Duration: Annual

Nativity: Native

Lifeform: Forb/Herb

General: Annual herb, from a slender taproot; stems prostrate to decumbent or sometimes ascending, 5-20 cm long, pilose.

Leaves: Opposite along the stems, on petioles less than 1 mm long; blade elliptic to obovate-oblong, 3-11 mm long and 2-8 mm wide, with a strongly asymmetric base and often a red spot in the center; margins sharply serrate on larger leaves, and often entire or with 2-5 small serrations near the tip on smaller leaves; stipules distinct, triangular, 2 mm long, divided into 3 narrow teeth near the tip.

Flowers: Has the highly modified flower structure characteristic of Euphorbias. Structures called cyathia appear to be individual flowers, but are composed of fused-together bracts forming a cup (involucre), with peripheral nectary glands which are often subtended by petal-like bracts called petaloid appendages. Within the cup there is a ring of inconspicuous male flowers, each reduced to a single stamen. Out of the middle protrudes a single, stalked female flower which lacks petals. In E. serrula, the cyathia (flower structures) are solitary or clustered in the nodes near branch tips. Involucres are cone-shaped to bell-shaped, 1 mm high, and glabrous to sparsely pilose, with 4 greenish-yellow glands around the edge, each with a white to pink petaloid appendage usually wider than the gland; 7-15 staminate flowers.

Fruits: Capsules obtusely triangular in cross section, 2-3 mm long, usually slightly wider than long, glabrous; containing 3 white to light brown seeds, these broadly ellipsoid to ovoid, 3–4-angled in cross section, 1.5 mm long, smooth to minutely rugulose or wit

Ecology: Found in gravelly soil from 2,500-5,000 ft (762-1524 m); flowers March-September.

Distribution: AZ, NM, s TX; south to s MEX.

Notes: This species belongs to the Chamaesyce subgenus of Euphorbia. Some treatments, even recent ones, continue to treat Chamaesyce as a separate genus even though molecular evidence places it within Euphorbia. Chamaesyce spp are distinct based on their leaves which are always opposite and and often have asymmetric bases; cyathia (flower structures) in leaf axils, not at branch tips, and usually with petaloid appendages; and stipules present and not gland-like. E. serrula is fairly common, especially in southern Arizona and southern New Mexico. Look for a ground-hugging annual with hairy stems and hairless leaves (use your hand lens); small leaves less than 1.5 cm long, with at least a few teeth around the edges; and seed pods 2-3 mm long. It is wise to make a collection whenever ID to species is needed, as Chamaesyces are difficult to identify in the field, and multiple species of the genus will commonly grow side-by-side.

Ethnobotany: Unknown

Etymology: Euphorbia is named for Euphorbus, Greek physician of Juba II, King of Mauretania, while serrula means serrate like a saw.

Editor: SBuckley 2010, FSCoburn 2015, AHazelton 2017

Plant: Prostrate to ascending annual forb; herbage pilose, with milky sap

Leaves: leaves opposite, inequilateral at base, with serrulate margins, sometimes with red along midvein

Flowers: flowers monoecious borne in cyathia; petaloid appendages white; ovary and capsule glabrous, ~2mm long; seeds smooth.