native hay. 

 Linum sulcatum is a frequent species of the hay meadows which 

 occasionally becomes sufficiently abundant and aggregated to be 

 very conspicuous. This tendency stands in marked contrast to the 

 habits of the upland flax, L. rigidum. 



When such meadows as these are pastured to too great a degree 

 many of the valuable species become greatly reduced or killed out 

 entirely and then some of the secondary species with a ruderal ten- 

 dency become strongly emphasized. As the soil becomes tramped 

 and exposed by grazing animals, such species as Solidago rigida, 

 Euthamia gramini folia, and even Cleome serrulata come in and form 

 large dense patches. Occasionally it happens that the drier portions 

 of the hay meadows are pastured along with the uplands and with 

 the tramping-out of the meadow species an area is exposed to strong 

 wind action, and a great "blow plain" is produced which then re- 

 quires the presence of RedHeldia or Psoralea lanceolata to correct. 

 This condition is particularly liable to occur in places where stock 

 continually cross a portion of the meadow on their way to water. 



The hay, meadow association presents the nearest approach to 

 the typical sod associations of the prairie-grass formation to be 



