Usnea fragilescens Hav. in Lynge

Nomenclatural data
Skr. Vidensk.-Selsk. Christiana, Math.-Naturvidensk. Kl. 7: 230 (1921).
For a detailed description, see Clerc (1987b, 2007) and Purvis et al. (1992). Thallus shrubby to subpendant, with divergent branches, 3–5(–12) cm; base black, often with a “stalk” before branching; main branches elongate, somewhat inflated, secondary branches distinctly constricted at their base; fibrils few or numerous, rather long; papillae numerous; soralia scattered over branches, punctiform at first, and large when mature but remaining discrete; isidiomorphs few, usually seen on the tips of branches and absent on mature soralia. Cortex glossy, thin; medulla thick and loose.
Medullary chemistry
Chemotype (1) with stictic acid complex (K+ yellow to red, Pd+ orange) is known in Europe (Clerc (1987b); further chemical variability is reported from North America, Sonoran region (Clerc 2007): chemotype (2) with psoromic acid as a major substance (K+ yellow, Pd+ yellow); chemotype (3) with salazinic acid as a major substance (K+ yellow to red, Pd+ orange).
Most of saxicolous specimens belong to the main variety (U. f. var. fragilescens) while corticolous specimens are treated as U. f. var. mollis (Vain.) Clerc (Clerc (1987b). Morphologically var. mollis is shorter, more tufted and more isidiate.
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Ecology and distribution
Europe (map legend)
Grows on rocks and boulders as well as on trees (see Remarks), mainly on broad-leaved trees and Salix (Purvis et al. 1992). Reported in Europe: Belgium, France, Germany, Great Britain, Italy, Netherlands (probably extinct), Norway, Poland, Spain.