Flora of North America species comparison

Mentzelia decapetala is introduced in Grundy County, Illinois. It appears to be native throughout the rest of its distribution.

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

Species ca. 95 (85 in the flora).

Mentzelia is monophyletic and sister to subfam. Gronovioideae (M. L. Moody et al. 2001; L. Hufford et al. 2003). These studies recovered clades that correspond to the six described sections of Mentzelia, including the five treated here and the monospecific sect. Dendromentzelia Urban & Gilg, which consists only of M. arborescens Urban & Gilg of Michoacán and Oaxaca, Mexico.

Mentzelias in North America often are ruderal or found in low-productivity, disturbance-prone environments, such as arroyos, sand dunes, cliffs, or talus slopes. They frequently are associated with distinctive substrates, including gypsum, limestone, serpentine, or volcanic tuff, and some might be edaphically restricted (J. B. Glad 1976; H. J. Thompson and A. M. Powell 1981; C. M. Christy 1997; N. H. Holmgren and P. K. Holmgren 2002; Brokaw et al. 2015).

Changes in chromosome number, through both aneuploidy and polyploidy, have been important in the evolution of Mentzelia. Aneuploidy has been especially important in reproductive isolation in sections Bartonia (haploid chromosome numbers include 9, 10, 11, and 18) and Bicuspidaria (haploid chromosome numbers include 9, 10). In contrast, aneuploidy has not been found in sections Mentzelia or Trachyphytum, although polyploidy has been found in both. In sect. Mentzelia, polyploidy has been reported only in M. asperula. Polyploidy has played a more important role in reproductive isolation and diversification in sect. Trachyphytum, in which diploids, tetraploids, hexaploids, and octoploids have been reported (H. J. Thompson and H. Lewis 1953).

Floral forms differ among the sections of Mentzelia, which may be indicative of reproductive shifts that were important in the differentiation of the sectional lineages. Reproductive ecology has been investigated best in sect. Bicuspidaria, in which Xeralictus bicuspidariae, a robust oligolectic bee, is the main pollinator of M. hirsutissima, M. tricuspis, M. tridentata, and is one of three main pollinators of M. involucrata (G. S. Daniels 1970). Only M. reflexa, which has markedly different flowers compared to other species of sect. Bicuspidaria, is not pollinated by X. bicuspidariae. Instead, another oligolectic bee, Perdita koebelei, is its primary pollinator (Daniels). Bees have been suggested to be the primary pollinators of other sections of Mentzelia, except for the few large-flowered species of sect. Bartonia in which hawk moth pollination has been observed (H. J. Thompson 1960; A. R. Moldenke 1976; R. J. Hill 1977). Many polyploid species of sect. Trachyphytum have small flowers and reportedly self-fertilize (J. E. Zavortink 1966).

Seed surface features are important in Mentzelia taxonomy. In sections Dendromentzelia, Mentzelia, and Micromentzelia, which compose a basal grade in Mentzelia, seed coat cells are longer than wide. In contrast the derived clade consisting of sections Bartonia, Bicuspidaria, and Trachyphytum has seed coat cells that are polygonal and largely isodiametric. The shapes of anticlinal and outer periclinal walls of seed coat cells are similar in sections Bicuspidaria and Trachyphytum but differ among the other sections of North American species. Seed coat cell anticlinal wall shape, which is particularly useful in distinguishing species in sect. Bartonia, may be straight, wavy (curves along walls forming less than 45° angles), or sinuous (curves along walls forming at least 45° angles, often forming U-shaped undulations).

Leaf and bract margins in Mentzelia usually are toothed or lobed to some degree, ranging from serrate or dentate (divided relatively shallowly) through pinnate (defined here as divided partway to the midvein) to pinnatisect (divided nearly or to the midvein).

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