90. Silphium L. (rosinweed)
Plants perennial
herbs, with rhizomes or taproots. Stems erect or ascending, unbranched or more
commonly few- to several-branched toward the tip, with several longitudinal
lines or ridges, strongly 4-angled (square in cross-section) in S.
perfoliatum, variously glabrous to roughened or hairy. Leaves opposite or
alternate (rarely a few nodes with whorls of 3), and sometimes also basal
(nearly entirely basal in S. terebinthinaceum), sessile or the lowermost
leaves short- to long-petiolate, the bases sometimes slightly to relatively
strongly expanded and wrapping around or clasping the stem, often strongly
perfoliate (leaves opposite and with the bases of each pair fused into a leafy
cup around the stem) in S. perfoliatum. Leaf blades unlobed or deeply
pinnately lobed or divided, lanceolate to ovate, elliptic, or occasionally
nearly circular in outline, tapered to deeply cordate at the base, angled or
short- to long-tapered to a usually sharply pointed tip, the margins otherwise
entire or finely to coarsely sharply toothed, the surfaces variously glabrous
to roughened or hairy. Inflorescences panicles or loose, open clusters (rarely
of solitary heads) or narrow racemes or racemelike panicles, the heads with
short to long, usually bractless stalks. Heads radiate. Involucre 15–40 mm
long, cup-shaped to broadly cup-shaped or somewhat bell-shaped, the bracts in
mostly 2–4 unequal to subequal, overlapping series. Involucral bracts 11–45,
variously shaped, ascending or those of the outer series sometimes with the
tips loosely spreading, green and somewhat leaflike to purplish brown or
yellowish brown and somewhat hardened or leathery, the outer surface glabrous
or variously roughened or hairy, with several fine, often inconspicuous nerves.
Receptacle flat to slightly convex, not elongating as the fruits mature
(occasionally broadening somewhat as the fruits mature), with chaffy bracts
subtending the ray and disc florets, these narrowly oblong to linear,
relatively flat with the margins not or only slightly curled partially around
the florets. Ray florets (8–)13–35 in 2 or 3 overlapping marginal series,
pistillate (with a 2-branched style exserted from the short tube at flowering),
the corolla showy, 15–50 mm long, relatively slender, yellow, spreading to
slightly drooping at flowering, not persistent at fruiting. Disc florets 40 to
numerous (more than 150), staminate (with a small, stalklike ovary and an
undivided style), the corolla yellow (rarely white elsewhere), slender
throughout. Style branches with the sterile tip somewhat elongate and tapered.
Pappus absent or that of the ray florets of 2 short, triangular, awnlike
extensions of the winged angles of the fruit 1–5 mm long (the tip of the fruit
then appearing deeply notched), persistent at fruiting). Fruits obovate in
outline, strongly flattened, the surface smooth but usually with several fine,
faint lines, glabrous or less commonly finely hairy, brown to black, sometimes
with minute resinous dots. About 12 species, U.S., Canada.
Some species of Silphium
resemble members of the genus Helianthus but differ in their staminate
(vs. perfect) disc florets, pistillate (vs. sterile) ray florets, and
frequently in their relatively large, broad outer involucral bracts. The name
rosin weed refers to the gummy resin that oozes from damaged tissues. This
exudate was used by Native Americans and pioneers as a kind of chewing gum.
Most of the species also were utilized in various ways medicinally for urinary
tract infections and as a general analgesic, among other uses (Moerman, 1998).
Several species
of Antistrophus Walsh, a genus of small, gall-forming wasps in the
family Cynipidae, are obligate parasites on the stems of various Silphium
species (Krombein et al., 1979; Tooker and Hanks, 2004). The female wasp
oviposits eggs into various portions of the developing Silphium stem
(depending on the wasp and host species) and the larvae feed on a specialized
lining of nutritious cells within their chamber, which is visible externally as
an ellipsoidal to spherical swelling of the stem. In contrast, the larvae of
most other Silphium stem-boring insects, such as the beetle Mordellistema
aethiops Smith (Mordellidae), form galleries within the stem that do not
result in externally evident abnormalities like galls (Tooker and Hanks, 2004).
At maturity, the adult Antistrophus wasps bore through the stem, leaving
a small hole. Complicating the situation, other wasps in various families, such
as the Eurytomidae, Ormyridae, Eupelmidae, and Pteromalidae, oviposit their
eggs into existing stem galls, and their larvae parasitize the gall-forming Antistrophus
larvae (Tooker and Hanks, 2004). Apparently, in some cases, these secondary
gall inhabitants may themselves be parasitized by yet other wasps.