AMBROSIACE-ffi]. 187 



1, A. tricolor, Hook. Stem 2 ft. high : leaves ample, deltoid-cordate, 

 coarsely sinuate-dentate or slightly lobed, green above, white-cottony 

 beneath: involucral bracts 4 or 5, in one series, ovate, reflexed in fruit, 

 small by the side of the 4 — 6 clavate achenes. — Redwood forests. 



Suborder 4- Ambbosiaoe*. 



Heads small, greenish, the fertile flowers without corolla, or this reduced 

 to an obscure rudiment. Eays none. Staminate involucres mostly 

 forming a raceme above the axillary and few pistillate ones. Anthers 

 but slightly united or quite distinct. Pappus none. 



Heads all alike, and only in the leaf -axils Iva 28 



Staminate heads racemose above the others Ambrosia. 29 



Staminate heads glomerate; fertile head becoming a bur Xanthium 30 



28. IVA, Linn. Perennial herb with simple mostly alternate leaves, 

 and discoid heads nodding on short pedicels in their axils. Involucre 

 of few scales in 1 series, commonly joined into a cup. Marginal fl. 

 pistillate and with short tubular corolla; the other and more numerous 

 fl. staminate, with funnelform 5-lobed corolla and undivided style: 

 anthers nearly distinct. Receptacle with linear or spatulate scales 

 subtending the sterile fl. Achenes thick, naked. 



1. I. axillaris, Pursh. Branching sparingly, 1 — V/ % ft. high: leaves 

 from obovate and spatulate to broadly linear, sessile, entire, 1 in. long 

 or more: heads hemispherical: scales of involucre about 5, united at 

 base, or beyond the middle. Var. pubescens, Gray. Villous with loose 

 spreading hairs; the involucre turbinate, almost entire. — Solano Co. and 

 southward, mostly on subsaline plains, or near the coast. 



29. AMBROSIA, Dodoens. Weedy aromatic coarse perennials with 

 mostly alternate and pinnately divided leaves. Flowers unisexual, the 

 staminate heads several-flowered and arranged in erect spikes or racemes 

 resembling aments. Pistillate heads mostly in the axils of the upper 

 leaves, 1 — 4-flowered, their involucres closed and achene-like, in maturity 

 bearing protuberances or prickles. Achene ovoid or obovate, thick. 



* Fruit 1-seeded, more or less roughened, but not spinescent. 



1. A. psilostachya, DO. Stems erect, from horizontal rootstocks, 

 2 ft. high or more, with strigose pubescence and somewhat scabrous: 

 leaves once or twice pinnatifid: fr. mostly solitary in the axils, turgid- 

 obovoid, less than 2 lines long, obtusely short-pointed, rugose-reticulate, 

 either unarmed, or with 4 short blunt or sharp tubercles. — Borders of 

 fields in uncultivated land near the Bay; plentiful on Point Isabel. 



* * Fruit often more than 1-seeded, spinescent. 



2. A. bipinnatifida (Nutt.). Stems very stout, procumbent, 2—3 ft 

 long, somewhat hirsute: leaves ovate, 1 — 3 in. long, twice or thrice pin- 



