Guide to Hornworts of Oregon: Phaeoceros carolinianus (Michx.) Prosk.
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Hornworts 1b stalked ventral tubers absent > Hornworts 2b spores yellow > Phaeoceros 1b spores not brown > Phaeoceros 2b sporophytes long > Phaeoceros carolinianus
Synonym: Anthoceros carolinianus Michx.; Anthoceros laevis var. carolinianus (Michx.) Lindb.; Phaeoceros laevis var. carolinianus (Michx.) Prosk.
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 spores that are ornamented on their distal face with small bumps (papillae, called verrucae in some literature) seen under a light microscope, or spines as seen in SEM images. The proximal faces have a fine, even roughness and often (but not always) a cluster of papillae in the center of each face. Mature capsules are straight, 3-5 cm long, splitting to shed spores as they dry out.
Distribution: On seasonally moist, bare mineral soil, close to or actually in seepage areas or sheet drainage. Occasional but widespread in western Oregon, from coastal deflation plains to middle elevations in the western Cascades. Representative specimens examined: Coos Co.: Cape Arago, Sanborn s.n. (OSC); Curry Co.: Harris Wayside, Wagner 1986 (ORE); Grant Co.: North Fork Malheur River, Kofranek 6108 (DHW); Lane Co.: Lookout Creek, Andrews Experimental Forest, Wagner 1474 (ORE); Lincoln Co.: Yaquina Head, Wagner m2244 (OSC); Linn Co.: Santiam Highway milepost 48, Wagner m2687 (OSC); Tillamook Co.: Hug Point, Lyford 2170 (OSC).
Comments: Phaeoceros carolinianus is one of the more common yellow-spored hornworts of western Oregon. Phaeoceros pearsonii is equally abundant and seems to tolerate drier habitats than P. carolinianus. They may grow together in seepy, disturbed soil that dries out in summer.
There has been some discussion about the presence of P. mohrii and P. oreganus in our region (Doyle 2004, pers. comm.) but all specimens provisionally identified as one or the other of these species were treated under P. carolinianus in the keys to California hornworts (Doyle and Stotler 2006). They did not make any formal synonymies but left the question open for future resolution. See further discussion of the taxonomic issues under P. oreganus.
With careful observation using light microscopy it is possible to detect distinctive spore features. The proximal faces of P. carolinianus normally vary from nearly smooth to evenly but sparsely ornamented with very small papillae, only rarely as large as papillae on the distal face. In many specimens of P. carolinianus a few 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, nearly equal to 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. Whittemore (2009) uses the number of papillae (which he terms verrucae) to differentiate the two: more than 30 on each proximal face in P. oreganus and less than 20 papillae on proximal faces of P. carolinianus. This criterion was useful in naming the Kofranek specimen from Grant County, which has quite large papillae on the proximal face of many spores but those are mostly less than 20 in number. Also, its capsule length and dehiscence pattern fits P. carolinianus well. This is the only specimen of Phaeoceros I have seen that was found east of the Cascade Mountains in Oregon.
Phaeoceros carolinianus, McKenzie Bridge, Lane Co., Oregon.
Phaeoceros carolinianus, Sutton Beach, Lane Co., Oregon. DHW m2414a
Distal view of spores. Santiam highway, Linn Co., Oregon. DHW m2680.
Proximal view of spore. Santiam Highway, Linn Co., Oregon. DHW m2680.
first and third spores in distal view, second and fourth spores in proximal view.E Fk. S Fk. Willamette River, Lane Co., Oregon. DHW m1992.
Left two spores in proximal view, right spore in distal view. E Fk. S Fk. Willamette River, Lane Co., Oregon. DHW m1992.
Upper left and lower right in proximal view, lower left in distal view; with pseudoelaters. E Fk. S Fk. Willamette River, Lane Co., Oregon. DHW m1992.
SEM spore images: distal, proximal, and side views. Calif., Monterey Co., Santa Lucia Range, The Mitteldorf Preserve of The Big Sur Land Trust. Doyle 10890 (UC); © W.T. Doyle June 2007. Used with permission.
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