Guide to Hornworts of Oregon: Phaeoceros pearsonii (M.A. Howe) Prosk.
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Hornworts 1b stalked tubers absent > Hornworts 2b spores yellow > Phaeoceros 1a spores brown, bumpy > P. pearsonii
Synonym: Anthoceros pearsonii M.A. Howe Paraphymatoceros pearsonii (M. Howe) J.C. Villarreal & Cargill
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. The round chloroplasts frequently have a median constriction, as if dividing, in the mid thallus epidermis (illustrated here). Doyle and Stotler (2006) describe these as dumbbell shaped, while Whittemore (2009) says division may be complete and two chloroplasts found in median thallus cells. This behavior of chloroplasts is a diagnostic character for P. pearsonii. This species has distinctive spores for the genus; the color is more brown than yellow. The distal face of the spores is composed of about a dozen large bumps. Some spores have more discrete bumps than others, always few and large. The proximal faces may have a few small papillae clustered in the center (Wagner m2685) or be quite smooth proximally (Doyle 8423); both illustrated below. The population from Santiam Higway, with papillae on the proximal faces, is apparently unusual (Crandall-Stotler et al. 2008). This species has small tuberous thickenings along the margin, often becoming quite large in vigorous, mature specimens.
Distribution: On seasonally moist, bare mineral soil, either recently disturbed or where fairly stable. Quite common in SW Oregon, this tolerates drier conditions than any other hornwort. I have found it on open hillsides away from valley bottoms, stream sides, or seepy road cuts. Representative specimens examined: Benton Co.: Adair Village Sanborn 1907 (OSC); Douglas Co.: Boulder Creek, Wagner 3199 (ORE); Jackson Co.: Ferris Gulch, Wagner 9746 (OSC); Lane Co.: Wolf Creek, Wagner 7047 (OSC); Linn Co.: Santiam Highway at milepost 50, Wagner m2685 (DHW).
Comments: The odd spores, turning rather dark brown in some populations, might indicate this is an isolated species in the genus. They look very much like P. hallii in morphology, with similar coarse bumps distally, but their color is starkly different. These two have recently been shown to be closely related (Crandall-Stotler et al. 2008). Although their spores are very similar, P. pearsonii has pseudoelaters composed of two or more cells, typical for the genus, while P. hallii has unicellular pseudoelaters. This was recently segregated into the genus Paraphymatoceros (Villarreal Aguilar, J. C., D. C. Cargill, L. Söderström, A. Hagborg & M. J. von Konrat. 2015.)
Left: Sterile thalli growing on a shady, seepy soil bank. Sweet Creek, Lane Co.,Oregon. Right: Santiam Highway, Linn Co.,Oregon. DHW m2685.
Left: Sweet Creek, Lane Co.,Oregon. DHW m1251. Middle (mid-thallus cells) and right (marginal cells): Goodman Creek, Lane Co., Oregon. DHW m1782.
Antheridia in antheridial pits. Goodman Creek, Lane Co., Oregon. DHW m1782.
Spores and pseudoelaters. Santiam Highway, Linn Co.,Oregon. DHW m2685.
Upper two spores in distal view, spore just below middle in proximal view. Santiam Highway, Linn Co.,Oregon. DHW m2685.
Pseudoelaters and spore in proximal view. Santiam Highway, Linn Co.,Oregon. DHW m2685.
SEM spore images: distal, proximal, and low power views. Calif., Marin Co., Bolinas-Fairfax Rd. near Alpine Dam. Doyle 8423 (UC); © W.T. Doyle June 2007. Used with permission.
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