6. Geocarpon Mack.
One species, endemic to Missouri, Arkansas, Louisiana, and Texas.
Although several studies have confirmed Palmer and Steyermarks (1950) conclusion that Geocarpon is a member of the Caryophyllaceae (see Nienaber [2005] for a summary), the relationship of Geocarpon to other members of the family has yet to be fully elucidated. It should be noted that ontogenetically the flowers are perigynous and that the fused portion of the apparent calyx is a hypanthium with the stamens and staminodes attached along the inner surface. However, unlike Scleranthus (see treatment below), in Geocarpon the hypanthium tissue is undifferentiated from that of the sepals. To avoid confusing users of the present volume who attempt to identify a specimen of this group, the key to genera of Caryophyllaceae above and the species description below treat the flowers as though they were hypogynous with fused sepals, which is how they would likely be interpreted by all but the most observant biologists.