Rhexia
Sp. Pl. 1: 346. 1753.
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.
Distribution
North America, West Indies (Greater Antilles).
Discussion
Species 13 (13 in the flora).
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
None.
Lower Taxa
Key
| 1 | Petals golden-yellow. | Rhexia lutea |
| 1 | Petals lavender, lavender-rose, lavender-purple, purple, pink, or white. | > 2 |
| 2 | Inflorescences condensed, mostly obscured by foliaceous bracts; leaf blades ovate to short-elliptic or suborbiculate, 1–2 cm; anthers 1.2–2 mm; roots relatively short, fibrous, lignescent and non-tuberiferous. | > 3 |
| 2 | Inflorescences diffuse, not obscured by bracts; leaf blades lanceolate to elliptic, ovate, oblong, oblanceolate, linear, or spatulate, 1.5–7.5 cm; anthers 3–11 mm; roots relatively long, rhizomelike, sometimes tuberiferous, sometimes lignescent (except short and fibrous in R. alifanus). | > 4 |
| 3 | Calyx lobes deltate, apices obtuse; hypanthia villous-hirsute, hairs gland-tipped; seeds irregularly ridged. | Rhexia nuttallii |
| 3 | Calyx lobes oblong-lanceolate, apices acute; hypanthia mostly glabrous except along calyx lobes; seeds pebbled or with ridges of domelike processes. | Rhexia petiolata |
| 4 | Stem internodes usually glabrous. | > 5 |
| 4 | Stem internodes hirsute to villous or hispid-villous. | > 9 |
| 5 | Stem internodes and nodes glabrous; leaf blade margins entire or subentire, teeth remote, low, blunt not bristle-tipped, surfaces glabrous. | Rhexia alifanus |
| 5 | Stem internodes glabrous, nodes hirsute to villous; leaf blade margins serrate or crenulate, teeth bristle-tipped, surfaces glabrous, glabrate, strigose, hirsute, or villous. | > 6 |
| 6 | Anthers 3–3.5 mm. | Rhexia parviflora |
| 6 | Anthers 5–8 mm. | > 7 |
| 7 | Leaf blades 3–9 mm wide, margins shallowly serrate to barely crenulate; hypanthia hispid-hirsute at neck, rim, and calyx lobes, hairs eglandular, yellowish. | Rhexia aristosa |
| 7 | Leaf blades (5–)8–20(–35) mm wide, margins serrate or finely serrate; hypanthia glabrous, glabrate, hirsute-villous, or sparsely villous, hairs gland-tipped. | > 8 |
| 8 | Leaf blades (7–)10–20(–35) mm wide; stem faces subequal, angles narrowly winged. | Rhexia virginica |
| 8 | Leaf blades (5–)8–15(–20) mm wide; stem faces strongly unequal. | Rhexia mariana |
| 9 | Stem faces unequal. | > 10 |
| 9 | Stem faces subequal. | > 12 |
| 10 | Hypanthia glabrous or glabrate (except calyx rims and lobes); petals 2–2.5 cm. | Rhexia nashii |
| 10 | Hypanthia usually hirsute-villous or hirsute; petals 1.2–2 cm. | > 11 |
| 11 | Hypanthia (10–)14–15(–16) mm. | Rhexia cubensis |
| 11 | Hypanthia 6–10 mm. | Rhexia mariana |
| 12 | Anthers 3–3.5 mm; petals white or pale lavender. | Rhexia parviflora |
| 12 | Anthers 5–8 mm; petals pink to lavender-rose, lavender-purple, or purple. | > 13 |
| 13 | Leaf blades 1–5 mm wide, margins entire or minutely crenulate, ciliate, with gland-tipped hairs; petals 1.1–1.2 cm; hypanthia (4–)5–7(–8) mm. | Rhexia salicifolia |
| 13 | Leaf blades (7–)10–20(–35) mm wide, margins serrate to serrulate, not ciliate; petals 1.2–2 cm; hypanthia 6–10 mm. | > 14 |
| 14 | Stem angles narrowly winged; stems usually unbranched or few-branched proximally; stem internodes and hypanthia usually sparsely villous, sometimes glabrous. | Rhexia virginica |
| 14 | Stem angles sharp, without wings or very narrowly winged; stems unbranched or few- to several-branched distally; stem internodes hirsute-villous, hypanthia hirsute-villous, glabrous, or glabrate. | > 15 |
| 15 | Seed surfaces with papillae in concentric rows; Atlantic coast states. | Rhexia ventricosa |
| 15 | Seed surfaces irregularly ridged in concentric rows or with laterally flattened, domelike processes; c, s United States. | Rhexia interior |