AMBROSIA CEAE. 9! I 



receptacles chaffy; fertile heads mostly solitary, ovoid or obovoid, reticulated, 

 short -pointed, unarmed, or with about 4 short tubercles, pubescent, 3-4 mm. long. 

 In moist open soil, 111. to the N. W. Terr., Tex., Mex. and Cal. July-Oct. 



Hispid or tomentose branching herbs, with the aspect of Ambrosias, sometimes 

 woody at the base, with mostly alternate lobed or divided leaves, and small monoe- 

 cious greenish heads of discoid flowers, the staminate in terminal spikes or racemes, 

 the pistillate solitary or clustered in the upper axils. Involucre of the pistillate 

 heads ovoid or globose, closed, i-4-celled, i-4-beaked, armed with several rows of 

 spines and forming a bur in fruit; corolla none or rudimentary; style deeply bifid, 

 its branches exserted ; stamens none; achenes obovoid, thick, solitary in the cells; 

 pappus none. Staminate heads sessile, or short-peduncled, their involucres broadly 

 hemispheric, open, 5~i2-lobed; receptacle chaffy; corolla regular, the tube short, 

 the limb 5 -lobed; style undivided; anthers scarcely coherent; mucronate-tipped. 

 [In honor of Joseph Gaertner, 1732-1791, German botanist.] About 15 species, 

 natives of America. In addition to the following, some 8 others occur in the west- 

 ern and southwestern United States. 



Plant hirsute; annual; spines of the fruiting involucre long, flat. i. G. acanthicarpa. 

 Leaves densely white-tomentose beneath; spines short, conic; perennials. 



Leaves bipinnatifid. 2. G. discolor. 



Leaves pinnately divided, the terminal segment large. 3. G. tomentosa. 



1. Gaertneria acanthicarpa (Hook.) Britton. HOOKER'S GAERTNERIA. 

 (I. F. f. 3595-) Annual, erect or diffuse, paniculately branched, 3-6 dm. high; 

 stem hirsute or hispid. Lower and basal leaves slender-petioled, bipinnatifid, 5-10 

 cm. long, the upper short-petioled or sessile, once pinnatifid, or merely lobed; 

 racemes of sterile heads usually numerous, 3-7 cm. long; fruiting involucres clust- 

 ered in the axils, 6-8 mm. long, commonly I -flowered, armed with numerous long 

 flat straight spines. N. W. Terr, to Kans., Tex., Br. Col. and Cal. July-Sept. 



2. Gaertneria discolor (Nutt. ) Kuntze. WHITE-LEAVED GAERTNERIA. (I. F. 

 f. 3596.) Erect or ascending from perennial rootstocks, branched, about 3 dm. 

 high. Leaves nearly all bipinnatifid, petioled, densely white-tomentose beneath, 

 green and pubescent or glabrate above, 5-12 cm. long; sterile racemes narrow, 

 commonly solitary, 3-5 cm. long; fruiting involucres clustered in the axils, finely 

 canescent, about 4 mm. long, mostly 2-flowered, armed with short, sharp, conic 

 spines. In dry soil, S. Dak. to Wyo., Colo, and N. Mex. Aug.-Sept. 



3. Gaertneria tomentosa (A. Gray) Kuntze. WOOLLY GAERTNERIA. (I. F. 

 f. 3597.) Erect from a deep perennial root, usually branched at the base, 3-9 dm. 

 high. Leaves pinnately lobed or divided, finely and densely tomentose on both 

 sides, or ashy above, the terminal segment lanceolate or oblong-lanceolate, acum- 

 inate, serrulate or entire, very much larger than the 2-6 rather distant narrow lat- 

 eral ones; sterile racemes solitary, 5-10 cm. long; fruiting involucres solitary, or 

 2-3 together in the upper axils, ovoid, finely canescent or glabrate, 2-flowered, 

 about 6 mm. long, armed with subulate-conic, very acute, sometimes curved spines. 

 On rich prairies and along rivers, western Neb., Kans. and Colo. Aug.-Sept. 



4. XANTHIUM L, 



Monoecious annual branching coarse rough or spiny herbs, with alternate lobed 

 or dentate leaves, and rather small heads of greenish discoid flowers, the staminate 

 ones capitate-clustered at the ends of the branches, the pistillate axillary. Stam- 

 inate heads with a short involucre of I to 3 series of distinct bracts ; receptacle 

 cylindric, chaffy; corollas tubular, 5 -toothed; anthers not coherent, mucronate at 

 the apex; filaments monadelphous; style slender, undivided. Pistillate heads of 

 an ovoid or oblong, closed involucre, covered with hooked spines, i-2-beaked, 

 2-celled, each cavity containing one obovoid or oblong achene; corolla none; pap- 

 pus none; style 2-cleft, its branches exserted. [Greek, yellow, from its yielding 

 a yellow hair-dye.] About 15 species, of wide geographic distribution. 

 Leaves lanceolate, not cordate ; axils bearing 3-divided spines, i. X. spinosum. 

 Leaves orbicular or broadly ovate, cordate, or truncate ; axils not spiny. 



Bur 12-20 mm. long, usually nearly glabrous ; beaks straight or nearly so. 



2. X.glabratum 

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