Annuals or perennials, 10-50(-70+) cm. Stems erect or decumbent, branched from bases and/or distally (sometimes rooting at proximal nodes). Leaves cauline; opposite; petiolate or sessile; blades (1- or 3-nerved) lanceolate to lance-linear, bases cuneate, margins serrate to subentire, faces sparsely scabrellous. Heads radiate, in loose, corymbiform arrays or borne singly. Involucres hemispheric, 3-5 mm diam. Phyllaries persistent, 8-12+ in 2-3 series (lanceolate to linear, subequal, thin-herbaceous, spreading in fruit). Receptacles flat to convex, paleate (paleae linear to filiform, not conduplicate, falling with fruits). Ray florets 20-40 (in 2-3+ series), pistillate, fertile; corollas white or whitish. Disc florets 15-30+, bisexual, fertile; corollas white or whitish, tubes much shorter than ampliate, cylindric throats, lobes 4-5, ± deltate. Cypselae obcompressed, weakly 3-4-angled (not winged, epidermes usually corky and rugose to tuberculate); pappi persistent, coroniform (sometimes with 2 teeth). x = 11.
Heads radiate, the rays pistillate, usually white, very short; invol bracts 1-2-seriate, herbaceous or herbaceous-tipped, subequal, or the inner narrower and shorter; receptacle flat or slightly convex, its bracts slender and fragile, or the inner ones wanting; disk-fls perfect and fertile, 4(5)-toothed; style-branches flattened, with short, obtuse, hairy appendage; achenes thick, commonly transversely rugose, 3-4-angled, those of the disk somewhat compressed at right angles to the invol bracts; pappus none or an obscure crown or 2 short awns; branching herbs with opposite simple lvs. 4, mostly tropical.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.