AMBROSIACEAE. 



VOL. III. 



4. Ambrosia psilostachya DC. Western Rag- 

 weed. Fig. 4128. 



Ambrosia psilostachya DC. Prodr. 5: 526. 1836. 



Similar to the preceding species, but perennial by 

 long rootstocks, the leaves thick, the pubescence stri- 

 gose or hispid. Stems usually much branched, 2-6 

 high, rather stout; leaves i-2-pinnatifid, 2'-$' long, 

 the lobes acutish ; racemes of sterile heads several or 

 numerous, 2'-6' long, the involucres campanulate, the 

 receptacles chaffy ; fertile heads mostly solitary, ovoid 

 or obovoid, reticulated, short-pointed, unarmed, or 

 with about 4 short tubercles, pubescent, ii"-2" long. 



In moist open soil, Illinois to Saskatchewan, Texas, 

 Mexico and California. July-Oct. 



3. GAERTNERIA Med. Act. Pal. 3 : 244. 



1785. Phil. Bot. 45. 1789. 

 [FRANSERIA Cav. Icon. 2 : 78. pi. 200. 1793.] 



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

 the base, with mostly alternate lobed or divided leaves, and small monoecious greenish heads 

 of discoidal '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 obo- 

 void, 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 25 species, natives of America. In addition to the following, some 12 others occur in the 

 western and southwestern United States. Type species : Xanthium fruticosum L. f. 



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. 



i. Gaertneria acanthicarpa (Hook.) 



Britton. Hooker's Gaertneria. 



Fig. 4129. 



Ambrosia acanthicarpa Hook. Fl. Bor. Am. i : 309. 



1833- 

 Fransena Hookenana Nutt. Trans. Am. Phil. Soc. 



(II.) 7: 345- 1841. 

 Gaertneria acanthicarpa Britton, Mem. Torr. Club 



5: 332. 1894. 

 Franseria acanthicarpa Coville, Contr. Nat. Herb. 



4: 129. 1893. 



Annual, erect or diffuse, paniculately branched, 

 i-2 high; stem hirsute or hispid. Lower and 

 basal leaves slender-petioled, bipinnatifid, 2'-4' 

 long, the upper short-petioled or sessile, once- 

 pinnatifid, or merely lobed; racemes of sterile 

 heads usually numerous, i'-3' long; fruiting 

 involucres clustered in the axils, 3"~4" long, 

 commonly i-flowered, armed with numerous 

 long flat straight spines. 



In moist soil, Saskatchewan to western Ne- 

 braska and Texas, west to British Columbia and 

 California. Sand-bur. Recorded from Minnesota. July-Sept. 



Gaertneria tenuifolia (A. Gray) Kuntze, a southwestern perennial species with pinnately 

 dissected leaves, the fruiting involucre only about i" long, is recorded as extending eastward into 

 Kansas. 



