Título-BPH: Syst. Bot. Monogr.

Artemisia subgenus Tridentatae -Asteraceae-Asteroideae-Anthemideae-Artemisiinae- comprises 13 species, including 12 subspecies, of shrubs endemic to western North America, including the coastal areas of Baja California, the grasslands of the Great Plains, the basalt scablands of the Columbia Plateau, the western shrub lands of Canada, and the warm deserts of the Colorado Plateaus. The Tridentatae lineage underwent a period of rapid diversification and expansion, espcially since the last glacial period. The greatest abundance of shrbs occurs within the arid Great Bsin, a cold desert that was occupied by Pleistocene lakes. Taxa aparently representing ancestral lineages -A. rigidia and A. Tripartita- occur outside the margins of this inland desert. In spite of the extraordinary ecological specializations among the taxa, there is relatively little genetics diferentiation, and morphological differences are often subtle. Differences in soil type, temperature, and moisture regimes distinguish the habitats of species as well as subspecies. Hybridization between species is rare, although hybridization among taxa primarily include discontinuities in growth form, the shape of the crown, the structure of the inflorescence, and habit -evergreen or deciduous, "root sprouting" or not-. Differences in leaf anatomy are significant and physioloically correlated, helping to define species boundaries but of no utility in field identification. Pollen varies notably in shape and size, and may prove to be usful in distinguishing species in stratigraphic profiles. Floral morphology varies little -florets and cypselae are nearly idenical-, but sexual arrangement within floral heads -capitula- defines sections: sect. Tridentatae is homogamous -all florets are pistillate-. An expanded circumscription and the geographic range of subg. Tridentatae is proposed. In order to accommodate morphological differences while keeping alliances indicated by moleular studies, subg. Tridentatae is divide into two new sections: sect. Tridentatae -10 species- and sect. Nebulosae; the latter includes A. californica and A. nesiotica -formerly placed in subg. Artemisia-, and A. filifolia -formerly placed in subg. Dracunculus-. Morphology and anatomy, ploidy levels, phytogography, and phylogeny are discussed. Full synonymies and descriptions are privided for all taxa, as well as a key, specimen citations, illustrations, and maps.