Do ferns hybridize? One might as well ask, "Is the Pope Catholic?" Ferns are notorious for their propensity to develop "hybrid complexes," in which species cross-fertilize in multiple combinations and the hybrids establish true-breeding sexual or asexual populations. Known examples in Oregon's flora include Polystichum (sword-fern, holly-fern), in which P. californicum, P. kruckebergii, and P. scopulinum are polyploid species of hybrid origin, and Polypodium (polypody), with P. hesperium representing a similarly fertile polyploid hybrid. Other Oregon ferns that may be involved in hybrid complexes include species of Asplenium, Cystopteris, Dryopteris, Gymnocarpium, and Woodsia.
The fact that particular species of ferns have originated through hybridization is usually not mentioned in standard floristic references, but an outstanding exception is the recently published Flora of North America, Volume 2 (1993). In this volume, which covers North American pteridophytes and gymnosperms, there are charts showing species' relationships within many genera, citing numerous hybrids which may be sterile, sexually fertile, or fertile by asexual means.
Species of the genus Cheilanthes are called "lip-ferns" because the margins of the leaflets roll over and cover the spore-cases like lips covering a mouthful of teeth. Flora of the Pacific Northwest mentions two Oregon species, C. feei Moore (Fee's lip-fern) and C. gracillima D. C. Eaton (lace lip-fern). The former reaches the state only in Wallowa County, while the latter occurs commonly in rocky sites throughout the Cascade Range and Siskiyous, as well as disjunctly in the Wallowas. In Peck's Manual of the Higher Plants of Oregon a third species, C. intertexta (Maxon) Maxon (coastal lip-fern), is cited from the "Siskiyou Mtns." southward to California. However, the only recorded sighting of C. intertexta in Oregon is a 1930s collection by F. H. Heckner from "lava country east of Brownsboro, Jackson Co.," a site in the south Cascade Range, not the Siskiyous. The species has long been listed (e.g. ONHP List 2) as rare in Oregon but stable elsewhere.
In June of last year, Richard Brock made a collection of Cheilanthes on Heppsie Mountain, east of Brownsboro, which resembled C. intertexta and therefore might comprise the second known occurrence of this species in Oregon. However, when I studied this specimen at Richard's request, I came to the surprising conclusion that it is not C. intertexta after all, but rather the closely related taxon C. covillei Maxon! This species is best known from desert regions of Arizona and southern California, but according to The Jepson Manual—Higher Plants of California, it extends northward to the Sierra Nevada and North Coast Range in California. Its key differences from C. intertexta are the shape and degree of dissection of the tiny scales covering the underside of the leaflets. As shown in the accompanying drawings, the scales of C. covillei are ovate, hairy at the base, and overlapped to form a continuous cover over the spore-cases. In C. intertexta the scales are much narrower and irregular in shape, with hairs on the base and margins. Cheilanthes gracillima is closely related to these two species, but it has matted hairs and narrow scales among the spore-cases, and its leaves are only twice-pinnate rather than three times divided.
The final piece of the lip-fern puzzle in Jackson County came into place recently, when Richard Callagan showed me a collection of Cheilanthes he had made in 1998, on rock outcrops in the Little Butte Creek region south of Heppsie Mountain. His plants represent C. intertexta and are similar to the 1930s collection by F. H. Heckner from "east of Brownsboro."
To return to the theme of hybridization with which I began this article, C. gracillima, C. covillei, and C. intertexta—the three lip-ferns now known to occur in eastern Jackson County—comprise a unique "hybrid complex." According to Flora of North America and other references, C. intertexta evolved as a fertile polyploid hybrid from the cross of C. gracillima times C. covillei. Like many other hybrid fern species, C. intertexta has an ancient origin and behaves like an independent entity, able to grow alongside its parental species and remain genetically isolated from them (although sterile offspring sometimes are produced from back-crossing with C. gracillima). Future field studies in the southern Cascades of Oregon might uncover sites where two or more of these lip-fern taxa grow together, possibly with back-cross hybridization adding to the morphological complexity of the group.
 Cheilanthes gracillima showing undersurface of leaf. Drawing by Jeanne R. Janish from Hitchcock et al. 1969, Vascular Plants of the Pacific Northwest, courtesy of University of Washington Press. |