Plants annual. Taproots filiform. Stems 1-100+, erect to ascending or sprawling, green, 3-40+ cm; internodes terete to ellipsoid, 2-8 times as long as leaves, dull, retrorsely pubescent throughout, sometimes also stipitate-glandu-lar. Leaves often connate basally, with scarious or mostly herba-ceous sheath 0.2-0.3 mm, petiolate (proximal leaves) or usually sessile; petiole 1-4 mm; blade 3-5 veined, elliptic to broadly ovate or rarely orbiculate, 2-7 × 1-4 mm, herbaceous, margins flat, herbaceous, dull, ciliate especially proximally, apex acute to acuminate, pustulate, sparsely minutely pubescent or glabrous; axillary leaf clusters absent. Inflorescences terminal, open, leafy, 3-50+-flowered cymes. Pedicels erect or ascending in fruit, 1-12 mm, retrorsely pubescent. Flowers: sepals green, often prominently 3-veined, not keeled, lanceolate to ovate-lanceolate (herbaceous portion narrowly elliptic to broadly lanceolate), 2-3 mm, to 4 mm in fruit, apex narrowly acute to acuminate, ± minutely pustulate, stipitate-glandular; petals oblong, 0.6-2.7 mm, 5- 4 times as long as sepals, apex obtuse to rounded. Capsules loosely enclosed by calyx, ovoid to cylindric-ovoid, 3-3.5 mm, 5-1 5 times as long as sepals. Seeds 10-15, ashy black, reniform, plump, 0.4-0.6 mm, not shiny, with low-elongate, prominent tubercules. Variation in Arenaria serpyllifolia in the broad sense is treated in various ways. The two varieties recognized here have been treated also as subspecies (e.g., A. O. Chater and G. Halliday 1993) or species (e.g., M. N. Abuhadra 2000; B. Jonsell 2001). Jonsell admitted that accepting them as species is questionable; while the morphological differences are slight (see esp. Abuhadra), the ploidy-level difference (2n = 40 in var. serpyllifolia vs. 2n = 20 in var. tenuior) is important.
Diffuse, delicate or wiry, puberulent annual 5-30 cm; lvs usually 8-10 pairs, 3-8 נ3-5 mm, ovate, acute, sparsely scabrid-puberulent, 3-5-nerved, mostly much shorter than the internodes; infl short or extending to the middle of the stem; bracts leafy, pedicels slender, 4-8 mm; sep 2.5-4 mm, lance-ovate, acuminate, 3-5-nerved, somewhat carinate, scarious-margined, scabrid- puberulent or often glandular; pet usually shorter than the sep; fr ovoid-conic, ±exceeding the sep, dehiscent to an uncertain depth by 6 teeth or valves; seed plump, 0.4-0.6 mm, gray-black or reddish-brown, tessellate- tuberculate; tetraploid on x=10. Native of Eurasia, now found throughout most of temperate N. Amer. as a weed in sandy or stony places. May-Aug. A delicate, diploid phase with relatively small lvs, fls and frs, less common in our range than typical A. serpyllifolia, is var. tenuior Mert. & Koch. (A. leptoclados)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.