344 FLORA OF TEXAS. 



stem l°-2° high, slender, much branched; annual, stiff 

 hairs; involucre scales 4-5, obovate, ciliate; fertile fiowers 

 1-3; leaves entire, narrow-linear. 



** Scales of the involucre 6-9, wibticated, in 2-4 raws. 



I. IMBRICATA, Walt. Stem l°-2° high; leaves fleshy, 

 lanceolate, lower ones slightly serrate, upper ones alternate 

 and entire ; outer scales of the involucre orbicular, inner 

 obovate, toothed-margined ; fertile jloivers 2-4, the short 

 corolla 5-parted. Somewhat shrubby; leaves V long. 



AMBROSIA, Tourn. 



Heads monoecious, in racemes or spikes, the upper ones 

 sterile, nodding, the lower pistillate and fertile; involucre 

 of the sterile flowers hemispherical, composed of 7-12 

 united scales, 5-20 flowered; receptacle naked or with 

 slender chaff; corolla 5-toothed; involucre of the fertile 

 flowers l-flowered, ovoid or turbinate, entire, closed, point- 

 ed, commonly with a row of tubercles or spines near the 

 apex ; corolla and stamens none ; aclienia globose or ovoid ; 

 'pappus none. — Herhs; leaves mostly pinnately lobed; fertile 

 fioicers single or clustered at the base of the sterile spike, 

 or in the axils of the upper leaves, bracted; flowers whitish. 



* Leaves undivided or S-5-lobed, opposite; receptacle naked. 



A. TiiiFiDA, L. Stem tall (6°-10°), 4-sided, rough-hairy; 

 leaves rough, palmately 3-5 lobed, with the lobes ovate- 

 lanceolate and serrate, or all undivided; /^-z/zY obovate, 

 6-toothed around the base of the conically beaked apex, 

 clustered. 



* Leaves pinnately lobed; tlie upper ones mostly alternate; receptacle commonly 



chaffy. 



A. CRiTHMiFOLiA, DC. Stem prostrate and shrubby at 

 the base ; the branches velvety pubescent ; leaves bipinnati- 

 fid, thickish, softly pubescent; spikes few, the terminal 



