A. SPINESCENS. .133 



villous; principal leaves petioled, flabellate in outline, 0.5 to 2 cm. long including the peti- 

 ole, 0.5 to 1.5 cm. wide, pedately 3- to 5-divided, and the divisions again cleft into linear- 

 spatulate lobes, densely villous; upper leaves similar but less divided, those of the inflor- 

 escence about equaling the heads; inflorescence consisting of numerous short racemes in 

 the axils and at the ends of the short branches, sometimes reduced to only 1 or several 

 heads, leafy-bracted, 1 to 5 cm. long, 0.5 to 1 cm. broad, the persistent rachis transformed 

 into a slender rigid spine after the heads have fallen ; heads heterogamous, short-pedun- 

 cled or subsessile, nodding; involucre broadly turbinate, 2 to 3.5 mm. high, 3 to 4.5 mm. 

 broad; bracts 4 to 8, all much alike and scarcely unequal, obovate-cuneate or nearly orbic- 

 ular with a cuneate base, rather thick and herbaceous, with narrow scarious margins, 

 densely villous ; receptacle naked ; ray-flowers 2 to 6, fertile, corolla very slender, about 1 

 mm. long, 2- or 3-toothed, long-hairy; disk-flowers 5 to 13, sterile, corolla funnelform 

 with narrow tube, 5-toothed, 2 to 3 mm. long, copiously clothed with long flaccid hairs 

 except on the limb ; style of disk-flowers undivided at the expanded and radiately penicil- 

 late summit, included ; achenes of ray-flowers ellipsoid, densely arachnoid-hairy, achenes 

 of disk-flowers wanting. {Picrothamnus desertorum Nuttall, Trans. Am. Phil. Soc. II, 

 7:417, 1841, not A. desertorum Sprengel, Syst. Veg. 3:490, 1825-28.) 



Common on arid plains and slopes from Montana and Colorado to New Mexico, eastern 

 California (from the San Bernardino Mountains north), eastern Oregon, and Idaho. 

 Type locaUty, Rocky Mountain plains, in arid deserts, toward the north sources of the 

 Platte River. Collections: Beaverhead County, Montana, Tweedy 19 (according to 

 Rydberg, Mem. N. Y. Bot. Card. 1:427, 1900); Palisade, western Colorado, May 29, 

 1894, Crandall (NY); Steamboat Mountain, Sweetwater County, Wyoming, Nelson 

 7047 (Gr, NY, UC); Marysvale, Utah, Jones 5326 (NY, UC, US); Navajo Indian Reser- 

 vation, Arizona, Standley 74-79 (US) ; Rabbit Springs, north base San Bernardino Moun- 

 tains, California, April 25, 1915, Shreve (UC); Truckee Valley, Nevada, Kennedy 1966 

 (UC); Malheur Butte, eastern Oregon, Leiherg 2020 (UC, US); near Pocatello, Idaho, 

 Palmer 14 (NY, SF, US). 



RELATIONSHIPS. 



Although Picrothamnus is sometimes taken as a separate monotypic genus because 

 of the spiny habit and notably pubescent flowers, it is in all essentials an Artemisia of 

 the section Dracunculus. The only differentiating characters are those of habit and the 

 pubescence of the flowers, as has been already indicated (p. 33). Its nearest relatives are 

 doubtless species of central Eurasia that do not extend to this continent. There are, 

 however, two cosmopolitan species, namely, A. dracunculus and A. campestris, with 

 which it is in agreement in all important technical characters and with which it shares 

 the almost unique feature of a tendency towards a fusion of the style-branches of the 

 disk-flowers. Whether it really is most closely connected with these can be determined 

 only after a detailed study of all the Old World Dracunculi. Whatever its origin, A. 

 spinescens is now sharply set off from all other species both morphologically and geo- 

 graphically. No other American species approaches it in its spinescent twigs, although 

 several, especially pygmaea, pedatifida, rigida, and the dwarf varieties of tridentata, all 

 but one members of another section, assume a dwarfed and rigid habit as a result of 

 similar unfavorable environments. 



The degree of variation in involucre and floral characters is indicated by table 12. The 

 most striking feature here brought out is the remarkably low variability, indicating a 

 stable non-plastic species. Even the small differences in the number of flowers may be 

 due to the falling-out of these in some cases where the count runs low. Perhaps it is 

 because of its fixed characters and consequent incapacity for adaptation to new environ- 

 ments that the species has produced no forms that have received taxonomic recognition. 



