908 



FLORA. 



involucre oblong, 4-5 mm. thick. 10-14 mm. high, very hirsute, its piindpa 

 bracts 8-io; flowers light yellow; pappus straw-color. On dry prairies, Ohio t. 

 Minn, and Neb., south to Ky., La. and Kans. Aug.-Sept. 



9. Nabalus racemosus (Michx.j DC. GLAUCOUS WHITE-LETTUCE. (I. F. f. 

 3584.) Stem virgate, striate, 3-18 dm. high. Leaves thickish, glabrous and 

 glaucous, the lower and basal ones oval, oblong, oblanceolate, or obovate, dentate 

 or denticulate, 1-2 cm. long, mostly obtuse, tapering into long margined petioles; 

 upper leaves sessile, smaller and partly clasping, lanceolate to ovate-lanceolate, 

 denticulate or entire, mostly acute; heads numerous, erect, spreading, or slightly 

 drooping, 1 2-1 6- flowered, 4-6 mm. broad; involucre oblong-cylindric, hirsute, 

 10-12 mm. long, 3-5 mm. thick, longer than the hirsute peduncle, its principal 

 bracts 8- 10, with several small outer ones; flowers purplish. In moist open places, 

 N. B. and Anticosti to the N. W. Terr., south to southern N. Y., N. J., Mo. and 

 Colo. Aug.-Sept. 



Nabalus racembsus pinnati'fidus (A. Gray) Britton. Leaves pinnatifid or pinnately 

 lobed. Hackensack marshes, N. J.; near Mt. Vernon, N. Y. 



Prenanthes Main^nsis A. Gray, from northern Me., is probably a hybrid between A\ 

 racemosus and N. trtfoliolatus. 



10. Nabalus crepidincus (Michx.) DC. CORYMBED RATTLESNAKE-ROOT. 

 (I. F. f. 3585.) Stem glabrous or nearly so below, sometimes puberulent above, 

 stout, 1.5-2.7 m. high. Leaves thin, the basal and lower ones hastate, ovate, oblong, 

 or deltoid, sharply dentate, lobed, or incised, sometimes 25 cm. long, usually with 

 broadly winged petioles; the upper much smaller, sessile or short- petioled, nar- 

 rowed at the base, not clasping, ovate, deltoid, or lanceolate, acute; heads numer- 

 ous, pendulous, short-pedunclcd, 8-12 mm. broad; involucre oblong or oblong- 

 campanulate, hirsute, 10-15 mm. long, about 6 mm. thick, dark green or purplish, 

 its principal bracts 12-15; flowers cream-color; pappus brown. In fields and 

 thickets, western Penn. and N. Y. to Ky., Minn, and Kans. Aug. -Oct. 



Family 4. AMBROSlACEAE Reichenb. 

 Ragweed Family. 



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, or in 

 separate heads. Receptacle chaffy. Pistillate flowers with no corolla, 

 or this reduced to a short tube or ring; calyx adnate to the i-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 connivent, not truly syngenesious, 

 with short inflexed appendages ; ovary rudimentary; summit of the style 

 often hairy or penicillate. Eight genera and about 55 species, mostly 

 natives of America. 

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



i. Iva. 



Staminate and pistillate flowers in separate heads, the Staminate mostly uppermost; in- 

 volucre 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 L. 



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. Involucre hemispheric or cup-shaped, its bracts few, rounded. 



