Euphorbia obtusata Pursh

Euphorbia obtusata plant

Family - Euphorbiaceae

Stems - Branching at the base, from a stout but short taproot, herbaceous, erect, simple, often reddish, glabrescent below, fistulose, with small patches of multicellular hairs near the apex in the internodes, with white milky sap.

Euphorbia obtusata stem

Leaves - Alternate (except in the inflorescence), sessile, oblong to oblanceolate, rounded to blunt at the apex, appearing glabrous but actually with a fine arachnoid pubescence (use a lens to see), irregular serrulate, to 5cm long, -2cm broad. Leaves in inflorescence opposite to sub-opposite, broadly ovate.

Euphorbia obtusata leaves

Inflorescence - Loose cymose arrangement of cyathia. Each division of cyme subtended by a pair of opposite bracts. Peduncles glabrous. Pedicels to 3mm long, broadening at the apex, glabrous.

Flowers - Glands of cyathia 4, red, truncate at the apex, 1-1.2mm broad, imperfectly surrounding the cyathia. Stamens +/-15, erect, exserted at the time of flowering. Filaments translucent to pale yellow, compressed, 1.5-2mm long. Anthers biglobose, yellow, .5mm broad. Ovary (in flower) -1.5mm in diameter, globose, green, tuberculate-papillate, quickly enlarging in fruit, with 3 ovules. Placentation axile. Styles 3, reflexed and resting on the surface of the ovary, green, glabrous, branched in the apical 2/3 (the branches spreading). Stigmas red, .2mm long.

Euphorbia obtusata flowerArrow shows gland of cyathium...

Euphorbia obtusata flower...and divided style.

Flowering - May - July.

Habitat - Low woods, thickets, fallow fields, sandy ground, gravel bars.

Origin - Native to U.S.

Other info. - This non-distinct species can be found mainly in the southeastern half of Missouri. Like most plants in the family, this plant bleeds copious amounts of milky sap when injured. Many plants in the genus Euphorbia can be difficult to ID in the field. E. obtusata can be identified by its alternate leaves (which have minute teeth along their margins), its erect growth habit, and the reddish glands around the apex of its cyathia.

Photographs taken near Tom's Creek, NC., 4-20-03.


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