The primary leaves are more or less appressed, linear-subulate, 0.5 to 1 cm long, 

 and terminate in mucronate or spinose tips. Their vestiture is glabrate to sparsely 

 floccose. The secondary leaves are linear-filiform to slightly clavate, 3 to 10 mm 

 long, and glabrous. They terminate in blunt tips. 



Flower heads are arranged in compact, corymb-like clusters terminating the branches. 

 Four glabrous or canescent, lanceolate to obovate, involucral bracts subtend each head 

 which contains four golden yellow to cream disc flowers. Slender, densely hairy achenes, 

 3 to 4 mm long, are produced. The achene hairs overlap and blend with the bristly pappus, 

 but do not obscure it. 



The flowering pattern of this species is just the opposite of gray horsebrush. 

 Little-leaf horsebrush begins flowering first during late April in the southern part 

 of its range in the Mohave Desert and progresses northward to Idaho where it flowers 

 in mid-June (Strother 1974). Seeds on the average are slightly larger than those of 

 T. canescens. 



Hybridization. --Tetradymia glahrata resembles T. aanesaens in being a polyploid 

 species. Populations of this species with diploid (2n = 60) and hexaploid (2n = 180) 

 numbers have been found. Sporadic aneuploid-diploid (2n = 62) individuals occur 

 (Strother 1974) . 



Distribution and habitat . --Lit tie- leai horsebrush is most abundant in the sage- 

 brush desert of the Great Basin but is found scattered in dry open places in foothills 

 associated with sagebrush, creosote {Larrea tridentata) , Joshua trees {Yuaca brevifolia] , 

 and pinyon and juniper trees from eastern Oregon and western Idaho southward to north- 

 eastern California, Nevada, and Utah between 800 and 2,400 meters (2,600 and 8,000 

 feet) . Its type locality is reported as the Sierra Nevada (Abrams and Ferris 1960) . 



Use . --Tetradymia glabrata is not ordinarily palatable to livestock. However, on 

 overgrazed, depleted rangelands where more desirable browse is not available, this 

 plant may be consumed. If large enough quantities are eaten in the spring when plants 

 are most toxic, poisoning may result. 



Johnson (1974a) reported an extremely variable hepatotoxic response of sheep to 

 T. glabrata and found that plants in the same stage of development from different 

 localities varied in their toxicity. This variation should allow use of less toxic, 

 more palatable forms that may have potential use in revegetation of the harsh, arid 

 sites where Tetradymia grows. Johnson (1974b") stated that prior consumption of 

 Artemisia species (^4. nova and A. tridentata) might be necessary for expression of 

 toxic effects of T. glabrata (bighead/photosensitization) . 



Tetradymia nuttallii Torr. & Gray (Nuttall horsebrush) 



This species is a rigidly branched, spiny shrub to 12 dm high. Its young stems 

 are woolly-canescent with glabrescent streaks along the internodes below the spines 

 (primary leaves). The stems become glabrate in age. 



The primary leaves transform into straight or recurved spines 0.5 to 2.5 cm long 

 (fig. 61). The sparsely tomentose to nearly glabrous spatulate, secondary leaves are 

 1 to 2 cm long and are borne in fascicles in the axils of the spines. 



Heads containing 4 bright yellow disc flowers each are arranged into terminal, 

 compact, corymb-like clusters. Four linear-oblong involucral bracts 4 to 8 mm long 

 subtend each head. The densely white-hirsute to tomentose achenes are 4 to 6 mm 



61 



