23. Heterotheca Cass. (golden aster, camphor weed)

Plants annual or perennial herbs, usually with taproots, sometimes with rhizomes, the stem bases sometimes somewhat woody. Stems 1 to several, erect or ascending, with usually several to numerous ascending to loosely ascending branches above the midpoint, sometimes only few-branched toward the tip, with fine, longitudinal ridges or grooves, moderately to densely hairy, sometimes also glandular. Basal leaves often withered or absent by flowering time, the blade narrowly to broadly oblanceolate, tapered at the base to a winged petiole, the margins entire, shallowly undulate, or variously toothed. Stem leaves slightly to moderately reduced toward the tip of the stem, sessile, the blade narrowly oblanceolate to oblong-lanceolate, oblong-ovate, or ovate, bluntly or sharply pointed at the tip, tapered to shallowly cordate at the base, sometimes clasping the stem, the margins entire or variously toothed, the surfaces and especially the margins moderately to densely hairy, sometimes also glandular. Inflorescences of solitary heads at the branch tips or of small, loose clusters, sometimes appearing paniculate, the branch tips and stalks short to relatively long, usually with few to several linear bracts 0.5–1.5 cm long. Heads radiate, not or only slightly sticky, sometimes aromatic but not resinous. Involucre 4–10 mm long, cup-shaped to slightly bell-shaped. Involucral bracts in 3–6 unequal, overlapping series, narrowly lanceolate or narrowly triangular-lanceolate to linear, tapered to an ascending to loosely ascending, slender, sharply pointed tip, with a green central stripe nearly the entire length or only above the midpoint (sometimes difficult to observe in H. canescens), the marginal and basal areas whitish or straw-colored, the outer surface usually densely hairy, sometimes also glandular. Receptacle flat or slightly convex, usually with low, toothlike ridges around the attachment points of the florets. Ray florets 10–35 (absent elsewhere), pistillate, the corolla 4–15 mm long, yellow, not persistent at fruiting. Disc florets 15–65, perfect, the corolla 3–9 mm long, yellow, not persistent at fruiting. Pappus of the ray and disc florets similar or dissimilar (sometimes absent in ray florets), a low crown of several shorter, white to off-white scales or bristles 0.2–1.0 mm long and numerous (25–45) longer, finely barbed bristles 5–9 mm long, these usually white when young but sometimes turning straw-colored to light orangish brown as the fruits mature. Fruits 1.2–4.0 mm long, sometimes of 2 types, those of the disc florets somewhat flattened, those of the ray florets sometimes more or less 3- or 4-angled in cross-section, both types obovate to narrowly obovate in outline, the surface glabrous or moderately to densely pubescent with fine, appressed, silvery hairs, light tan to grayish tan. About 28 species, U.S., Canada, Mexico.

Traditionally, most botanists separated this complex into two genera, Chrysopsis (Nutt.) Elliott and Heterotheca, with Chrysopsis comprising species with a pappus similar in disc and ray florets and producing only one type of fruit and Heterotheca in the strict sense confined to species having ray florets lacking a pappus or nearly so and producing angled (vs. flattened) ray fruits. Beginning with the work of Shinners (1951), some botanists developed arguments for treating the entire complex as a single genus under the name Heterotheca (Harms, 1963, 1965a, 1968, 1974), citing exceptions to this rule. More recently, Semple (1977, 1996) and Semple et al. (1980) developed cytological, morphological, and molecular data to support the hypothesis that there exist three main lineages within the complex, as well as a few miscellaneous, small segregates. As circumscribed by Semple and his colleagues, Chrysopsis consists of about eleven non-Missouri species native to the southeastern United States. Semple (1996) excluded the two species of Bradburia, one of which does occur in Missouri, and which some botanists continue to accept as a primitive member of Chrysopsis (Nesom, 2000). See the treatment of Bradburia for further discussion. Semple et al. (1980) and Semple (1996) further segregated a group of about seven species of so-called grass-leaved golden asters as the genus Pityopsis Nutt. These also grow in the southeastern United States, with one species also in the Neotropics, and none occurs in Missouri. The confusing array of treatments for Heterotheca and its relatives in the botanical literature is sufficient to cause temporary lightheadedness in botanists, many of whom nevertheless have strong opinions on the appropriate classification.