1. Sonchus arvensis L. (field sow thistle, perennial sow thistle)
Pl. 262 e, f;
Map 1096
Plants
perennial, with deep-set, branched rhizomes. Stems 40–150 cm long, often
somewhat glaucous. Leaves with the clasping basal lobes rounded or pointed, the
margins with the teeth having relatively stiff, short, slender prickles at the
tips, the upper surface glabrous, not or only slightly shiny, the undersurface
glabrous or rarely sparsely pubescent with minute, inconspicuous, unbranched
hairs. Basal and lower stem leaves 6–40 cm long, usually irregularly and deeply
lobed. Median and upper stem leaves gradually reduced in size, variously
shallowly or deeply lobed, sometimes unlobed and merely toothed. Inflorescence
branches glabrous or sparsely to moderately pubescent with spreading,
gland-tipped hairs, occasionally with minute, branched, cobwebby to woolly
hairs toward the tip. Flowering heads 2.5–4.5 cm in diameter (measured across
the spreading corollas). Involucre (10–)14–22 mm long, glabrous or sparsely to
moderately pubescent with a central band of spreading, gland-tipped hairs,
occasionally with minute, branched, cobwebby to woolly hairs toward the base.
Corollas 12–25 mm long, bright yellow to orangish yellow. Pappus 8–14 mm long.
Fruits 2.5–3.5 mm long, noticeably 5–8-ribbed on each face, also finely
cross-wrinkled, reddish brown to dark brown. 2n=36, 54. July–October.
Introduced,
uncommon, known thus far only from Marion and St. Louis Counties
and the city of St. Louis (native of Europe,
introduced widely in North America).
Railroads, gardens, and disturbed areas.
Most botanists
recognize two subspecies based mainly upon differences in pubescence patterns
of the inflorescence (and underlying differences in ploidy). Both types appear
to be widespread weeds in North America,
although usually they are not found in mixed populations. The paucity of
documented chromosome counts makes it impossible to determine whether the
ploidy differences are consistently correlated with the morphological
characters of the subspecies.