Flora of North America species comparison

Zeltnera arizonica and Z. calycosa appear to intergrade in western Texas and Coahuila, Mexico, as noted by C. R. Broome (1973), and have often been treated as varieties of a single species. Zeltnera arizonica was subsumed in undivided Centaurium calycosum by N. H. Holmgren (1984b), who attributed its allegedly distinguishing features largely to environmental effects, whereas B. L. Turner (1993d) considered the resemblance between these taxa to be superficial and Z. arizonica (as Centaurium) appropriately recognized at species rank. From studies for this flora, acceptance of this species seems warranted.

In Zeltnera arizonica, the relatively sparse branches generally spread at 10–20°, whereas in Z. calycosa the usually denser branches spread at 20–60°.

Zeltnera arizonica is highly variable in the proportionate width of its corolla lobes. Some plants in the western part of its range resemble Z. exaltata vegetatively but differ in having corolla lobes much longer in proportion to the tube.

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

The inflorescence of Zeltnera trichantha is generally corymboid, but the level at which plants first branch varies. Plants branching at or near the base, so that the whole plant above ground is obconic, plants branching only in the upper fifth or less, and a complete range of intermediates may occur within a single population.

Unequivocal Zeltnera trichantha occurs in the Inner North Coast Range and San Francisco Bay region of California, from San Mateo and Stanislaus counties north to Tehama County, often on alkaline flats and in serpentine soils. This species can generally be recognized by a distinctive combination of relatively dense, obconic inflorescences; all flowers sessile or on true pedicels usually less than 4 mm, rarely to 6 mm; narrow, acute to acuminate corolla lobes, notably contrasting with the obtuse or abruptly acute corolla-lobe apices prevalent among the other Zeltnera species in the flora area; styles that are longer and more slender than those of most Zeltnera species, often extending 6–11 mm beyond the throat of the corolla; and cuneate stigmas 0.2–0.3 mm wide at the summit, which remain more or less appressed to each other throughout much of the life of the flower. Basal leaves are absent at flowering time. Usually, but less consistently, the cauline leaves are elliptic, widest near the middle, tapering toward the base, and acute to acuminate at the apex. Elsewhere in California, plants of Z. venusta with narrow corolla lobes often resemble Z. trichantha to various degrees. These species are contrasted in the discussion of Z. venusta. Other species, notably Z. muehlenbergii, occasionally approach Z. trichantha in habit, but, like Z. venusta, differ in having two distinctly separate, fan-shaped stigmas on a shallowly cleft style.

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