16. Conyza
Less.
Plants annual,
slender or robust, with taproots, the vegetative portions glabrous or more
commonly sparsely to densely hairy. Stems 1 or few to several, erect to
spreading, usually much-branched either throughout or only above the midpoint,
finely to coarsely angled or longitudinally lined. Basal leaves absent at
flowering. Stem leaves somewhat reduced above the lower stem or relatively
uniform, narrowly linear to oblong-lanceolate or oblanceolate, mostly sharply
pointed at the tip, more or less tapered to a sessile or short-petiolate, often
slightly expanded but nonclasping base, the margins entire or few-toothed
toward the tip. Inflorescences panicles or sometimes appearing as racemes, the
heads long-stalked to nearly sessile along the branches and/or in loose
clusters at the branch tips, these usually with few to several minute, linear
bracts. Heads appearing discoid but actually radiate (see below), not sticky or
resinous. Involucre 1.5–4.0 mm long, narrowly to broadly cup-shaped or
slightly bell-shaped, occasionally appearing nearly urn-shaped at flowering.
Involucral bracts in 2–4 more or less unequal, overlapping series,
narrowly lanceolate to linear, the tip ascending, with a slender to relatively
broad, green or brown central stripe (usually with a slender, yellowish
midvein), this sometimes not extending to the bract tip, and with relatively
slender, thin, pale margins. Receptacle flat or nearly so, relatively smooth.
Ray florets 18–40 in 1–3 series, pistillate, the corolla
inconspicuous, the strap-shaped portion 0.2–0.8 mm long, sometimes
reduced to a short fringe at the tip of the slender tube, white or pink to
light purple, shed before fruiting. Disc florets relatively few (less than 30),
perfect, the corolla 1.5–2.5 mm long, yellow, sometimes turning
pinkish-tinged after the pollen has been shed, shed before fruiting. Pappus of
the ray and disc florets similar, of numerous (15–25) capillary
bristles, 2–3 mm long, usually white, less commonly somewhat pinkish-
or yellowish-tinged. Fruits 1.0–1.5 mm long, narrowly oblong in outline
(slightly tapered at the base), flattened, the angles usually with
inconspicuous nerves (these occasionally appearing as narrow wings), the
surface glabrous or more commonly sparsely to moderately and minutely hairy,
light tan to pale grayish brown. Twenty-five to 40 species, nearly worldwide,
mostly in tropical or warm-temperate regions.
Cronquist (1943)
segregated Conyza from Erigeron, based primarily upon its reduced
ray corollas and relatively few disc florets. Nesom (1990b) reexamined generic
limits in the group and suggested some additional morphological distinctions,
including that most species of Erigeron have 1-nerved (vs. 3-nerved)
involucral bracts (very difficult to observe without the use of a histological
clearing agent) and that, in Conyza, the pappus bristles tend to
elongate slightly as the fruits mature. However, preliminary molecular studies
(Noyes and Rieseberg, 1999; Noyes, 2000a) have suggested that Conyza
represents a mere specialized group within the Erigeron lineage that
possibly would be better resubmerged within that genus.