136 GENUS ARTEMISIA. 



wide, obtusely 3-toothed (rarely 4- to 7-toothed) or 3- to 5-cleft or parted at the truncate 

 summit, canescent or silvery with a very fine and close tomentum, slightly if at all 

 viscid; upper leaves linear or cuneate-linear, mostly entire, acute, pubescent like the 

 lower ones; inflorescence a leafy-bracted panicle, this commonly diffusely branched 

 and open, then 10 to 25 cm. long by 2 to 10 cm. broad, but sometimes much narrowed 

 or even spike-like and only 0.5 to 2 cm. broad; heads homogamous, sessile, erect, or 

 the branches of the inflorescence sometimes drooping, usually 2 in each peduncled 

 cluster; involucre ovoid or campanulate, 3 to 4.5 or rarely 5 mm. high, 1.8 to 3.5 or 

 rarely 4 mm. broad; bracts 8 to 18, the outer very short and orbicular-ovate or occasion- 

 ally narrowed to an herbaceous tip, the inner elliptic-spatulate, obtuse, all with broad 

 scarious margin, canescent to nearly glabrous; ray-flowers wanting; disk-flowers 3 to 12, 

 or in subspecies rothrocki even to 20, fertile, corolla funnelform, 5-toothed, 2 to 3.3 mm. 

 long, resinous-glandular especially on the tube; style-branches flat, enlarged at summit, 

 where the margins are erose; achenes cylindric-turbinate, the summit slightly contracted 

 or broader and with a raised border, 4- or 5-angled or ribbed, resinous-granuliferous, 

 pubescent only in subspecies parishi. 



The most common and widely distributed shrub of western North America, espe- 

 cially on arid plains of the Great Basin, but ranging to timber-line in the mountains; 

 central Montana to North Dakota, eastern Colorado, New Mexico, Lower California, 

 and eastern British Columbia. 



SUBSPECIES. 

 The number of well-marked variations is not so great as might be expected in a plant 

 of such wide distribution. This may be due to an inherent lack of plasticity or it may be 

 because of the immense abundance and nearly continuous distribution over most of 

 the area covered. This continuity provides for copious interbreeding and the consequent 

 swamping of most variations almost as rapidly as they occur. The commonly recognized 

 variations, which are more or less fixed, notwithstanding the influence just mentioned, 

 are provided for in the following key, and their characters are also indicated in the 

 diagram, page 141. Minor additional variations, for the most part fluctuating or plainly 

 ecologic, are of course numerous. 



Key to the Subspecies of Artemisia tridentata. 



Achenes conspicuously arachnoid-villous. A tall, rare shrub of southern California and 



western Nevada (6) parishi (p. 137). 



Achenes glabrous or only resinous-granuliferous. 



Heads 1.5 to 2.5 mm. broad, with usually 3 to 5 or rarely 6 to 9 flowers; inflorescence spike- 

 like to diffusely paniculate; shrubs either low or tall. 



Inflorescence 1.5 to 7 cm. broad, loosely paniculate; shrubs normally tall (a) typica (p. 136). 



Inflorescence only 0.5 to 2 or rarely 3 cm. broad, narrowly paniculate or spike-like; 

 shrubs often low. 



Leaves merely 3-toothed; involucre greenish yellow (c) nova (p. 137). 



Leaves in part cleft into linear lobes 0.5 to 1 cm. long; involucre canescent (d) trifida (p. 137). 



Heads 2.5 to 4.5 mm. broad with 6 to 20 or rarely only 5 flowers; inflorescence less than 

 3 cm. broad, often spike-like; shrubs mostly of low stature. 

 Pubescence gray or dull white, the hairs closely appressed; leaves cuneate or spatulate 

 in outline. 



Leaves mostly 1.5 cm. or less long; flowers 5 to 8 (e; arbuscula (p. 138). 



Leaves 1.5 to 3 cm. long; flowers 8 to 20, rarely 6 or 7 (/} rothrocki (p. 138). 



I^ibescence white, loose and floccose; leaves narrowly linear, only slightly dilated at 



summit even when cleft " (g) bolanderi (p. 139) . 



25o. Artemisia tridentata typica. — Shrub normally 4 to 40 but even to 50 dm. 

 high, sometimes reduced to 1 dm. under unfavorable conditions, usually with definite 

 trunk or several ascending trunk-like branches; leaves narrowly cuneate, 1.5 to 4 cm. long, 

 3-toothed (rarely 4- to 7-toothed) at the apex or the uppermost linear or oblanceolate 

 and entire; inflorescence openly paniculate, 1.5 to 10 cm. broad; heads ovoid, about 2 

 to 2.5 mm. broad; involucre canescent; flowers 4 to 6; corolla 2 to 2.5 mm. long; achenes 



