Guide to Hornworts of Oregon: Phaeoceros oreganus (Aust.) Hässel de Menéndez
Hornworts 1b ventral tubers absent > Hornworts 2b spores yellow to brown > Phaeoceros 1b spores yellow > Phaeoceros 2b sporophytes short > Phaeoceros 3b spores echinate > P. oreganus
Synonyms: Anthoceros oreganus Aust.; Anthoceros mohrii Aust. (?); Phaeoceros mohrii (Aust.) Hässel de Menéndez (?)
Special status: None
Recognition: Solitary chloroplasts in young epidermal cells characterize the family; yellow spores and a smooth thallus lacking stalked ventral tubers characterize the genus. This species has bright yellow (to brownish or yellow brown in California, Doyle, pers. comm.) spores that are distally ornamented with numerous small papillae. The capsules are shorter than all other species of the genus except P. hallii. Both it and P. oreganus have capsules no longer than 2 cm and usually distinctly curved. It is typical for capsules to remain mostly closed at the end of the season, even when dried out. Tubers are lacking (Schuster 1992, as P. mohrii). The spores are very similar to P. carolinianus in distal view, somewhat more evenly and densely papillate but not different enough to appreciate without side by side comparison of several examples of both species. The proximal faces exhibit a difference that is more useful for separation. The proximal faces of P. carolinianus vary from nearly smooth to evenly but sparsely ornamented with very small papillae. In many specimens of P. carolinianus a small number of papillae are clustered near the center of the proximal faces. This central clustering is not seen in P. oreganus. Instead, P. oreganus has prominent papillae, at least half the size of papillae on the distal face, which are scattered across the entire proximal faces. Variation in density and prominence of papillae is found in many specimens so that it is necessary to observe many spores, ideally twenty or more in proximal view, before deciding whether the papillar character better fits P. oreganus or P. carolinianus.
Distribution: On wet soil next to seasonal seeps, springs or vernal ponds. Not common, I have seen it only once in Oregon, in the west Eugene wetlands, Lane County, DHW m2691a (DHW).
Comments: Doyle and Stotler (2006) treated both P. oreganus and P. mohrii under P. carolinianus because intermediate specimens between these and typical P. carolinianus were observed. They did not make any formal synonymies but left the question open for future resolution. The plants I found at the west Eugene wetland site are quite distinctive, looking in the field like P. hallii, not P. carolinianus. Late in the season (July) I was unable to tell P. hallii from P. oreganus in the field. Only later microscopical examination of spore differences worked to separate these two. Phaeoceros oreganus was by far the more abundant at this locality in 2010.
Because spores of P. carolinianus are so similar to those of P. oreganus, the short capsules are an especially useful character. The west Eugene specimens I studied match the description of P. mohrii in Schuster (1992) very closely in capsule morphology and the spore morphology is likewise identical. Although Hässel de Menéndez treated P. mohrii and P. oreganus as distinct species, Schuster opined that the differences were not sound and placed them in synonymy. It is not clear why he did not then use P. oreganus instead of P. mohrii since he observed that P. oreganus has priority if the two are synonymous.
I have studied a collection from San Diego County, California, identified in 1995 by Doyle as Phaeoceros mohrii (Doyle 7194), which matches the west Eugene material in both spore morphology and stature. Other collections from California and Missouri which I have examined show consistent features which match the Oregon material. This convinces me that P. oreganus/P. mohrii deserves recognition as (a) distinct species from P. carolinianus even though I have personally detected no basis for evaluating whether or not P. mohrii is distinct from P. oreganus. Whittmore (2009) mentions the lack of taxonomic clarity on this issue and uses P. oreganus in his treatment of California species of Phaeoceros; my treatment is essentially identical to his. Crandall-Stotler has pointed out (pers. comm.) that if P. mohrii does turn out to be, indeed, distinct from P. oreganus, it will in all likelihood be recognized to be a southeastern species and the species that occurs in Oregon obviously will continue to be called P. oreganus.
Phaeoceros oreganus. Spores: left, distal view; right, proximal view. Coyote Prairie, Lane County, Oregon. DHW m2691a.
Phaeoceros oreganus. Spores and pseudoelaters. Coyote Prairie, Lane County, Oregon. DHW m2691a.
Probably Phaeoceros oreganus, possibly Phaeoceros hallii, but spore morphology not verified. Coyote Prairie, Lane County, Oregon. Photo by Ed Alverson, May, 2011.
Phaeoceros oreganus. Above, proximal view; below, distal view. San Diego County, California. Doyle 7194.