2. Chenopodium ambrosioides L. (Mexican tea, wormseed)

Ambrina ambrosioides (L.) Spach

Dysphania ambrosioides (L.) Mosyakin & Clemants

Pl. 352 i–l; Map 1522

Plants annual (biennial or short-lived perennial herbs farther south), usually with an unpleasant odor. Stems 25–100 cm long, spreading to ascending, usually much-branched, with sparse to moderate, sessile, yellowish resin glands, sometimes reddish-tinged or reddish-striped. Leaves sessile (uppermost) to long-petiolate (lowermost). Leaf blades 1–14 cm long (mostly over 5 cm long), those of well-developed leaves mostly 2–5 times as long as wide, linear (uppermost leaves) to oblong, lanceolate, or ovate, bluntly to sharply pointed at the tip, rounded or angled at the base, entire to irregularly lobed, yellowish green to green and herbaceous in texture, the margins often also somewhat wavy or with shallow, irregular, narrow teeth, the surfaces lacking hairs and mealiness, but with sparse to moderate, sessile, yellowish resin glands. Venation noticeably branched, often with 3 main veins from the base. Inflorescences axillary and terminal spikes, the terminal ones sometimes arranged into small panicles, the spikes relatively short and dense, with small clusters of flowers. Flowers not all maturing at the same time. Calyx (4)5-lobed to below the midpoint, covering the fruit at maturity, the lobes 0.7–1.0 mm long, ovate, bluntly pointed at the tip, flat to rounded dorsally, glabrous or with sparse to moderate, short, fine, nonglandular hairs. Stamens (4)5. Stigmas 3. Fruits 0.6–1.0 mm long, ovoid, the seeds positioned horizontally or vertically, the wall thin, papery, and somewhat translucent, smooth to finely wrinkled, easily separated from the seed. Seeds reddish brown to dark brown, shiny, smooth to faintly wrinkled, rounded along the rim. 2n=16, 32, 36, 48(?), 64. July–November.

Introduced, scattered nearly throughout the state, but more common south of the Missouri River (native of tropical America; naturalized widely in the U.S. and Canada). Banks of rivers; also crop fields, fallow fields, gardens, roadsides, railroads, and open, disturbed areas.

The resinous odor exuded by this species has been described as somewhat reminiscent to that of kerosene. Plants have been used medicinally for a variety of ailments, but principally to treat worms. The active ingredient, a volatile oil called ascaridol, is considered poisonous in larger doses. In portions of Latin America, the species is also cultivated for use as a flavoring for other foods and to prepare a tea. The oils in and on the foliage are allelopathic, retarding the germination and growth of other plant species (Clemants, 1992). Some botanists refer the roughly 32 species of Chenopodium with resinous glands rather than farina to the genus Dysphania R. Br. in the belief that these species are more closely related to Atriplex than the remainder of Chenopodium (Mosyakin and Clemants, 2002). In Missouri, these include C. ambrosioides, C. botrys, and C. pumilio. The situation requires further study.

The infraspecific taxonomy of this polyploid complex requires further study. Several variants have been given formal taxonomic recognition in some older literature. Two of these that are treated in many floristic manuals are accepted here, but readers should note the presence of both varieties in some populations and the existence of occasional morphologically intermediate plants.