THE SHORT-GRASS PLAINS. 143 



pond, it is bordered by a zone of hydroid raderals and subruderals, followed 

 by a broad band of pure buffalo-grass, the whole set in a matrix of grama. In 

 the fall or in years of drought the pond dries to a bed of mud or moist soil, 

 over which the ruderals extend, followed by the slower invasion of Bulbilis. 

 When drought, cultivation, or drainage leads to the final drying-up of the 

 play a, the buffalo-grass sooner or later takes entire possession. It is invaded 

 at the same time by grama along the upper edge, but the ordinary drainage 

 into the depression keeps the center more or less permanently in the Bulbilis 

 stage. 



The other dominants are also subclimax to Bouteloua gracilis, and hence, 

 in an arid climate, are somewhat more mesophytic. The equivalence of 

 Muhlenbergia is very close to that of Bouteloua gracilis, while that of B. hirsuta 

 is less so. Even in the latter instance, the difference in requirements must be 

 regarded as slight, since the two are often associated. As in all such cases, 

 however, it must be borne in mind that the mixing is due rather to the ability 

 of B. gracilis to invade slightly better conditions than that of B. hirsuta to 

 enter slightly poorer ones. While Hilaria jamesii is clearly subclimax, its 

 factor relations are somewhat obscured by its more or less halophytic nature 

 (plate 27). 



SOCIETIES. 



The density of the sod and the effect of the superficial roots upon water 

 penetration explain the relatively small number of societies and the general 

 lack of conspicuous or distinctive character. These factors naturally owe their 

 effectiveness to the low rainfall, the average over much of the area being from 

 10 to 15 inches, and to the high evaporation. As a consequence, while many 

 of the subdominants of the other associations occur in the short-grass plains, 

 they attain a relatively feeble expression, and then only where the dominants 

 have been more or less disturbed. It is not at all infrequent to find a Bou- 

 teloua plain stretching in all directions without a single conspicuous society to 

 relieve the monotony. Wherever the soil becomes somewhat sandy or the 

 rainfall greater, the water penetration increases correspondingly, and societies 

 become more prominent. As a consequence, the actual number of subdomi- 

 nants throughout the association is much greater than their diminished 

 importance or extent would indicate. There are fewer mixed societies, and 

 both the growth-form and abundance of particular subdominants are reduced. 



Prevernal Societies: Estival Societies: Estival Societies — continued. 



Leucocrinum montanum. Psoralea tenuiflora. Astragalus bisulcatus. 



Anemone patens. Petalostemon candidus. Ipomoea leptophylla. 



Townsendia exscapa. Petalostemon purpureus. Gaura coccinea. 



Vernal Societies: Lepachys columnaris. Erigeron pumilus. 



Senecio aureus. Malvastrum coccineum. Linum rigidum. 



Astragalus drummondii. Opuntia polyacantha. Dalea laxiflora. 



Aragalus lamberti. Opuntia mesacantha. Menohx serrulata. 



Euphorbia robusta. Luoinus argenteus Artemisia canadensis 



Sonhora sericea Mipinus argenteus Actinella nchardsomi. 



Penttemon unliateralis. Thelesperma gracile. Haplopappus spinulosus. 



Pentstemon coeruleus. Carduus plattensis. Hedeoma drummondu. 



Arenaria fendleri Helianthus pumilus. Lepachys tagetes. 



Erysimum asperum. Chrysopsis villosa. Gymnolomia multiflora. 



Lithospermum linearifolium. Polygala alba. Aster bigelovii. 



Krynitzkia virgata. Zinnia grandiflora. Aster tanacetifona. 



