2. Bartonia paniculata (Michaux) Muhlenberg, Cat. Pl. Amer. Sept. 16. 1813.
[E F]
Branched or twining bartonia, panicled screwstem, bartonie paniculée
Centaurella paniculata Michaux, Fl. Bor.-Amer. 1: 98, plate 12, fig. 1. 1803
Herbs decumbent to erect or ± twining, yellowish green to purplish, 3–52 cm. Leaves all alternate or distal or occasionally most leaves opposite or subopposite, closely spaced near base of stem, gradually more widely spaced distally; blade 0.5–3 mm. Inflorescences racemoid cymes, or thyrses with branching variable, often arcuate-ascending. Flowers: calyx lobed nearly to base or some or all lobes proximally connate, lobes lanceolate to ovate, 1–3.2 × 0.3–1.1 mm, that is, 0.5–0.8 times the length of corolla, apex acute to acuminate; corolla white or occasionally pale yellow to green, distally often purple-tinged, 2–6.2 mm, lobes oblong-lanceolate, 1.5–4 × 0.7–2 mm, margins entire, apices acute to acuminate, not mucronate; anthers not recurving or coiling, yellow or purple, 0.3–0.9 mm, apex rounded to obtuse, sometimes mucronate; style absent; stigmas decurrent along sutures of ovary. Capsules dehiscent from apex.
Subspecies 3 (3 in the flora): c, e North America.
Bartonia paniculata is less well differentiated morphologically from B. virginica than some references indicate. The stems of some plants of subsp. paniculata are sinuous or occasionally twine around grass culms, but many are as straight and erect as those of B. virginica. Conversely, the stems of B. virginica are often decumbent and occasionally somewhat sinuous. Both species vary in the extent of purple suffusion in the stems. The leaves of some plants of B. paniculata are all widely spaced and distinctly alternate, but other plants have mostly opposite or subopposite leaves and/or closely spaced proximal leaves, as B. virginica has been characterized. The inflorescence branches of B. paniculata are often relatively long and divergent or distally arcuate-ascending but are sometimes short and/or strongly ascending their whole length as in B. virginica. Most such specimens represent variability in vegetative morphology within the respective species rather than true intergradation or hybridization. Corolla, anther, and stylar characters reliably distinguish these two species and indicate that they generally retain their distinctness where sympatric. Occasional plants have been found, mostly on the Atlantic plain, that are intermediate in corolla and vegetative morphology and at least in some cases are sterile. K. G. Mathews et al. (2009) found a few plants combining traits of both B. paniculata and B. virginica but no plants exhibiting intermediate character states.
Bartonia paniculata subsp. paniculata and subsp. iodandra intergrade. Plants combining deeply four-parted, narrowly lobed calyces with various degrees of purple suffusion in the anthers are relatively frequent along the Atlantic coast from New Jersey to Nova Scotia and occasionally occur south to Maryland. Plants variously combining traits attributed to subsp. paniculata and subsp. iodandra have been called var. intermedia. Most such plants are included in subsp. iodandra here, following J. M. Gillett (1959). Plants intermediate between subsp. paniculata and subsp. texana have been reported from Caddo Parish, Louisiana (K. G. Mathews et al. 2009).
