Ekman, S 1997: In: Tibell, L/Hedberg, I (eds.): Lichen Studies Dedicated to Rolf Santesson. Symbolae Botanicae Upsalienses, Acta Universitatis Upsaliensis, Uppsala, pp. 17-28.
Thallus thin, smooth to wrinkled, pale grey to green-grey, esorediate. Apothecia always present, minute (less than 0.4 mm diam.), convex, pale yellow (rarely almost white), thinly pruinose, internally without pigment but with crystal-inspersed proper exciple and epithecium. Spores consistently 3-septate, bacilliform. Pycnidia have not been reported from this species previously, and are hence described here in detail: Pycnidia (observed in Muhr 9351, LD) half-immersed in thallus, 100-125 µm diam., unilocular, very pale yellowish in upper part, colourless below. Pycnidial wall without pigment but inspersed with pale yellow crystals in upper part (similar to epithecium). Conidia bacilliform, narrowly ellipsoid, or narrowly drop-shaped, 4-7.5 X 1.2-1.5 µm.
CHEMISTRY. Trace of usnic acid (from apothecial initials and pycnidia?) and zeorin in thallus and large amounts of usnic acid in apothecia (HPTLC!).
HABITAT AND DISTRIBUTION. Presently known from Finland, Sweden, Norway, and northwestern Canada (Santesson 1993, Ahti et al. 1973). Cliostomum pallens is an overlooked, boreal species found on the trunks, branches, and twigs of various species of confiers and deciduous trees and shrubs under well-lit as well as shady conditions.
NOTE. Cliostomum pallens approaches C. vitellinum in having small convex apothecia, almost the same shape and size of the spores, unpigmented pycnidia, essentially the same shape of the conidia (i.e., a higher length/width ratio than in other species), and in sometimes containing zeorin in the thallus. C. pallens is separated from C. vitellinum on account of the paler apothecia, consistently 3-septate spores, and in containing large amounts of usnic acid in the apothecia. The pale colour of the apothecia and the presence of usnic acid is more reminiscent of C. corrugatum and C. leprosum. C. pallens appears to form a link between the rather deviant C. vitellinum and the other species of Cliostomum.
Thompson, J., 1997. American Arctic Lichens: The Microlichens.
Thallus thin, smooth or verruculose, to dispersed, white. Apothecia tiny, 0.2-0.4 mm, broadly adnate, becoming convex and the margin thinning; exciple pale; disk flesh-colored to white, pruinose or bare; hypothecium straw-yellow; epihymenium yellowish; hymenium 40-45 µm, 1+ persistently blue; paraphyses simple, 1.5 µm, tips slightly thicker; spores oblong or elongate-oblong, the tips rounded, 3-septate, 9-18 x 2-3 µm.
Reactions: thallus K+ pale yellow, C+ yellow.
This species grows on a variety of woody plants, Alnus, Salix, Betula, Picea, and Abies. It is known from Scandinavia and Finland and was recently reported from the Reindeer Preserve by Ahti etal. (1973).