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Published In: Species Plantarum 2: 919. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

Project Name Data (Last Modified On 8/11/2017)
Acceptance : Accepted
Project Data     (Last Modified On 8/10/2009)

 

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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.

 

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1 1. Blades of all or most of the leaves deeply (rarely shallowly) lobed or divided ... 3. S. LACINIATUM

Silphium laciniatum
2 1. Leaf blades unlobed, the margins entire or finely to coarsely toothed

3 2. Main leaves basal, often 1 or a few attached just above the stem base, long-petiolate, the median and upper portions of the stem naked or with widely spaced, much-reduced, sessile, bractlike leaves ... 6. S. TEREBINTHINACEUM

Silphium terebinthinaceum
4 2. Leaves well distributed along the stems

5 3. Stems strongly 4-angled (square in cross-section); leaves opposite, most or rarely only the uppermost leaves perfoliate (the bases of adjacent leaves in each pair fused into a leafy cup around the stem) ... 4. S. PERFOLIATUM

Silphium perfoliatum
6 3. Stems not or only slightly angled (more or less circular in cross-section); leaves alternate and/or opposite, the bases not perfoliate, although sometimes cordate and clasping the stems

7 4. Stems glabrous or moderately to densely hairy or roughened-hairy, the hairs very short (mostly less than 0.5 mm long); ray florets (17–)20–35 ... 2. S. INTEGRIFOLIUM

Silphium integrifolium
8 4. Stems moderately to densely hairy or roughened-hairy, most of the hairs longer (at least some of the hairs 0.8–2.0 mm long, especially toward the stem tip); ray florets (8–)13–35

9 5. Ray florets (8–)13–17(–20), the corolla 15–30 mm long ... 1. S. ASTERISCUS

Silphium asteriscus
10 5. Ray florets 20–35, the corolla 30–45 mm long ... 5. S. RADULA Silphium radula
 
 
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