538 Part IV. Chapter 3. 
longiseta, Psoralea tenuiflora, Plantago Purshii. In general it may be said, that the mesa top is 
dominated by the typice Bouteloua oligostachya formation, the crests by B. hirsuta, the 
low ground with this formation alternating with Muhlenbergia gracillima, or Andropogon oecci- 
nn the month of August, the rainfall decreases markedly and is only slight in je 
es are Artemisia frigida, Gutierrezia euthamiae (= 
spartioides, Chrysopsis villosa, Bigelovia graveolens var. 
The secondary species are Liatris (Lacinaria) punctata, 
zz 
dentale. 
ember. e. principal autumnal speci 
sarothrae), a squarrosa, Senecio 
glabrata (= Chrysothamnus graveolens). 
Eriogonum Jamesii, Aster polycephalus, Munroa squarrosa, Artemisia canadensis, 
plattensis, Eurotia lanata. Hence from a study of the composition of this formation it appea 
that it is typic of the high plains. 
Fig. 28. Yucca angustifolia Pursh (= Y. glauca Nutt.) in the foreground and Opuntia arborescens 
Engelm, in the background. Photograph by Dr. H. L. Smantz with permission of United States 
Department of Agriculture, 
Agropyron Formation of.level, sandy and gumbo Plains. On low hillsides 
the sod is more or less thick, but on level stretches it is sparse. The con- 
trolling grass of this formation is Agropyron spicatum associated with Festuca 
tenella (= F. octoflora), F. ovina, Koeleria cristata, Elymus elymoides, Buchloe 
dactyloides, Boutcloua oligostachys with large patches of Monarda citriodora, 
Hedeoma Drummondi, Malvastrum coccineum, Plantago Purshii and Rumex 
venosus. On less sandy situations near the hills Malvastrum coccineum is almost 
controlling. Astragalus microlobus is often copiously distributed over definite 
areas sprawling in the Agropyron sod. 
