Meadow beauty, deergrass [Greek rhexis, rupture, alluding to reputed astringent property to cure wounds]

Herbs, sometimes suffrutescent; caudices relatively short and woody when present, sometimes becoming spongy-thickened when submerged; roots fibrous, root tubers produced by the roots in some species on the primary root at base of stem or at irregular positions on secondary roots. Stems erect to ascending-erect, ± 4-angled, faces subequal or unequal, if unequal then 1 opposing set convex, broader, the other concave, narrower, the different morphologies alternating at 90° at each node, usually hirsute to villous, hairs bristlelike, often gland-tipped; bark (if any) thin, exfoliating distally. Leaves petiolate, sessile, or subsessile; blade usually 3-veined (1-veined in R. cubensis), without strong cross veins, margins entire, subentire, serrate, serrulate, or crenulate, usually with bristle-tipped teeth, rarely gland-tipped, surfaces strigose, villous, hirsute, or glabrous. Inflorescences terminal, cymes or appearing as secund racemes (through abortion of inner branches); bracts often deciduous, subfoliaceous. Flowers subsessile or short-pedicellate; hypanthium urceolate to campanulate; calyx 4-lobed, lobes triangular; petals fugacious, 4, ascending or spreading, asymmetric, lavender, lavender-rose, lavender-purple, purple, pink, white, or yellow, short-clawed, midvein extending as a slender, multicellular hair; stamens 8, subequal, in 2 whorls, connective bases appendaged; anthers straight or downcurved, linear to linear-lanceolate, 4-locular (1-locular at anthesis through breakdown of septae), apically to subapically poricidal; ovary inferior, adnate to floral tube except apically, 4-locular; style exserted, curved, linear. Fruits capsules loculicidal, enclosed within hypanthium, dehiscent. Seeds cochleate (except cuneate-prismatic in R. alifanus). x = 11.

Species 13 (13 in the flora): North America, West Indies (Greater Antilles).

Hybrids are common between some of the species with elongate anthers (sect. Rhexia) and identifications sometimes are arbitrary. There is no evidence that the other species are involved in hybridization. Tuberous swellings on the roots are produced in some species of sect. Rhexia; presence of these root tubers appears to vary within and among populations.

Rhexia alifanus (sect. Cymborhexia) is the only species of the genus with glabrous, isofacial, subentire, and glaucous leaf blades, caducous bracts, and relatively large, oblong-cuneate, subprismatic seeds (versus small, cochleate seeds in the other species), but it shares with sect. Rhexia (perhaps as a plesiomorphic feature) elongate, curved, small-pored anthers.

Polyploidy occurs in many species of sect. Rhexia. In R. cubensis (2x, 4x, 6x), R. nashii (4x, 6x), and R. virginica (2x, 4x), conspecific plants of different ploidy apparently occur sympatrically and are completely reproductively isolated (no seeds formed in experimental crosses), but there are no obvious morphological differences among them (see chromosome counts, geography, and crossing data in R. Kral and P. E. Bostick 1969). The phylogenetic analysis by G. M. Ionta et al. (2007) indicates that the evolutionary origin of R. cubensis, R. nashii, R. parviflora, and R. salicifolia (sect. Rhexia), as well as R. lutea, probably was through hybridization.

The biological situation is different in Rhexia mariana (in the broad sense, as interpreted by R. Kral and P. E. Bostick 1969). Variety mariana is diploid and is completely reproductively isolated from the two tetraploid varieties (vars. interior and ventricosa); experimental crosses between vars. interior and ventricosa fail to produce viable seeds. Varieties interior and ventricosa are morphologically similar and completely allopatric; each differs from var. mariana in a prominent feature of stem morphology—typical R. mariana has unequal stem faces while each of the two tetraploids has equal faces. These two taxa are treated here at specific rank, apart from R. mariana in the strict sense, following C. W. James (1956).

Based on molecular evidence (G. Clausing and S. S. Renner 2001), Rhexia appears to be sister to Arthrostemma Pavon ex D. Don, a genus of perennial herbs native to Central America and northwestern South America. Arthrostemma ciliatum Pavon ex D. Don has become a damaging invader on the Hawaiian Islands.

The key to Rhexia species below is artificial, a nearly inevitable requirement to separate some of the species.

SELECTED REFERENCES Ionta, G. M. et al. 2007. Phylogenetic relationships in Rhexia (Melastomataceae): Evidence from DNA sequence data and morphology. Int. J. Pl. Sci. 168: 1055–1066. James, C. W. 1956. A revision of Rhexia (Melastomataceae). Brittonia 8: 201–230. Kral, R. and P. E. Bostick. 1969. The genus Rhexia (Melastomataceae). Sida 3: 387–440. Nesom, G. L. 2012. Infrageneric classification of Rhexia (Melastomataceae). Phytoneuron 2012-15: 1–9.