18. Erigeron L. (fleabane)
Plants annual or
perennial herbs, lacking taproots, sometimes with rhizomes or stolons,
variously hairy. Stems 1 or few to several, erect or ascending, unbranched or more
commonly branched, finely to coarsely angled or longitudinally lined. Basal
leaves absent or more commonly present at flowering. Stem leaves progressively
reduced toward the tip, sessile or short-petiolate, the blade linear to
lanceolate, oblanceolate, or oblong-lanceolate, the base sometimes somewhat
clasping the stem, the margins entire or relatively few-toothed above the
midpoint. Inflorescences panicles or sometimes (in E. pulchellus) a
small cluster or solitary head at the stem or branch tips, the heads
long-stalked to nearly sessile, the inflorescence branches with few to several
small, leaflike bracts. Heads radiate (rarely discoid in E. strigosus),
not sticky or resinous. Involucre cup-shaped to broadly cup-shaped or slightly
bell-shaped. Involucral bracts in 2 or 3(4) equal or subequal, overlapping
series, narrowly oblanceolate to narrowly lanceolate or linear, the tip
ascending (sometimes loosely so in E. pulchellus), with a relatively
broad (slender elsewhere), uniform, green central stripe (often with a slender,
yellow or orange midvein) and with relatively narrow, thin, pale margins,
sometimes purplish-tinged toward the tip. Receptacle flat or shallowly convex,
relatively smooth (the veins leading to the florets often appearing as raised
points after the fruits have been shed). Ray florets usually numerous (50–400)
in 1–4 series, pistillate, the corolla usually conspicuous (occasionally a few
of the innermost ray florets with reduced or absent corollas), relatively
slender, white or often tinged with pink, lavender, or blue, especially with
age or on the undersurface, shed before fruiting. Disc florets numerous (more
than 100), perfect, yellow, shed before fruiting. Pappus of the ray and disc
florets similar or of 2 types, of relatively few to numerous (8–30) capillary
bristles and usually also an outer series of fewer short bristles or narrow
scales, usually white, the pappus of the ray florets sometimes with only the
outer, shorter series present. Style branches with the sterile tip (beyond the
stigmatic lines) 0.1–0.3 mm long, lanceolate to broadly triangular. Fruits
narrowly oblong-obovate (slightly tapered toward the base) in outline,
flattened, the angles usually with thickened nerves or ribs, rarely with an
additional nerve on each face, the surface sparsely hairy or glabrous, pale tan
to light yellowish brown. About 390 species, nearly worldwide, most diverse in
temperate montane regions.
Thus far, most
botanists have resisted the temptation to subdivide Erigeron into
smaller genera. In fact, the segregates Conyza and Aphanostephus,
which Cronquist (1943, 1947c) and Shinners (1946b) retained as genera separate
from Erigeron, should perhaps be reintegrated into the genus, based on
molecular data (Noyes, 2000a). In temperate North America, where more than 170
species occur, the greatest diversity is in the western mountains, where
numerous narrowly endemic taxa grow. The molecular studies of Noyes (2000a),
which involved samples from nearly 70 species spread across the taxonomic and geographic
diversity of Erigeron, provided support for the natural (monophyletic)
circumscription of the genus and suggested a North American origin for it.
That said, the
morphological characters separating Erigeron from some other genera of
the tribe Astereae are rendered less effective by variation within some species
in each of the groups. In the eastern half of the United States, the general
rule of thumb has been that a plant flowering before July with relatively
numerous ray florets and subequal involucral bracts having uniform, green
central stripes is likely an Erigeron, whereas a plant flowering after
July with somewhat fewer ray florets and unequal involucral bracts in which the
green stripe is expanded toward the tip is probably an aster (Doellingeria,
Eurybia, Symphyotrichum). However, in individual species one or more of
these characters are difficult to interpret or break down, and the two groups
have overlapping flowering periods from about August to October. In the western
United States, the differences between the two groups are further blurred by
the presence of other related genera, such as Machaeranthera Nees and
its segregates. In Missouri, care also must be taken to avoid confusion between
Erigeron and Boltonia, which differ primarily in their pappus
types and in the generally later flowering period in the latter.