with dense, long, soft hair. Blooming occurs during August and September. Cleaned 

 seed averages 3,140 per gram (1,425,000 per pound). 



Eybvldization. — Chrysotharmus linifolius has a chromosome number of 2n = 18 

 (Anderson 1966 and 1971). Hall and Clements (1923) reported that intermediates showing 

 all gradations in characteristics between C. linifolius and C. viscidiflovus ssp. 

 typiaus {viscidiflovus) have been noted in the area around Point of Rocks, Wyoming. 



Distribution and feziiitat. --Spreading rabbitbrush occurs in Wyoming, Colorado, New 

 Mexico, Utah, and Nevada. It is most abundant in alkaline soils along roadcuts, barrow 

 pits, ditches, streams, and washes in the Upper Colorado River drainage. 



Use.--l\\e aggressive underground spreading characteristics of the species make it 

 a valuable stabilizer of disturbed alkaline soils (McArthur and others 1974; Plummer 

 1977) . 



Chrysothamnus nauseosus (Pallas) Britt. (rubber rabbitbrush) 



Rubber rabbitbrush is a shrub usually 3 to 20 dm high, but it varies from dwarf 

 forms to types over 3 ra high. Usually, several erect stems arise from the base and 

 these branch to form rounded bushes. Branches are covered with a green, yellow-green, 

 gray-green to white feltlike tomentum usually infiltrated with a resinous gum, making 

 the plant somewhat sticky. This coating is often mistaken for part of the bark, but 

 can be discerned by scraping with a knife edge or a fingernail. 



Leaves are nearly filiform in some subspecies to broadly linear in others. Leaves 

 vary from 18 to 63 mm long and are covered with a tomentose vestiture. They are not 

 twisted or gland-dotted. 



The heads of this species are usually arranged into a cymose inflorescence. Each 

 head bears 20 to 25 glabrous to densely tometose involucral bracts arranged in up to 5 

 vertical rows (fig. 36a). Rubber rabbitbrush blooms from August to October. Plants at 

 higher elevations bloom earlier than those at lower elevations. Most forms of C. nau- 

 seosus are among the latest bloomers of the genus. Close examination of the buds and 

 flowers in various stages of development revealed that the anthers open to release pol- 

 len during late bud and just as the buds are opening. As the bud continues to open, the 

 anther tube is pushed above the corolla by elongating filaments. At the same time, the 

 styles elongate and push the stigmas up through the anther tube until the stigmas are 

 fully exserted at full bloom. The stigmas then separate to expose their receptive inner 

 surface (fig. 38). By this stage of development, most, if not all, pollen from the 

 anther tube of the same flower has been shed. The later maturing stigmas, particularly 

 from the first flowers to bloom on an individual shrub, indicate that Chrysotharmus is 

 predominantly self-fertilized (Anderson 1966) because pollen from later developing 

 flowers in the same head, or from other heads on the same bush, may be transferred to 

 receptive flowers on that bush. This genus is not apomictic (Anderson 1966). Fruits 

 mature in October and November. Cleaned seed averages 1,530 per gram (693,000 per pound) 

 (Plummer and others 1968) . 



We have recently discovered that galls induced by tephritid flies are differen- 

 tially distributed among the subspecies of rubber rabbitbrush (McArthur and others 

 1979). Over much of its range, ssp. albioaulis is infested with a persistent round 

 stem gall, 0.3 to 1.2 cm in diameter (fig. 43). In our observations, this gall was 

 restricted to ssp. albioaulis. Subspecies consimilis and graveolens, on the other hand, 

 have a less persistent, fluffy stem gall reminiscent of a ball of cotton about 0.7 to 

 1.4 cm in diameter (fig. 43). This gall is found also on ssp. albioaulis in southern 

 areas (near Kanab, Utah, and Colorado City, Arizona). The round gall is absent in these 



43 



