Figure 26. — Typical leaves 

 of basin big sagebrush 

 (k. tridentata ssp. 

 tridentataJ . The scale 

 is in mm. 



Heads of this species contain 3 to 8 disc flowers each and are arranged into leafy 

 panicles with erect or sometimes drooping branches. In some forms, the inflorescence 

 becomes spikate. Blooming occurs from July to October. Seeds (fig. 27) mature in Octo- 

 ber, November, and December. There are about 5,510 seeds per gram (2,500,000 per pound) 

 (Plummer and others 1968; Deitschman 1974). Big sagebrush plants often live 100 years. 

 Specimens from several sites were found to be more than 200 years old (Ferguson 1964) . 



Hybridisation. --Artemisia tridentata has a basic chromosome number of a: = 9. Both 



diploid (2n = 18) and tetraploid (2n = 36) forms are known (Ward 1953; Taylor and others 

 1964; Winward 1970; Kelsey and others 1975; McArthur and Plummer 1978; McArthur and Pope, 



data on file at the Shrub Sciences Laboratory, Provo, Utah). In many cases there is no 



morphological difference between the diploid and tetraploid plants. In fact they may 



grow together. Subspecies tridentata and vaseyana both have diploid, tetraploid, and 

 mixed populations. Subspecies wyomingensis is now known only as a tetraploid. 



This species readily crosses both intraspecif ical ly and interspecif ical ly (table 

 2) . In their chromatographic work. Hanks and others (1973) found evidence of natural 

 hybridization among the following species and subspecies: A. tridentata ssp, vaseyana 

 with A. tridentata ssp. tridentata, A. arbuscula ssp. arbuscula, A. nova, A. tripartita 



Figure 27 . — Basin big sage- 

 brush (A. tridentata ssp. 

 tridentataJ seed (7X). 



27 



