Plants annual or biennial, gla-brous and somewhat glaucous; tap-root stout. Stems erect, sparingly branched below inflorescence, robust, to 65 cm. Leaves: basal few, usually withering by time of anthesis, blade spatulate; cauline numerous, 2 per node, blade 1-veined, ovate-lanceolate to oblan-ceolate, 3-7 cm × 7-30 mm, margins entire, apex acute. Inflorescences many-flowered, open, bracteate; primary branches racemose, elongate, with sessile or shortly pedunculate cymes of 1-6 flowers per node; bracts narrowly lanceolate, 3-12 mm, hyaline-margined, apex acute. Pedicels ascending, ± straight, 1-2 times calyx, broadening at calyx base. Flowers: calyx often obscurely ca. 20-veined, elliptic, abruptly contracted at base, opening constricted to 2 its diam., slightly inflated in flower, 7-10 × 3-4 mm, in fruit tightly enveloping capsule, ovoid, thin, enlarging to 9-13 × 5-7 mm, herbaceous, margins narrow, membranous, dentate with broadly triangular lobes to 1 mm, glabrous, veins obscure, usually purple tinged, without conspicuous, pale commissures, longitudinal, parallel, not obviously reticulate; petals white, clawed, claw equaling calyx, limb deeply 2-fid into 2 spatulate lobes, to 5 mm, appendages ca. 0.5 mm; stamens exserted, to 2 times length of calyx; filaments usually dark purple; stigmas 3, exserted, to 2 times length of calyx. Capsules ovoid, equaling calyx and sometimes splitting it, opening by 6 recurved, narrowly lanceolate teeth; carpophore ca. 1 mm. Seeds grayish brown, plump, broadly reniform, 0.6-1 mm, with concentric rings of papillae; papillae slightly longer than broad. 2n = 24.
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A related biennial sp. of se. Europe, is also occasionally intr. in our range. It has broadly ovate lvs and a smaller, less inflated cal, only 10–12 mm in fr, with 10 long and 10 short veins and few or no anastomosing lateral veins.