36. Hieracium L. (hawkweed)
Plants perennial
herbs, with rhizomes or a short, hard rootstock and fibrous roots. Latex white.
Stems usually solitary (sometimes few to several in H. caespitosum),
erect or strongly ascending, unbranched below the inflorescence, finely ridged,
often noticeably spreading-hairy (the hairs with a bulbous or slightly expanded
base), at least toward the base, sometimes also with minute, stellate hairs
and/or gland-tipped hairs. Leaves basal and sometimes also alternate, hairy or
sometimes nearly glabrous, sessile or short- to long-petiolate. Leaf blades
unlobed, narrowly oblanceolate to obovate, those of the uppermost leaves
sometimes oblong-lanceolate to oblong-ovate, the margins otherwise entire, with
1 main vein visible and often also a pinnate pattern of secondary veins (these
sometimes forming a network), occasionally also a faint network of anastomosing
tertiary veins. Stem leaves gradually reduced in size, sometimes only produced
toward the stem base, without a pair of narrowly triangular, clasping lobes at
the base. Inflorescences terminal panicles or racemes, sometimes appearing as
loose clusters at the stem tip. Involucre not or only slightly elongating as
the fruits mature, narrowly to broadly cup-shaped, the bracts 20–35 or more,
arranged variously, generally in 1 or 2 inner series and usually 1 or more
additional shorter, outer series, the inner series more or less similar in
size, the narrow margins sometimes thin and pale, the tip ascending at
flowering. Receptacle with minute, broadly triangular scales around the base of
each floret, these fused into an irregular low ridge or wing. Ligulate florets
20 to more than 100 per head. Corollas light yellow to bright yellow (orange to
reddish orange elsewhere). Pappus of more or less numerous bristles, these
appearing smooth but microscopically barbed, white or straw-colored to light
yellowish to orangish brown. Fruits nearly cylindrical to narrowly
oblong-elliptic in outline, not beaked, not flattened, circular or finely
angled in cross-section, with (8–)10 longitudinal ribs, these appearing smooth
or nearly so (microscopically cross-wrinkled), glabrous, purplish brown to
nearly black, the pappus attached to a relatively broad but not or only
slightly expanded tip. About 100 to many more than 1,000 species, North America
to South America, Caribbean Islands, Europe, Asia, Africa.
The species of Hieracium
native to North America are generally well behaved biologically, aside from
occasional interspecific hybridization. However, in the Old World, particularly
in Europe, where polyploidy and apomixis are pervasive, the formal taxonomic
description of various minor forms and races as so-called microspecies has led
to an incredible nomenclatural proliferation involving thousands of published
species epithets. This has tended to preclude precise estimates of species
numbers in the genus.
Unfortunately,
several of the Old World species have become important weeds of pastures and
natural grasslands in the United States. Thus far, only one of these has begun
to make inroads into Missouri (see below).