Geocarpon MacKenzie was originally a monotypic genus of plant described by K.K. MacKenzie in 1914. It was placed in the family Caryophyllaceae. However, recent phylogenetic work suggests that it should be placed in the genus Mononeuria and it is now known as Mononeuria minima. This new scientific classification has not been updated in the Federal Register, so for regulatory purposes this species retains its original scientific name (Geocarpon minimum) and is still referred to commonly as Geocarpon. A recent study described the genetic variability among geographically isolated populations and populations that occur in differing habitats. Researchers at the Missouri Botanical Gardens determined that genetic diversity within populations was very low and that most populations are highly genetically divergent from one another.
Geocarpon is a small succulent-like forb. It has a slender taproot and erect to spreading-ascending stems that may be unbranched or branched from near the base. It has sessile, opposite, oblong-triangular, and fleshy leaves. Apetalous, sessile flowers occur singly in leaf axils at alternate nodes. Each flower has five triangular sepals atop a hypanthium, with five stamens alternating with the sepals, and three styles.
Stems, leaves and sepals are usually light green when plants are young, but as they mature, they transition to darker green or greenish brown suffused with reds and then to reddish-purple at full maturity.
Geocarpon emerges as early as November in the form of small winter rosettes, although lack of soil moisture during this period may delay germination until early spring. The flowering and fruiting period when plants are most visible ranges from January to early June, although March and April are the most common survey dates reported throughout the range. The entire flowering period lasts about a month.
Geocarpon is an annual plant and has seeds that often germinate in late fall or early winter, and complete their life cycle by late spring. The life span for this species varies depending on when seeds germinate and the favorability of climatic conditions, like precipitation and temperature. The period when plants are easily visible is approximately one month.
Genetic studies indicate that Geocarpon is an obligate self-pollinator. This supports previous research hypothesizing self-pollination due to the lack of observed pollinators and a lack of obvious adaptations such as nectaries or a showy calyx or corolla that might attract pollinators. Seeds are present in the seed bank for an indeterminant period, although observations indicate that they remain viable for at least several years. In years with inadequate moisture, seeds may remain dormant. They have been observed to germinate in subsequent years when conditions improve. Genetic studies indicate that seed dispersal is highly localized and likely occurs by gravity and sheet flow or wind. Occasional wider dispersal may occur via inadvertent movement by humans or animals.
Although there are other species within the same genus, no other plants are similar in appearance within the microhabitats occupied by this species.
Geocarpon occurs within highly mineralized soils within two distinct microhabitat types. In the Ozark and Cross Timbers ecoregions, it occurs in mineralized sandy soils dispersed within rocky outcrops in sandstone glades. In the Arkansas Valley and South Central Plains ecoregions it occurs in saline prairie habitats along the margins of highly mineralized areas of bare soil known as slick spots.
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