190 BOTANICAL GAZETTE [September 



and the Artemisia frigida consocies may be mentioned specially. 

 The last is closely allied to the Gutierrezia- Artemisia association of 

 the plains, very widespread, and continuing, at the expense of 

 short-grass, with heavy grazing. In mountain-front and plains 

 the primitive bunch-grass association, with Panicum virgatum, 

 Sporobolus cryptandrus, Stipa Vaseyi, Eriocoma, etc., occupies sandy 

 or loose-soil habitats recently disturbed. 



Chrysothamnus-Sarcobatus association. — The shrubby composite, 

 Chrysothamnus (rabbit-brush), and the chenopodiaceous grease- 

 wood occupy loose soil, mostly alkaline areas, on certain slopes 

 in the mountain-front, and are particularly abundant in stream- 

 bottoms in the southern plains. 



Cercocarpus association. — Mountain-mahogany, of the rose 

 family, is the only dominant in the open shrub growth of the 

 mountain-front and outer foothills, in very dry exposed situations 

 and usually stony soil. In the interstices between shrubs are 

 plants of primitive grassland or mixed short-grass. 



Mixed shrub association. — This is a heterogeneous assemblage of 

 shrubs, ranging from xerophytic, like Rhus trilobata, to relatively 

 mesophytic forms, like Crataegus coloradensis, in sheltered situa- 

 tions. The same species range through a variety of habitat condi- 

 tions, and may form a community either as shrubs or trees. The 

 mixed shrub grades into the canyon forest. 



Arctostaphylos association. — The well known and widespread 

 bearberry forms its characteristic mats in the foothills, mostly on 

 compacted gravelly floors. It is more abundant in the upper 

 foothills, in open places among the scattered pines. Its congener, 

 Juniperus communis sibirica, is present but infrequent. 



Ceanothus association. — Ceanothus Fendleri forms low matlike 

 ground-cover in the lower foothills, similar to that of Arctostaphylos, 

 though it is not evergreen, is of more southerly distribution, and 

 ranges into drier and more exposed habitats. It favors the estab- 

 lishment of seedling mesophytes, and plays a part in revegetation 

 of burned areas. 



Symphoricarpos association. — The buckbrush, as it is called, 

 occupies moist fine soil, and invades grassland in the mountain- 

 front and foothills, as well as in the eastern prairie, in favorable 



