Flora of North America species comparison

H. J. Thompson and W. R. Ernst (1967) called attention to two different floral size morphs among populations of Eucnide bartonioides, and Turner recognized them formally as different varieties. Thompson, in a personal communication to Turner, suggested that the small-flowered var. edwardsiana may be a highly autogamous derivative of the larger flowered form, and observations for this treatment showed the stigma of this form to be closely surrounded by dehisced anthers. Turner indicated that var. edwardsiana, which is found on the Edwards Plateau (Comal, Edwards, Hays, Kerr, Llano, and Travis counties), grades into var. bartonioides in Edwards County. Flowers of var. edwardsiana from Edwards County are at the large end of their size range, and a survey of specimens showed that both relatively large-flowered plants and more typical forms of var. edwardsiana occur near each other in some localities. Turner characterized var. edwardsiana as perennial, but most of the examined specimens appear to be annuals. Across its range in Mexico, E. bartonioides displays variation in flower size, with small-flowered plants that appear to be annuals found in different regions. These are likely to be separate evolutionary origins of small-flowered annuals that converge on the characteristics of var. edwardsiana.

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

Species 14 (4 in the flora).

Eucnide was placed in subfam. Mentzelioideae by I. Urban and E. Gilg (1900), Gilg (1925b), and H. J. Thompson and W. R. Ernst (1967); however, this subfamily is paraphyletic. Eucnide has been placed in molecular phylogenetic studies as sister to the rest of Loasaceae (L. Hufford et al. 2003).

Three sections of Eucnide were recognized by H. J. Thompson and W. R. Ernst (1967). Section Mentzeliopsis H. J. Thompson & W. R. Ernst, consisting only of E. urens, was distinguished on the basis of its floral architecture in which all stamens are shorter than the style and not exserted beyond the corolla. The numerous stamens of E. urens are also tightly positioned around the style. Section Sympetaleia (A. Gray) H. J. Thompson & W. R. Ernst consists of three species that are endemic to the Baja California Peninsula and surrounding islands, except for E. rupestris, which has a distribution that extends into extreme southern California and southwestern Arizona and to Sinaloa and Sonora, Mexico. Species of sect. Sympetaleia have stamens with monothecate, bisporangiate anthers in contrast to other members of the genus, which have more conventional bithecate, tetrasporangiate anthers. All other species of Eucnide were placed in sect. Eucnide by Thompson and Ernst.

Among the North American species of Eucnide, E. urens is the only species found in the Mojave Desert, where it is centered (H. J. Thompson and W. R. Ernst 1967). The other North American species are found in the Chihuahuan and Sonoran deserts and in adjacent areas. All species are found in similar cliff or rocky slope habitats (uncommonly in arroyos and washes). Some species, such as E. bartonioides, have fruit pedicels that are negatively phototropic and elongate extensively, which appear to be adaptations for dispersal on cliffs (Thompson and Ernst).

Most species of Eucnide are self-pollinating (H. J. Thompson and W. R. Ernst 1967), although they generally have some spatial separation between the stigmas and anthers soon after the flowers open that allows for cross-pollination (L. Hufford 1988). Only taxa with the largest flowers, such as E. bartonioides var. bartonioides (possibly pollinated by hawk moths) and E. urens (pollinated by the melittid bee, Hesperaster laticeps), appear to be strictly outcrossing (Thompson and Ernst).

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