Arenaria serpyllifolia L. var. serpyllifolia, thymeleaf sandwort. Annual, fine–taprooted, 1—several–stemmed at base, divaricately branched in upper canopy, ascending to spreading, in range to 15 cm tall; shoots somewhat wispy, puberulent.
Stems cylindric, slender, 0.3—0.6 mm diameter but bulbous and ± 1 mm diameter just above each node, green aging reddish purple, internodes mostly 7—15 mm long.
Leaves opposite decussate, simple, petiolate (lower leaves) and sessile with pair fused across node, without stipules; petiole hemicylindric, to 2.5 mm long, with midvein raised on lower surface, glabrous to sparsely short–hairy or margin ± short–ciliate; blade ascending to spreading from fused bases, ovate, elliptic, obovate, or roundish, (1—)2—5.5 × (1—)1.5—3.5 mm, tapered at base, entire and typically short–ciliate on margins, acute at tip, 3–veined or 5–veined from base and raised slightly on lower surface, lower puberulent with mostly radiating or somewhat ascending hairs.
Inflorescence dichasial cyme with unequal branches, terminal or axillary, bracteate, puberulent and with longer, radiating, short–glandular hairs on all green structures; bracts subtending peduncle and each fork 2, leaflike, sessile and fused across node, short–ciliate on margins; peduncle < 8 mm long, distinctly glandular hairy but not viscid. 5–veined with principal veins somewhat raised on the lower surface; most bracts upper surface glabrous or essentially so, lower surface sparsely short–hairy; pedicel to 2 mm long, puberulent and short glandular–hairy.
Flower bisexual, radial, 5—6 mm across (calyx), dish–shaped; hypanthium short; sepals 5, ± free, overlapping, acuminate–lanceolate to narrowly ovate, at anthesis 2.8—3.5 × ± 1 mm increasing slightly in fruit, green and white–membranous on margins (wider on inner sepals), sometimes reddish purple at tip and submarginally near tip, short glandular–hairy on lower (outer surface), glandular hairs at base, faintly 3–veined with veins becoming more visible in fruit (outer sepal 1–veined); petals 5, short–clawed, elliptic or obovate, 1—1.4 × 0.6—0.7 mm, white (veins obscure), obtuse at tip; stamens 10 in 2 whorls, free, formed on rim of hypanthium; filaments unequal, suberect and 1—1.2 mm (outer stamens) and spreading and < 1 mm (inner stamens), white, glabrous; anthers dorsifixed, dithecal, 0.25 mm long, pinkish (outer stamens) and white and often appearing sterile (inner stamens), longitudinally dehiscent; pollen colorless; nectaries semicircular projections at bases of outer filaments to 0.3 mm long, green, producing nectar on sepal bases; pistil 1, very short–stalked (stipe or gynophore); ovary superior, broadly ovoid, 0.6—0.7 × 0.6—0.7 mm long, light green above midpoint and whitish below midpoint with 6 faint lines, glabrous, 1–chambered with 20+ ovules attached to base on a central post; styles 3, spreading, ± 0.7 mm long, translucent–white, stigmatic on inner side of styles and especially on spreading tips, conspicuously papillate.
Fruit capsule, loosely enclosed by glandular clayx, dehiscent at top by 6 terminal teeth, in range 12—21–seeded, before dehiscent ovoid (gourd–shaped), in range 2.2—2.9 × 1.3—1.5 mm becoming urn–shaped with spreading teeth, maturing reddish but drying straw–colored, smooth, teeth acute and ± 0.6 mm long; seeds long–stalked (funiculus) on central postlike placenta; fruit stipe inconspicuous; calyx in range ≥ immature and dehiscent fruit.
Seed sowbuglike with strongly curved embryo, in range 0.45—0.55 × 0.4—0.45 mm, brown and not shiny with each cell minutely papillate and papillae blackish.
A. C. Gibson