48. Taraxacum F.H. Wigg. (dandelion)
Plants perennial
herbs, with somewhat fleshy taproots. Latex white. Stems 1 to several, continuing
to elongate as the heads mature, erect or ascending, unbranched, hollow, finely
ridged or nearly smooth, glabrous or with patches of fine, white, cobwebby
hairs, sometimes purplish-tinged or purplish-mottled. Leaves all basal (see
discussion under T. erythrospermum), short- to long-petiolate. Leaf
blades shallowly to deeply and irregularly pinnately lobed, sometimes nearly
compound, narrowly oblanceolate to obovate or elliptic in outline, the lobes
triangular with angled sinuses, irregularly toothed, glabrous or 1 or both
surfaces sparsely to moderately pubescent along the veins with fine,
irregularly curled, white, sometimes somewhat cobwebby hairs, sometimes
pinkish- to purplish-tinged toward the base. Venation of 1 main vein and a
network of anastomosing secondary and tertiary veins. Heads solitary at the
stem tips. Involucre elongating somewhat as the fruits mature, somewhat
urn-shaped to cup-shaped at flowering, the bracts in 1 inner series of 11–23
and 1 or 2 additional outer series of 11–17, usually glabrous, often
purplish-tinged, especially toward the tip; those of the inner series similar
in size and shape, sometimes more or less fused along the margins toward the
base when young, lanceolate, with well-differentiated, thin, pale margins, the
tip sharply pointed or minutely notched, ascending at flowering; those of the
outer series less than to more than half as long as the inner series, ovate to
narrowly ovate, mostly becoming reflexed as the heads first develop. Receptacle
naked. Ligulate florets 40–120 or more per head. Corollas bright yellow,
sometimes purplish- or grayish-brown-tinged on the outer surface. Pappus of
numerous bristles, these smooth or microscopically barbed, white. Fruits with
the body oblanceolate in outline, tapered to a slender beak usually more than
twice as long as the body, not or only slightly flattened, with 4 or 5 rounded
or flattened ribs, these sometimes with 1 or 2 shallow longitudinal grooves,
smooth or appearing smooth or more commonly minutely pebbled or roughened, with
several rows of prominent barbs toward the tip, glabrous, variously colored,
the pappus attached to a relatively broad, expanded, disclike tip. About 60 to
more than 2,000 species, North America, South America, Europe, Asia, introduced worldwide.
The taxonomy of
the weedy dandelions is in a state of perpetual confusion. As in Hieracium,
botanists working in the Old World centers of taxonomic diversity for the genus
have described large numbers of apomictic polyploid races as microspecies,
which has tended to confound application of species-level nomenclature to the
introduced populations in North America. The
two taxa present in Missouri
have been assigned to two different sections, sect. Erythrosperma (H.
Lindb.) Dahlst. and sect. Ruderalia Kirschner, H. Øllg. &
Štěpánek, which are characterized by large numbers of weedy polyploid
apomictic taxa within their native ranges. In fact, ecological and biochemical
studies of nonnative populations in the northwestern United States have indicated
that the two taxa present in Missouri may not be particularly distinct
morphologically, biochemically, or ecologically (Taylor, 1987). Conversely,
genetic studies (King and Schaal, 1990; King, 1993) have documented
surprisingly high genetic diversity among North American dandelions, as well as
intertaxon hybridization, due in part to low levels of outcrossing sexual
reproduction in these predominantly apomictic plants (Valentine and Richards,
1967). The nomenclature and taxonomy of both of the Missouri dandelions remains controversial,
with most recent authors provisionally relying on traditional names and
circumscriptions for the species until the taxonomic situation with these
complexes within their native ranges becomes stabilized. See further discussion
below.