Herbs, annual, winter annual, or perennial, sometimes mat-forming. Taproots filiform to stout and woody, perennial plants often with branched caudex or with rhizomes or trailing stems. Stems ascending to erect or prostrate, simple or branched, ± terete. Leaves mostly connate proximally, petiolate (M. cumberlandensis, M. godfreyi proximal leaves) or sessile; blade 1-3-veined, sometimes obscurely so, filiform-linear to subulate, lanceolate or oblanceolate, rarely to ovate, herbaceous to succulent, apex blunt, rounded, or obtuse to acute, acuminate, or spinescent. Inflorescences terminal, open or seldom congested cymes or flowers solitary and terminal or axillary, rarely absent; bracts paired, herbaceous or scarious, rarely absent (M. pusilla, M. rossii). Pedicels erect to arcuate-spreading, rarely reflexed (M. drummondii) in fruit. Flowers: perianth and androecium perigynous; hypanthium usually disc-, occasionally dish- or cup-shaped; sepals 5, distinct, green (herbaceous portion purple in M. arctica, M. macrocarpa, and M. rossii), linear, lanceolate, or oblong to elliptic, ovate, or broadly ovate, 1.5-6(-9) mm, margins herbaceous or silvery and scarious, apex rounded or obtuse to acute, acuminate, or spinescent, sometimes hooded; petals 5 or rarely absent, white, rarely pink (M. biflora) or lilac (M. marcescens), clawed (M. glabra, M. groenlandica) or not, blade apex entire, emarginate, or notched; nectaries 5, at base of filaments opposite sepals, sometimes prominent and 2-lobed; stamens 10 (8-10 in M. godfreyi), arising from hypanthium; filaments distinct; staminodes absent; styles 3 (to 4 in M. cumberlandensis, M. godfreyi), filiform, 0.6-2.5 mm, glabrous proximally; stigmas 3 (to 4 in M. cumberlandensis, M. godfreyi), linear along adaxial surface of styles, minutely papillate (30×). Capsules ovoid to broadly ellipsoid or rarely globose, opening by 3 incurved or erect to recurved valves; carpophore absent or sometimes present. Seeds 1-25, reddish brown to brown or black (or rarely yellowish or purplish brown), spherical or suborbiculate to reniform or obliquely triangular, plump or variously compressed, smooth, reticulate, tuberculate, muriculate-papillate, or rarely with long marginal papillae (M. macrocarpa), marginal wing absent (present in M. douglasii), app