COMPOSITAE. 257 



Leaves lanceolate, bright green above, white-tomentose 



beneath, the axils bearing spines. X. spinosum. 



Leaves ovate to orbicular; axils without spines. 



Body of the burs 2.5-3 cm. long; prickles about 100, 



hispid, strongly hooked. X. speciosum. 



Body of the burs 1.5-2 cm. long. 



Prickles about 20, about as long as the diameter of the 



body of the narrowly oblong bur. X. oligacanthum. 



Prickles 50-70. 



Burs oblong or slightly ovate; prickles about 50, 

 each two thirds as long as the diameter of the 

 bur. X. affine. 



Burs ovate; prickles about 70, each one half as 



long as the diameter of the bur. X. varians. 



Xanthium spinosum L. Chinese Thistle. Much branched, the stems 

 decumbent or ascending, pubescent, 30-60 cm. high; leaves ovate-lanceolate, 

 acute or acuminate, entire or 3-lobed, cuneate at base, sessile or nearly so, 

 white-tomentose beneath, green and shiny above, 5-12 cm. long; axils bearing 

 stout 2- or 3-parted yellow spines; burs oblong-cylindric, solitary or 2 or 3 in 

 the upper axils, 1-1.5 cm. long, armed with slender rather weak prickles and 

 beaked with 1 or 2 stouter ones. Sparingly introduced. 



Xanthium speciosum Kearney. Cocklebur. Coarse and stout, 15-90 cm. 

 high, scabrous throughout; leaves broadly ovate, coarsely toothed or some- 

 what angularly lobed, cuneate at base, 5-10 cm. long, on stout, usually longer 

 petioles; burs about 4 cm. long, armed with numerous hooked spines, the two 

 terminal ones stoutest, all of them glandular-hispid, except at their apexes, 

 and about 1 cm. long. Common along Snake River. 



Xanthium oligacanthum Piper. Stem erect or spreading, 30-60 cm. high, 

 sparsely hispid; leaves reniform-orbicular, obscurely lobed and crenate, 

 harshly scabrous, hispid on both faces, the petioles as long as or longer than 

 the blades; burs oblong, the body 1.5 cm. long, 5-7 mm. thick, the stout beaks 

 somewhat incurved; prickles 15-25, uncinate-tipped, about as long as the 

 diameter of the bur; surface of the bur and base of prickles pubescent. Bolles, 

 Piper; Waitsburg, Horner. 



Xanthium affine Greene. Similar to X. oligacanthum in foliage and habit; 

 beaks of the burs slender with long incurving tips; prickles numerous, all 

 hooked at the tip, two thirds as long as the diameter of the bur, their bases 

 as well as the spaces between covered with stout hairs. Common about 

 Spokane. 



Xanthium varians Greene. Very similar to X. affine; beaks of the burs 

 shorter and stouter; prickles less numerous, half as long as the diameter of the 

 bur, hirsute at base as is also the body of the bur. Waitsburg, Horner. 



383. AMBROSIA. Ragweed. 



Annual or perennial herbs; leaves pinnately-lobed, at least the 

 lower opposite; heads small, greenish, monoecious; pistillate heads 

 1-flowered, in terminal bractless racemes or spikes; involucral- 

 bracts of staminate heads united; pistillate flowers enclosed in an 

 akene-like 1-celled involucre, usually armed with a single series of 

 tubercles or prickles; pappus none; corollas of staminate flowers 

 broad, 5 -toothed ; of the pistillate none; akenes ovoid or obovoid, 

 thick. 



