1. Galeopsis tetrahit L. (common hemp nettle)

Pl. 433 j, k; Map 1946

Plants annual, with taproots. Stems 20–80 cm long, erect or ascending, relatively sharply 4-angled, unbranched or more commonly branched, sparsely to moderately pubescent with relatively long, stiff, more or less spreading, multicellular hairs, mostly along the angles, also with shorter, finer, gland-tipped hairs, especially around the somewhat swollen nodes. Leaves opposite, short- to long-petiolate. Leaf blades 2–10 cm long, 0.8–5.0 cm wide, lanceolate or narrowly elliptic-lanceolate to rhombic-elliptic or ovate, angled or rounded at the base, angled or tapered to a sharply pointed tip, unlobed, the margins coarsely and bluntly toothed, also hairy, the upper surface moderately pubescent with stiff, multicellular hairs, the undersurface moderately pubescent with similar, but usually shorter, finer, mostly unicellular hairs, both surfaces also with usually inconspicuous sessile glands. Inflorescences axillary, dense clusters of 6–12 flowers per node, these sessile. Bractlets more or less leaflike, 3–12 mm long, narrowly oblong-elliptic or sometimes 3-lobed, spinescent at the tips, shorter than to longer than the adjacent calyces. Calyces 7–12 mm long at flowering, actinomorphic, lacking a lateral projection, symmetric at the base, more or less cylindric to narrowly bell-shaped, the tube strongly 10-nerved (-ribbed) and with finer nerves between the ribs, finely hairy in the mouth, the lobes about as long as the tube and more or less equally ascending, narrowly triangular to triangular, tapered to spinescent tips, moderately pubescent with fine, straight, mostly multicellular hairs on the outer surface and the inner surface of the lobes, becoming slightly enlarged (to 16 mm) and papery at fruiting. Corollas 15–22 mm long, zygomorphic, variously white, pink, or light purple (at least the lower lip usually partially white), the lower lip usually with darker nerves, also a pair of small nipplelike projections (often associated with yellow spots) on the upper surface toward the base, the outer surface moderately to densely pubescent with straight, mostly multicellular, spreading hairs, the tube narrowly funnelform, 2-lipped, the lips shorter than the tube, the upper lip occasionally shallowly notched at the broadly rounded tip, slightly hooded, the lower lip 3-lobed with the central lobe spreading and somewhat arched, longer than the 2, spreading, lateral lobes. Stamens 4, not exserted (curved under the upper lip), the filaments of 2 lengths, the anthers small, the connective somewhat expanded, the pollen sacs 2, oriented vertically and spreading, white or purplish-tinged. Ovary deeply lobed, the style appearing nearly basal from a deep apical notch. Style not exserted, with 2 slender branches at the tip. Fruits dry schizocarps, separating into 1–4 nutlets, these 2.5–3.5 mm long, ovoid to broadly ovoid, the surface brown with irregular, lighter, tan to yellow mottling, glabrous, smooth or the mottled areas appearing slightly raised. 2n=32. June–September.

Introduced, uncommon, known thus far only from the city of St. Louis (native of Europe; introduced widely in the northern U.S., Canada). Railroads.

Galeopsis tetrahit is an allotetraploid species that arose in nature as a hybrid between two diploid progenitors, G. pubescens Besser and G. speciosa Mill. with a subsequent doubling of the chromosome number. The ancestry and cytological behavior of this taxon and its progenitors were studied by Müntzing (1930, 1932), who also successfully synthesized the hybrid through a breeding program. According to Mabberley (1997), it was the first naturally occurring allotetraploid to be artificially recreated by crossing of the parents under controlled conditions.

Burrows and Tyrl (2001) noted that when nutlets of this species contaminating animal feed are ingested in even fairly low doses, they can result in a variety of symptoms and occasional fatalities.

The Eurasian G. bifida Boenn., which has smaller corollas (to 15 mm) with the middle lobe of the lower lip notched or more deeply divided, sometimes has been treated as G. tetrahit var. bifida (Boenn.) Kudô. This taxon escapes sproadically in the northern United States, but has not been reported from Missouri. Several other infraspecific taxa have been named within the native range of G. tetrahit, but require further taxonomic study.