1. Malvastrum angustum A. Gray (narrow-leaved false mallow, hispid false mallow)

M. hispidum (Pursh) Hochr., misapplied

Sphaeralcea angusta (A. Gray) Fernald

Pl. 454 c, d; Map 2062

Plants annual (perennial herbs and shrubs elsewhere), densely pubescent with stellate hairs throughout (also with simple hairs along the leaf margins). Stems 40–140 cm long, ascending to erect, often branched in the basal half. Leaves short-petiolate, the blades 1.8–5.5 cm long, 0.5–1.3 cm wide, linear to narrowly oblong-elliptic or lanceolate, rounded or narrowed at the base, mostly pointed at the tip, unlobed, the margins usually toothed. Stipules shriveling at maturity, 2–7 mm long, linear. Flowers solitary or in small clusters in the leaf axils, the bractlets subtending the calyx (2)3, 3–7 mm long, linear. Calyces 3.5–7.0 mm long at flowering, becoming enlarged and inflated to 9.0–12.0 mm at fruiting, the sepals fused below the midpoint, the lobes ascending, broadly ovate-cordate and overlapping, giving the calyx a strongly angular-winged appearance. Petals inconspicuous, 2.5–5.0 mm long, the tips shallowly and somewhat asymmetrically notched, the margin otherwise entire or nearly so, yellow. Stamens numerous, the staminal column circular in cross-section, without a low crown of teeth at the tip, the anthers yellow. Pistils with 5(6) locules, the carpels arranged in a ring. Styles fused most of their length, each branch with a globose terminal stigma. Fruits deeply lobed schizocarps breaking into 5(6) mericarps. Mericarps dehiscing longitudinally from the tip along the dorsal and ventral surfaces, 2.6–3.0 mm long wedge-shaped, tan to brown, the dorsal surface lacking a longitudinal groove, beakless, nearly circular to broadly kidney-shaped in profile, not differentiated into sterile and fertile cells, 1-seeded, the sides smooth-walled to finely roughened. Seeds 2.0–2.5 mm long, kidney-shaped, glabrous, dark reddish brown to black. 2n=36. July–September.

Scattered nearly throughout the state, but apparently absent from the Mississippi Lowlands Division and northern portions of the Glaciated Plains (Illinois to Alabama west to Nebraska, Kansas, and Oklahoma). Upland prairies, glades, and tops of bluffs, frequently but not exclusively on calcareous substrates, less commonly stream banks; also roadsides and open moist to dry disturbed areas.

Malvastrum angustum is among the few species (including also Plantago cordata; Plantaginaceae) that are reasonably widespread in Missouri but quite uncommon elsewhere in their distributional ranges. Recent field work in Missouri indicates that, although some historical populations have disappeared, it is still well represented in the state’s flora. This species has been classified in several genera over the years, including being lumped into Sida and Sphaeralcea or segregated from the rest of Malvastrum in the monotypic Sidopsis Rydb. Bates (1967) and S. R. Hill (1982) reviewed the generic taxonomy and relationships, both concluding that the species is best treated as a specialized annual species within Malvastrum. The proper species epithet for the plant also has been a controversial topic. The taxon has long been referred to as M. hispidum (based on Sida hispida Pursh), but S. R. Hill (2002) suggested that this name actually refers to a species of true Sida from the southeastern United States. He advocated adopting the next earliest name, M. angustum, for the taxon, but this has only recently begun to be adopted by other botanists.

Steyermark (1963) suggested that the corolla is sometimes absent, but this has not been confirmed by subsequent workers. S. R. Hill (1982) used garden transplants of Missouri plants to show that the flowers are open for a brief period in late morning and early afternoon, that they are effectively self-pollinated, and that some corollas remain closed and the flowers are then cleistogamous. He also noted that plants are often relatively short-lived, producing mature fruits about three months after the seeds germinate.