1. Panicum acuminatum Sw.

Pl. 164 a–f; Map 662

P. lanuginosum Elliott

Dichanthelium acuminatum (Sw.) Gould & C.A. Clark

D. lanuginosum (Elliott) Gould

Plants perennial, without rhizomes, forming tufts. Flowering stems 5–75 cm long, erect to spreading, initially unbranched, but becoming highly branched during the summer and autumn, the nodes and internodes glabrous or variously hairy. Foliage in a dense rosette of shorter, somewhat broader leaves at the base and longer, narrower leaves well distributed along the main stems, those of the later branches usually appearing as axillary fascicles of even shorter, narrower leaves. Leaf sheaths glabrous or hairy, the ligule 2–5 mm long, a conspicuous line or band of stiff hairs. Leaf blades of the main stems 2–12 cm long, 3–9(–12) mm wide, abruptly rounded to truncate at the base, glabrous or variously hairy, sometimes with only a few hairs along the margins at the base. Inflorescences 2–9 cm long, the earlier ones larger and more open with mostly spreading to loosely ascending branches (later ones sometimes reduced to small clusters of long‑stalked spikelets), the branches mostly rebranched 1 or more times, not spikelike or 1‑sided, the spikelets appearing mostly long‑stalked at the tips of the branches. Spikelets 1.2–2.2 mm long, ovate to elliptic in outline. Lower glume 0.3–1.1 mm long, usually less than 1/3 as long as the spikelet, broadly ovate, bluntly to sharply pointed at the tip, glabrous or sparsely hairy, nerveless or faintly 1–5‑nerved. Second glume 0.8–2.3 mm long, elliptic, rounded at the tip, 5–9‑nerved, hairy. Lowermost floret usually sterile and with a reduced, inconspicuous palea, the lemma 0.8–2.3 mm long, elliptic, rounded at the tip, 5–9‑nerved, hairy. Fertile floret 0.5–2.0 mm long, elliptic, rounded to bluntly pointed at the tip. Anthers 0.2–0.8 mm long. 2n=18. May–September (vernal), June–November (autumnal).

Common throughout Missouri (U.S. and adjacent Canada; Mexico, Central America, South America, Caribbean Islands; introduced in Hawaii). Upland prairies, glades (on both calcareous and acidic substrates), savannas, mesic to dry upland forests, bottomland forests, and banks of streams, spring branches, and rivers; also pastures, old fields, roadsides, railroads, and open, disturbed areas.

The taxonomy of the widespread, polymorphic P. acuminatum complex is not well understood and detailed populational studies are urgently needed to unravel the complex patterns of morphological variation present in specimens from Missouri. The present treatment follows that of Hansen and Wunderlin (1988) in restricting the name P. ovale to plants with noticeably longer spikelets (2.2–3.0 mm) than are present on any of the materials from Missouri. Other authors, such as Gould and Clark (1978) and Lelong (1984, 1986), have attempted to separate this taxon from P. acuminatum based on either the presence of a “double ligule” (a line of shorter hairs in front of longer hairs) or different spikelet measurements, with the results that plants referred to as P. villosissimum by Steyermark (1963) were grouped with P. ovale. However, there appears to be continuous variation for spikelet length within the Missouri members of the complex and the double ligule character is difficult to apply for many specimens. Thus, true P. ovale is excluded from the state’s flora, even though Gould and Clark (1978) included Missouri in the range of the species.

Within P. acuminatum, various authors have accepted as few as three and as many as eight weakly differentiated varieties (Gould and Clark, 1978; Freckmann, 1981; Lelong, 1984, 1986, Hansen and Wunderlin, 1988). Most of the confusion seems centered around specimens with pubescent stems and sheaths, whereas there is more general agreement on the taxonomy of more glabrous plants. Examination of a large number of specimens has forced me to agree with Hansen and Wunderlin (1988) in treating all of the material with strongly pubescent stems and sheaths under var. acuminatum. Although a minority of specimens with dense, soft, longer (2–5 mm) hairs (corresponding to Steyermark’s [1963] concept of P. praecocius and P. villosissimum) seem strikingly distinct, a number of specimens appear intermediate for hair length. The presence of pustular bases on some of the hairs appears linked with shorter, more appressed hairs, but these characters do not correlate well in Missouri materials. As noted above, further research is needed.