INTRODUCTION 



Sagebrush species of the genus Artemisia section Tridentatae^ occur discontinuous ly 

 as dominants or partial dominants on over one-third of that portion of the contiguous 

 United States west of 102° W. longitude (Beetle 1960). Some of these taxa also occur 

 in adjacent areas to the east and in Canada and Mexico. Sagebrush is an important con- 

 stituent on much of the West's rangeland. It serves as forage for wildlife and live- 

 stock and as cover for birds and small animals. Moreover, sagebrush has watershed and 

 recreation values . 



Section Tvidentatae is noted for both intraspecific and interspecific morphological 

 variation (Hall and Clements 1923; Ward 1953; Beetle 1960, 1970). Hybridization and, 

 to a lesser extent, introgression contribute to the morphological plasticity of the 

 Tridentatae . These processes have been important in the reticulate evolutionary past 

 of the section. Despite the variability of the group. Beetle's (Beetle 1960; Beetle and 

 Young 1965) taxonomic treatment approaches practical workability. 



Cytological studies were initiated by Diettert (1938). Diettert's and subsequent 

 studies show the Tridentatae to be a polyploid series based on the chromosome number of 

 x = 9 (Clausen and others 1940; Ward 1953; Taylor and others 1964; Winward 1970). 

 Diploid (n = 9) , tetraploid (n = 18) , hexaploid {n = 27) , and octoploid {n = 36) popula- 

 tions have been discovered. Although chromosome numbers for over 50 populations have 

 been determined, no detailed meiotic or karyotypic studies have been performed. The 

 chromosome numbers now available do not clarify the phylogenetic relationships within 

 the Tridentatae over most of its range. However, in areas of the Northwest, diploid 

 and tetraploid populations appear to be clearly separated by elevation (Ward 1953; 

 Taylor and others 1964) . 



^In this paper, section Tridentatae Rydb . is recognized over the analogous section 

 Seri-phidiwn Besser. Taxonomic treatment of species follows Beetle (1960) and Beetle 

 and Young (1965) . 



