37. SÍLPHIUM, L. ROSIN-WEED. Heads many-flowered, radiate; rays numerous, pistillate and fertile, their broad flat ovaries imbricated in 2 or 3 rows; disk-flowers apparently perfect, but with entire style and sterile. Scales of the broad and flattish involucre imbricated in several rows, thickish, broad and with loose leaf-like summits, except the innermost, which resemble the linear chaff of the flat receptacle. Achenes broad and flat, dorsally compressed, surrounded by a wing notched at the top, without pappus, or with 2 teeth confluent with the winged margin, the achene and its subtending chaff usually falling together; those of the disk sterile and stalk-like.--Coarse and tall rough perennial herbs, with copious resinous juice, and large corymbose-panicled, yellow-flowered heads. (Σίλφιον, the ancient name of some resinous plant, transferred by Linnæus to this American genus.) [*] _Stem terete, alternate-leaved (root very large and thick)._ 1. S. laciniàtum, L. (ROSIN-WEED. COMPASS-PLANT.) _Rough-bristly throughout_, stem stout (3--12° high), leafy; _leaves pinnately parted_, petioled but dilated and clasping at the base; _their divisions lanceolate or linear_, acute, _cut-lobed or pinnatifid_, rarely entire; heads few (1--2´ broad), sessile or short-peduncled along the naked summit; _scales ovate, tapering into long and spreading rigid points_; achenes broadly winged and deeply notched, 6´´ long.--Prairies, Mich. to Dak., and southward. July.--Lower and root-leaves vertical, 12--30´ long, ovate in outline; on the wide open prairies disposed to present their edges north and south; hence called _Compass-Plant_. 2. S. terebinthinàceum, L. (PRAIRIE DOCK.) _Stem smooth, slender_ (4--10° high), panicled at the summit and bearing several or many, large heads, leafless except toward the base; _leaves ovate_ and ovate-oblong, somewhat heart-shaped, _serrate-toothed_, thick, rough, especially beneath (1--2° long, on slender petioles); _scales roundish, obtuse_, smooth; achenes narrowly winged, slightly notched and 2-toothed.--Var. PINNATÍFIDUM, Gray, has the leaves deeply cut or pinnatifid, but varies into the ordinary form.--Prairies and oak-openings, Ohio and Mich. to Minn., and southward. July--Sept. [*][*] _Stem terete or slightly 4-angled, leafy; leaves undivided (not large), some opposite._ 3. S. trifoliàtum, L. _Stem smooth, often glaucous_, rather slender (4--7° high), branched above; stem-_leaves lanceolate, pointed, entire_ or scarcely serrate, rough, _short-petioled, in whorls of 3 or 4_, the uppermost opposite; heads loosely panicled; achenes rather broadly winged, and sharply 2-toothed at the top.--Dry plains and banks, Penn. to Ohio, and southward. Aug.