This is a contribution to an understanding of the floristic composition and ecology of the chief plant communities in the region. Three major grassland types are recognized and correlated with climatic factors, topography, and soil colour zones,— the Bouteloua–Stipa association in the dark brown soil zone, the Festuca–Danthonia association in the black soil zone, and the Agropyron–Stipa–Carex associes in the intermediate shallow black soil zone. Of some 250 species and varieties of vascular plants found in these grassland communities, approximately 125 are regarded as either common or characteristic. Poplar and coniferous vegetation is considered in relation to climate, physical features, and the associated grassland. The limber pine and Douglas fir communities of rocky ridges in the "parkland" are described. Prairie and poplar associations are compared with corresponding vegetational types described for other regions. Brief consideration is given to probable migration routes of certain species, and to current fluctuations and trends in the flora of the region.