292 AMBROSIACEAE. [Vol.. III. 



Family 42. AMBROSIACEAE Reiclienb. Consp. 112. 1828. 



Ragweed Family. 



Annual or perennial herbs, monoecious, or sometimes dioecious, many of them 

 weeds, rarely shrubby, with alternate leaves, or the lower opposite, and small 

 heads of greenish or white flowers subtended by an involucre of few, separate or 

 united bracts, the pistillate heads sometimes larger and nut-like or bur-like. 

 Staminate and pistillate flowers in the same heads, or in separate heads. Re- 

 ceptacle chaffy. Pistillate flowers with no corolla, or this reduced to a short 

 tube or ring; calyx adnate to the 1 -celled ovary, its limb none, or a mere 

 border; style 2 -cleft. Staminate flowers with a funnelform tubular or obconic 

 4-5-lobed corolla; stamens mostly 5, separate, or their anthers merely conni- 

 vent, not truly sjmgenesious, with short inflexed appendages; ovary rudimen- 

 tary; summit of the style often hairy or penicillate. 



Eight genera and about 55 species, mostly natives of America, a few only of the Old World. 



Staminate and pistillate flowers in the same heads; involucre of a few rounded bracts. 1. Iva. 

 .Staminate and pistillate flowers in separate heads, the staminate mostly uppermost; involucre of 

 the pistillate heads bur-like or nut-like. 

 Involucral bracts of the staminate heads united. 



Involucre of the pistillate heads with several tubercles or prickles in a single series. 



2. Ambrosia. 

 Involucre of the pistillate heads with numerous prickles in several series. 3. Gaertneria. 

 Involucral bracts of the staminate heads separate; involucre of pistillate heads an oblong bur. 



4. Xanthium. 



i. IVA I,. Sp. PI. 988. 1753. 



Puberulent or scabrous herbs, with thick opposite leaves, or the upper alternate, and small 

 nodding, axillary and solitary, spicate racemose or paniculate heads of greenish flowers. In- 

 volucre hemispheric or cup-shaped, its bracts few, rounded. Receptacle chaffy, the linear or 

 spatulate chaff enveloping the flowers. Marginal flowers 1-6, pistillate, fertile, their corollas 

 short, tubular or none. Disk-flowers perfect, sterile, their corollas funnelform, 5-lobed, their 

 styles undivided, dilated at the apex. Anthers entire at the base, yellow, scarcely coherent 

 with each other, tipped with mucronate appendages. Achenes compressed, obovoid, glabrous. 

 Pappus none. [Named after Ajuga Iva, from its similar smell.] 



About 12 species, natives of America. Besides the following, 6 others occur in the southern 

 and western United States. 



Heads spicate or racemose, each subtended by a linear or oblong leaf. 

 Heads solitary, pedicelled. 



Bracts of the involucre 4-5; heads i%"-2" high. 

 Leaves serrate, oval or oblong; eastern. 

 Leaves entire or nearly so, obovate or oblong; western. 

 Bracts of the involucre 6-9; heads 3" -4" high; southeastern. 

 Heads spicate-paniculate; leaves dentate. 

 Heads spicate-paniculate, not subtended by leaves. 



1. /. frutescens. 



2. /. axillaris. 



3. /. imbricata. 



4. /. cilia la. 



5. /. x'a?ithii/olia. 



i. Iva frutescens L,. Marsh Elder. 

 High-water Shrub. (Fig. 3586.) 



Iva frutescens L. Sp. PI. 989. 1753. 



Perennial, shrubby or herbaceous, somewhat 

 fleshy; stem paniculately branched above, mi- 

 nutely pubescent, or sometimes glabrous below, 

 3-i2 high. Leaves oval, oblong, or oblong- 

 lanceolate, all the lower ones opposite, short- 

 petioled, 3-nerved, acute or obtusish,serrate, nar- 

 rowed at the base, the lower 4 / -6 / long, i / -2 / wide, 

 the upper smaller and narrower, passing gradu- 

 ally into those of the racemose inflorescence 

 which are much longer than the short-pedi- 

 celled heads; involucre depressed-hemispheric, 

 its bracts about5,orbicular-obovate,separate; fer- 

 tile flowers about 5, their corollas tubular. 



Along salt marshes and on muddy sea-shores, 

 Massachusetts to Florida and Texas, the northern 

 plant mainly broader-leaved and less shrubby than 

 the southern. July-Sept. 



