Flora of North America species comparison

The only specimen from Nevada identified as Frasera paniculata has been reidentified as F. albomarginata var. albomarginata in studies for this flora.

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

Species 15 (15 in the flora).

The division of Swertia in the broad sense, which is sometimes circumscribed so as to include Frasera, is supported by molecular phylogenetic studies by P. Chassot et al. (2001), but as of this writing, the matter of the most appropriate segregation of genera is unsettled. The acceptance of generic status for Frasera, which has become more frequent in recent years, is provisional in this flora. Relatively few of the North American species were included in the study by Chassot et al., and its usefulness for generic delimitations is further impaired by the molecular techniques available at the time. Satisfactory morphological characterizations of Frasera and other genera that might be segregated from Swertia in the broad sense remain elusive. Some character states that have sometimes been said to distinguish Frasera from Swertia in the narrow sense, including the presence or absence of connate-sheathing leaf bases, four- versus five-merous flowers, and whether or not a distinct, slender style is present, are not consistently applicable in the flora area, and some extralimital species that clustered closely with S. perennis in the phylogenetic study cited above are morphologically similar to species treated as Frasera in the flora area. Further generic realignments in the complex are to be expected.

Characters associated with the nectaries are important in distinguishing among the species and varieties of Frasera, so herbarium specimens should be prepared so that the adaxial surfaces of some corollas are visible. The nectaries, which may be one or two on each corolla lobe, are located in pits, called foveae, which open adaxially. Two kinds of foveal morphology are present in North American Frasera. In species 1 through 5, the opening is immediately adaxial to the nectary, and a raised, fringed rim surrounds the fovea opening only. This foveal morphology is present in the type species of both generic names Frasera and Swertia. In species 6 through 15, including all of the species that have been placed in Leucocraspedum, the fovea is pocketlike, with the adaxial opening distal to the nectary or nectaries. The raised, fringed rim extends beyond the opening of the fovea, so as to surround both the opening and a distally adjacent portion of the adaxial corolla surface differentiated in color and texture. These structures associated with the nectaries are to be distinguished from the androecial coronas of some species, which consist of fimbriate ridges or entire, serrate, or laciniate scales between the bases of the filaments.

In the first group of species distinguished above, the ovary in species 1 and 2 tapers toward the stigmatic lobes with scarcely any differentiation of a style, as also occurs in Swertia perennis, whereas in species 3 through 5, including Frasera caroliniensis, the type of the generic name Frasera, a distinct, slender style is present. All species in the second group have distinct, slender styles. White leaf margins prevail in the second group of species but in the first group are present only in F. tubulosa.

The inflorescences of Frasera are thyrses or verticillasters, consisting of a central axis along which smaller thyrses (proximally) and/or dichasial or modified cymules are borne in pairs or whorls. The cymules of the larger plants are usually accompanied by flowers on pedicels that arise directly from the main axis. In F. coloradensis and F. parryi, the primary branches of the inflorescence may be nearly as long as the central axis. Small inflorescences, which occur frequently in F. ackermaniae and F. gypsicola and occasionally in F. albicaulis, may be racemoid. Inflorescences are described here as dense if the primary branches are closely spaced, less than 2 cm apart except sometimes at the proximal nodes, and relatively short, so that the inflorescences are usually less than 6 cm wide and/or more than five times as long as wide, with crowded flowers; as narrow but not dense in F. gypsicola, in which the inflorescence width is similar but the flowers are few and are not closely spaced; or as diffuse, if the branches are more widely separated and both the branches and the pedicels are longer and generally strongly divergent, so that the inflorescences are generally more than 6 cm wide and most of the flowers are well separated.

Plants of at least some of the monocarpic species of Frasera remain in a rosette stage for several to many years. Studies of F. caroliniensis and F. speciosa have shown that, within any one population, after several years in which all or most plants have remained in the rosette stage, all or most of the larger plants are likely to flower in the same year, while the smaller plants remain vegetative. The environmental factors that induce flowering in such species are not well understood. In some species, the plants are monocarpic with respect to the original crown, which dies after flowering, but prior to flowering may produce new crowns from adventitious buds on the roots. These may persist as independent plants and flower in later years, after having attained larger size (D. W. Inouye and O. R. Taylor 1980; P. F. Threadgill et al. 1981).

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