Flora of North America species comparison

The name Centaurium muehlenbergii has often been misapplied to C. tenuiflorum in the flora area (J. S. Pringle 2010b). This error accounts for reports of C. muehlenbergii in Louisiana, Mississippi, and Texas. The name C. floribundum, although typified by specimens referable to Zeltnera muehlenbergii, has also been misapplied to C. tenuiflorum, hence comments by some authors that C. floribundum should perhaps be included in C. tenuiflorum. This complex nomenclatural history has caused confusion with regard to the conservation status of plants called Centaurium or Z. muehlenbergii. From studies for this flora work, it appears that the true Z. muehlenbergii is appropriately of conservation concern.

The inflorescences of Zeltnera muehlenbergii are relatively open, in contrast to the dense, corymboid dichasia of Centaurium tenuiflorum. The proximal branching of the larger plants of Z. muehlenbergii is usually dichasial, with the central flower sessile or on a pedicel to 5 mm long or occasionally to 12 mm. The distal branching is mostly monochasial, with a branch developing only on one side of each flower; the inflorescences of small plants are often monochasial throughout. The flowers at the distal divisions are sessile or on pedicels to 4 mm. Both C. tenuiflorum and C. pulchellum, which are sometimes similar in aspect, differ from Z. muehlenbergii in the style and stigma characters noted in the descriptions of the respective genera (J. S. Pringle 2010b).

Some plants of Zeltnera davyi and exceptionally small plants of Z. exaltata look much like Z. muehlenbergii but consistently have proximal pedicels more than 4 mm long and often more than 12 mm. Even on small plants the pedicels of Z. exaltata are often longer than the flowers.

The name Monterey centaury is sometimes associated with this species, but that vernacular name came into use when the name Centaurium muehlenbergii was being misapplied to Zeltnera davyi. True Z. muehlenbergii has rarely been found as far south as the northern shore of Monterey Bay (J. S. Pringle 2010b).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Although the maximum flower size in Zeltnera venusta is the largest for any Zeltnera species in the flora area, both plant and flower size are highly variable in this species, and corolla size is not reliable for the identification of this species. Small, one-flowered plants and plants intermediate in stature and flower size may be present near larger plants in microhabitats where less time has elapsed between seed germination and flowering. The whitish corolla eye of Z. venusta is especially prominent, sometimes with rose-purple spots in the throat aligned with the midveins of the corolla lobes.

Plants of Zeltnera venusta with relatively large flowers and proportionately wide corolla lobes are readily identifiable. Corolla lobes, however, range from linear or narrowly oblong to widely elliptic, with apices ranging from rounded (usually associated with proportionately wide corolla lobes) to acute. The largest flowers and the proportionately widest corolla lobes tend to occur in populations near the Pacific coast, from Los Angeles County, California, to Baja California, Mexico. Variation in corolla-lobe shape is especially pronounced in Fresno County, California, where plants with wide, elliptic lobes rounded at the apex and plants with narrowly lanceolate, acute-tipped lobes are sympatric and occur with many intermediates. Plants and flowers tend to be smaller in populations northward and toward the interior, especially at higher elevations, but plants with relatively small, narrowly lobed corollas occur as far south as Riverside, San Bernardino, and San Diego counties in California. Plants approaching all extremes in length-width proportions and other aspects of shape sometimes occur within a local population, as indicated by herbarium specimens at DUKE and UC that represent sampling from single populations. In studies for this work, no distinct entities were discerned that could be segregated from Z. venusta or recognized as infraspecific taxa.

Plants of Zeltnera venusta with relatively wide corolla lobes may resemble the similarly large-flowered species Z. arizonica and Z. calycosa. The corolla lobes of such plants of Z. venusta widen abruptly above the base, whereas those of the other large-flowered species widen more gradually.

In parts of California, plants of Zeltnera venusta with narrow corolla lobes resemble Z. trichantha to various degrees. Relatively densely branched plants of Z. venusta are highly similar to Z. trichantha in aspect and branching pattern. Such plants are more common north of Yosemite National Park, especially in Shasta County, but occasionally occur south to western Riverside and San Bernardino counties.

Zeltnera trichantha and Z. venusta are largely allopatric, but their ranges overlap northward. Although C. R. Broome (1973) was unsuccessful in attempts to cross Z. trichantha with other California species, morphology suggests that natural hybridization occurs, especially in Lake, Sonoma, and Tehama counties, California, and that introgression may be extensive. Some suspected hybrids resemble Z. trichantha but combine the elliptic leaves and subsessile flowers usual in that species with wider, obtuse corolla lobes and/or shorter styles and with stigmas approaching those of Z. venusta in morphology. Other plants differ from typical Z. trichantha in their more open inflorescences and longer-pedicelled proximal flowers, or in having very small stigmas, like those usually occurring in Z. trichantha, on distinctly cleft styles, like those of Z. venusta.

The type specimens of the names Centaurium venustum subsp. abramsii Munz and Zeltnera abramsii (Munz) G. Mansion, and most of the other specimens that have been so identified, are interpreted here as probable hybrids between Z. trichantha and Z. venusta. These names are typified by plants from the vicinity of Redding, Shasta County, California, that combine traits of these two species. Subsequent reports of this taxon have largely been limited to scattered localities in this part of northern California. The corolla lobes of the type are narrow and lanceolate, as in Z. trichantha, rather than oblong, but are obtuse at the apex, as in Z. venusta. The stigmas are small, about 0.3 mm wide, as in Z. trichantha, but the style divides into two branches about 1 mm below the stigmas, as in Z. venusta. The largest seeds are about 0.4 mm long and dark brown, but seed size varies and some appear malformed. Other specimens from the vicinity of Redding are more characteristic of Z. venusta in the narrow sense, and both Z. trichantha and Z. venusta occur in Shasta County. Reports of Z. abramsii from elsewhere in California (G. Mansion 2004) may have been based on variants of other species, or plants representing other hybrid combinations.

In molecular studies by G. Mansion and L. Zeltner (2004), plants identified as Zeltnera abramsii appeared in a basal position in the clade comprising the Californian group, including Z. trichantha and Z. venusta. The apparently basal position of Z. abramsii is not irreconcilable with a hypothesis of its hybrid origin, however, as molecular phylogenetic studies using nrITS or chloroplast DNA are not well suited for the detection or confirmation of hybridization.

In southeastern California, Zeltnera venusta and Z. arizonica are sometimes similar in appearance, but their respective ranges are separated by a part of the Sonoran Desert. Generally, Z. venusta is single stemmed at the base and Z. arizonica is multistemmed, although occasional specimens of Z. venusta are multistemmed, perhaps as a response to injury, and the smallest plants of Z. arizonica are sometimes single stemmed. The corolla lobes of Z. venusta are usually conspicuously and abruptly narrowed at the base, thus differing from those of Z. arizonica, but the plants of Z. venusta with the narrowest corolla lobes show little difference from Z. arizonica in this respect.

Plants from Los Angeles and San Diego counties, California, with pedicels to 50 mm and sometimes with several stems from the base and/or with more or less persistent basal leaves, may be hybrids with the southwestern form of Zeltnera exaltata.

(Discussion copyrighted by Flora of North America; reprinted with permission.)