Torreya 14: 67. 1914.
Herbs, annual or winter annual. Taproots slender. Stems erect or spreading-ascending, simple or few-branched basally, terete. Leaves connate basally into sheath, sessile; blade 1-veined, narrowly oblong, not succulent, apex acute. Inflorescences axillary, flowers borne singly at alternate nodes; bracts paired at upper nodes, foliaceous. Pedicels: flowers sessile. Flowers: perianth and androecium perigynous; hypanthium green or reddish, cup-shaped, 10-veined, commisural veins branched ca. at base of sepals; sepals 5, connate proximally ca. 1 mm, green or reddish, 3-veined (lateral pair 1/8 length of midvein), triangular-ovate, margins white, scarious, apex acute; petals absent; nectaries unknown; stamens 5, arising between sepals; filaments distinct; staminodes 5, alternating with stamens, scalelike; styles 3, filiform, ca. 0.3 mm, without stiff hairs proximally; stigmas 3, linear along adaxial surface, minutely papillate (50×). Capsules ovoid, opening by 3 apical, spreading valves; carpophore absent. Seeds ca. 30(–60), yellowish green to brown, translucent, elliptic to ovoid, reniform, slightly laterally compressed, muriculate, marginal wing absent, appendage absent.
sc United States.
When MacKenzie described Geocarpon he placed it in Aizoaceae, thinking that it was closest to Cypselea among the North American representatives of that family. He noted that Geocarpon differs markedly in the absence of stipules and styles and in the valvular rather than circumscissile dehiscence of the fruit. Other authors following this alignment included M. L. Fernald (1950), H. A. Gleason (1952, vol. 2), and P. Wilson (1934). After closer scrutiny of several morphological characteristics, E. J. Palmer and J. A. Steyermark (1950) moved Geocarpon to Caryophyllaceae. Features they noted in suggesting this change included exstipulate leaves, connate sepals, five stamens and, particularly, five staminodes inserted perigynously on the floral tube opposite the calyx lobes, the unilocular ovary with numerous amphitropous ovules on a free-central placenta, and three sessile, recurved stigmas alternating with three bifid tips of the ovary valves. Within Caryophyllaceae, Palmer and Steyermark erected a new monotypic tribe, Geocarpeae, between Alsineae (subtribe Sabulinae) and Sclerantheae. In addition to the morphological evidence for placing Geocarpon in Caryophyllaceae, there are also supporting chemical and anatomical data. The reddish pigment in Geocarpon is an anthocyanin; no betalain-type pigments have been found (A. L. Bogle et al. 1971). Furthermore, H.-D. Behnke (1982) found that the sieve-element plastids of Geocarpon are the type found in Caryophyllaceae (PIIIc´f), not like those found in members of Aizoaceae.