Centaury [For Louis Zeltner, b. 1938, and his wife, Nicole Zeltner, b. 1934, Swiss botanists]

James S. Pringle

Herbs annual, biennial, or perennial, glabrous throughout except in Z. glandulifera. Leaves cauline, opposite, often also basal. Inflorescences dichasial or partly to largely monochasial cymes. Flowers 4- or 5-merous; calyx deeply lobed, lobes narrowly linear, usually ± ridged, distinctly keeled only in Z. davyi; corolla pink to rose-violet (white), with ± distinct white or occasionally pale green or pale yellow eye, salverform, tube generally constricted above ovary (scarcely so in Z. nudicaulis), lobes abruptly spreading, shorter to slightly longer than tube, margins entire or erose-tipped, neither adaxial scales or trichomes nor plicae between lobes present; stamens inserted in distal 1/2 of corolla tube, in some species all initially curved to one side; anthers distinct, coiling helically at dehiscence; ovary sessile; style deciduous, erect or initially deflexed away from stamens, distinct, not cleft or cleft to 1 mm or less; stigmas 2 or, if 1, 2-lobed, stigmas or lobes generally ± fan-shaped and divergent, lobes connivent and sometimes appearing as a subcapitate stigma only in Z. trichantha; nectaries absent. Capsules narrowly ellipsoid to ovoid. x = 17, 21; disploidy frequent.

Species ca. 25 (14 in the flora): North America, Mexico, West Indies, Central America, South America; temperate to tropical regions.

Plant size varies greatly in Zeltnera species, especially in Z. venusta and others that sometimes grow in intermittently moist habitats such as the edges of vernal pools. Small, one- or two-flowered plants, some only 3–10 cm, do not exhibit the characters of branching pattern, basal-leaf numbers or persistence, leaf and flower size, pedicel length, and orientation of styles and stamens by which the species are usually distinguished. It is scarcely feasible to identify such plants strictly from their own morphology; their provenance and the identity of larger plants in the vicinity must be considered. Branching pattern appears also to be affected by the density of the surrounding vegetation.

Some Zeltnera species are primarily outbreeding. They have large, showy corollas with conspicuous, sharply defined eyes. The style is at first deflexed in one direction and the stamens in the opposite. Later, the style becomes erect, and the stamens become radially disposed and erect to incurved. In these species, the stigma or stigmas are borne on a slender style often as long as or longer than the ovary at anthesis and much exceeding the stamens, although the latter are also distinctly exserted. Style length may vary among the flowers on a single plant, although true heterostyly is not known to occur in the genus. Smaller, presumably autogamous flowers may be present along with larger flowers. Other species are primarily, although probably not obligately, autogamous. They have smaller, often less brightly colored corollas with poorly defined eyes. The styles are shorter and generally erect, and the filaments may be erect or incurved throughout the life of the flower. Zeltnera arizonica, Z. beyrichii, Z. calycosa, Z. maryanniana, Z. trichantha, and Z. venusta exhibit the floral syndrome associated with outbreeding; the other taxa in the flora area, to various degrees, exhibit the morphology associated with autogamy (C. R. Broome 1973).

G. Mansion and L. Zeltner (2004) divided Zeltnera informally into a Californian group including species 1 through 6, a Texan group including species 7 through 14, and a Mexican group represented in the flora area only by Z. nudicaulis. Within the respective species-groups, especially the Californian group, some of the species are poorly defined morphologically. Some species, especially Z. venusta, vary greatly in the size and shape of the corolla lobes as well as in habit and vegetative characters and may thus resemble other species. Hybridization, intro­gression, and allopolyploidy are believed to account for some of the variation that has made Zeltnera problematic taxonomically, although some variation suspected of being due to hybrid­ization may merely represent variability within species. Some otherwise well-differentiated species tend to be more similar to each other in morphology where their ranges overlap; in such cases, similarity has been interpreted as being due more to convergent responses to selective pressures than to interspecific hybridization (C. R. Broome 1981). In some localities, populations of intermediates appear to be more or less stabilized. In the classification presented here, plants treated as or suspected of being hybrids and introgressants may be more numerous and more widely distributed than one would consider pragmatic. Nevertheless, this treatment seems to present a truer picture of the evolutionary situation than would the recognition of intermediates between sympatric species as additional species or subspecies. In California, intergradation has been reported to involve, in various combinations, Z. davyi, Z. exaltata, Z. muehlenbergii, Z. namophila, Z. trichantha, and Z. venusta. In Texas, Z. calycosa appears to hybridize with Z. beyrichii and Z. texensis.

SELECTED REFERENCES Mansion, G. 2004. A new classification of the polyphyletic genus Centaurium Hill (Chironiinae, Gentianaceae): Description of the New World endemic Zeltnera, and reinstatement of Gyrandra Griseb. and Schenkia Griseb. Taxon 53: 719–740. Mansion, G. and L. Zeltner. 2004. Phylogenetic relationships within the New World endemic Zeltnera (Gentianaceae-Chironiinae) inferred from molecular and karyological data. Amer. J. Bot. 91: 2069–2086.