Usnea mexicanaVain. Dansk bot. Ark.4(11): 3 (1926).
For a detailed description see Herrera-Campos et al. (1998), Truong et al. (2013b) and Herrera-Campos (2016).
Short Description. The very long thalli of U. mexicana are readily recognized by the stiff thallus with cylindrical to irregular branches and elongated fibrils (arranged in a conspicuous “fishbone-pattern”). Very characteristic is also the dull reddish brown to dark yellow pigmentation of the central axis (visible only in section). The branches of U. mexicana are segmented with ±thin annulations, eroded at their edges (most visible along the trunk and basal branches). The segments are often enlarging, with their medulla extruding and bead-like cortical regenerations in between the segments. The cortex of branches is cracked to areolate, sometimes bearing thin elongated pseudocyphellae. Soralia are minute to slightly enlarged, slightly stipitate, often crowded on lateral branches (but not fusing), with short isidiomorphs. In section the cortex appears mat to ±shiny and the medulla is compact with an A/M ratio > 2.5.
Chemistry. Medulla with ±diffractaic and ±constictic acid, or only with constictic acid [P± orange, K± yellow, C–, KC–]. The main chemotype in continental South America contains protocetraric acid [P+ red, K+ yellowish brown], whereas in Galapagos only the chemotype with ±diffractaic and ±constictic acid or constictic acid alone are currently known (Truong et al. 2013b).
Ecology and distribution. A pantropical species, known from southern North America, Central and South America (Belize, Bolivia, Brazil, Costa Rica, Colombia, Cuba, Ecuador, Jamaica, Mexico, Paraguay, Venezuela), tropical Africa, eastern Asia and northern Australia; first reported from the Galapagos by Truong et al. (2013b). In the Galapagos U. mexicana is a common and locally abundant species, especially throughout the transition zone, also in the humid zone, rarely in the dry zone, especially in open native vegetation (woodland with Bursera, Zanthoxylum, and/or Psidium galapageium), on bark (including cacti).
Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: pendulous, 5 to 50 cm long, stiff and barely ramified branching: anisotomic-dichotomous, parallel basal part: concolorous with branches to brownish blackish, often with multiple attachment points branches: cylindrical and distinctly segmented; lateral branches: not narrowed at point of attachment segments: terete, slitghtly ridged to weakly alate, smooth to conspicuously cracked and areolated, with eroded cortex at some places especially at the edges papillae: indistinct to verrucous, irregularly distributed, from scattered to abundant tubercles: absent fibercles: few to numerous fibrils: long (5-15 mm), abundant, usually in fish-bone like pattern soralia: punctiform, smaller than the diameter of the branch, plane to slightly stipitate, arising on fibercles and on cracks at the ridge of the ±alate branches, scattered to numerous isidiomorphs: nearly absent to numerous pseudocyphellae: inconspicuous, long and narrow on thin branches cortex: thin to moderately thick (3-10%), dull to shiny medulla: thin and compact, unpigmented axis: thick, with ochraceous brown pigment, fistulose in thickest branches, brittle Apothecia: not seen Spot tests: K-, C-, KC-, P+ orangish yellow, or K+ yellow turning red, C-, KC-, P + orangish yellow, C- Secondary metabolites: diffractaic acid (major), ±salazinic acid (major), ±constictic acid (minor) or salazinic acid (major), ±protocetraric acid (minor) or protocetraric acid (major). Substrate and ecology: on bark between 1200 and 1900 m in mixed conifer-oak forests World distribution: pan-tropical: southern North America, South America, Africa, eastern Asia and Australia Sonoran distribution: so far known only from Guadalupe Island in Baja California. Notes: Usnea mexicana is closely related to U. himantodes, from which it seems to differ only by the reproductive strategy (U. himantodes produces no soralia, only apothecia). The pendulous habitus, its brown pigmented and ±fistulous central axis and its chemistry are diagnostic characters for U. mexicana.