IvA. COMPOSITiE. 289 



tive, short, glabrous, not thickened at the summit. Achenium somewhat 

 compressed. 



79. PICROTHAMNUS. NuU. in trans. Amer. j)hil. soc. {n. ser.) 7. j;. 417. 



Fertile and sterile flowers in the same heads; the former 3-5, marginal, 

 with a very small obliquely truncate and obscurely 2-3-toothed corolla ; the 

 latter about 10, with an infundibuliform 5-toothed corolla, inflated at the sum- 

 mit. Scales of the depressed-hemispherical involucre about 5, orbicular, 

 distinct. Receptacle flat, naked. Sterile flowers destitute of ovaries; the 

 anthers slightly united, tipped with an inflexed mucronate appendage ; the 

 abortive style entire, whh a radiate-penicillate summit. Style in the fertile 

 flowers deeply 2-cleft ; the branches subulate-linear, glabrous. Achenia 

 obovoid-oblong, terete, entirely clothed, as well as their corollas, and the 

 tube of the sterile corolla, with very long and tortuous woolly hairs. — A low 

 rigid suffruticose plant, with the habit of Artemisia, canescently pubescent, 

 much branched ; the branches spinescent. Leaves small, alternate or 

 crowded, petioled, pedately or 2-ternately divided. Heads small, in short 

 and somewhat loose racemes or spikes; the rachis persistent and spinescent. 

 Flowers pale yellow. 



P. desertorum (Nutt. ! 1. c.) 



Arid deserts in the Rocky Mountains, towards the north sources of the 

 Platte, Nut tall .' — Plant bitter to the taste (whence the name), 4-18 inches 

 high. — Mr. Nuttall doubtfully refers it to Milleries, next to Clibadium : but 

 the heads and flowers entirely accord with Iva, except in the naked recepta- 

 cle, and the woolly hairs of the achenia and corolla. 



Div. I?. Ambrosie^, DC. — Fertile and sterile flowers in different 

 heads on the same individual; the former (1-4) often apetalous ; the latter 

 tubular. Scalesof the involucre in the fertile heads united into an ovate or 

 oblong persistent covering, including or closely investing the flowers and fruit, 

 often prickly or spinose. Pappus none. Anthers approximate, but distinct 

 or very slightly united : filaments inserted at the base of the corolla. 



80. AMBROSIA. Tourn. ; Linn.; G^ertn.fr.t. 164; Schkuhr, handb. t. 



292 ; DC. jmdr. 5. p. 524. 



Sterile heads occupying the upper portion, the fertile at the base of the ra- 

 cemes or spikes, or in the axils of the upper leaves. Sterile Fl. Involu- 

 cre flattish, hemispherical, or somewhat turbinate, composed of 7-12 united 

 scales, 5-20-flowered. Receptacle flattish, naked, or usually with filiform 

 chaff" among the flowers. Corolla infundibuliform or turbinate, 5-toothed. 

 Anthers tipped with a mucronate-setigerous indexed appendage. Ovary 

 none or rudimentary : abortive style included, minutely fimbriate or radiate- 

 penicillate at the summit. Fertile Fl. Involucre globose-ovoid, oblong, 

 or turbinate, closed, pointed, usually armed near the summit witli 4-8 tuber- 

 voL. II. — 37 



