tips of branches. Each head contains from 4 to 9 yellow disc flowers. Ray flowers are 

 lacking. Four to 6 equal involucral bracts with overlapping sides subtend each head. 

 The pappus consists of numerous bristles or scales or may be lacking. The ovary of 

 each flower develops into a glabrous to densely long-haired achene. 



This genus blooms from April to August depending on elevation, climatic conditions, 

 and species. Numerous small moths, bees, flies, and beetles visit the flowers. Al- 

 though numerous potential pollinators are available and the flowers are highly fertile, 

 seedlings are not commonly seen in nature. This is probably due to the harsh environ- 

 ment in which Tetradymia is usually found (Strother 1974). However, we recently ob- 

 served many young plants growing in a burned-over area near Wallsburg, Wasatch Co., 

 Utah. 



The basic chromosome number for horsebrush is a: = 30, and sporadically in some 

 species, a; = 31. Polyploidy occurs in the genus but has only been found in some spine- 

 less species {T. filifolia, T. canescens, T. glabvata) . Strother (1974) found no 

 correlation between either geography or elevation of populations and their chromosome 

 number. He only rarely found variation of chromosome numbers within colonies. 



Horsebrush provides some critically needed ground cover in the dry, sparsely 

 vegetated desert ranges where it grows. Although several species are poisonous to 

 sheep and have caused losses of thousands of animals in Utah and Nevada, horsebrush 

 is browsed particularly heavily during winter and early spring on desert ranges and 

 overgrazed areas where little else may be available this time of year. Most severe 

 losses have occurred when hungry animals have been trailed from winter to summer 

 ranges through stands of horsebrush without allowing the animals a chance to graze 

 other plants (Kingsbury 1964) . Considerable variation in toxicity within and between 

 species of Tetradymia has been noted (Johnson 1974a, 1974b) . Hopefully, through care- 

 ful selection and study, palatable atoxic forms may be discovered and developed. 



Four horsebrush species {T. oanesoens , T. glabvata, T. nuttallii , and T. spinosa) 

 are quite common in the Intermountain area. T. axillaris, a spiny species, occurs on 

 the edge of the Intermountain area in scattered populations from southwestern Utah to 

 southern California. Two other species are found southwest of the Intermountain area: 

 T. argyraea (southeastern California), and T. aomosa (southern California and Baja Calif- 

 fornia) (Strother 1974). 



Tetradymia canescens DC. (Gray horsebrush^) 



Gray horsebrush is a spineless, much branched shrub up to 8 dm high (fig. 60). 

 The branches are covered with a thick tomentum interrupted by glabrous to woolly 

 streaks running down the stem internodes from the base of the primary leaves. 



The primary leaves are linear-lanceolate to spatulate, 1 to 3 cm long, 2 to 4 mm 

 wide, and are covered with a gray canescent to tomentose vestiture. The secondary 

 leaves are shorter and narrower than the primary leaves, but are otherwise similar to 

 them. 



Flower heads are borne on the tips of numerous short branches in corymb-like 

 clusters. Each head contains 4 light yellow to cream disc flowers. Four lanceolate 

 or ovate to oblong, canescent, involucral bracts subtend each head. The achenes are 

 3 to 4 mm long and may be glabrous or hairy. The hairs when present are shorter than 

 the abundant, well-developed pappus of bristles. 



Other common names include spineless horsebrush and black sage. 



59 



