2. Sida spinosa L. (prickly sida)

Pl. 454 a, b; Map 2065

Stems 10–60 cm long, moderately to densely pubescent with minute stellate hairs. Leaves with the petiole longer than the axillary flower stalk. Leaf blades 0.5–5.0 cm long, linear to ovate, the base rounded to truncate or shallowly cordate, glabrous or pubescent with stellate hairs on the upper surface, pubescent with stellate hairs on the undersurface. Flowers solitary or more commonly in small clusters in the leaf axils and usually also in small terminal clusters. Calyces 5–7 mm long, the lobes angled to a sharply pointed tip, pubescent with minute stellate hairs. Petals 4–6 mm long. Pistils with 5 locules and style branches. Fruits with 5 mericarps. 2n=14, 28. June–October.

Introduced, common nearly throughout the state (native of the New and Old World tropics, naturalized widely in the eastern and central U.S.). Banks of streams and rivers and margins of ponds and lakes, rarely savannas; also roadsides, railroads, margins of crop fields, pastures, and open disturbed areas.

The species epithet and common name are in reference to a short spinelike projection at the base of the leaves in well-developed plants. The northern limits of the natural distribution of this species are unclear. In Missouri, it was present at least as early as 1833, but the oldest specimens originated from disturbed sites.