Rexiella sullivanii

Rexiella forms shrubby tufts of upright secondary branches known as pseudopodetia. The fragile primary crust quickly weathers away, so the eye is drawn to the green- to yellow-tinged pseudopodetia, which stand 2.5–14 cm tall and about 2–4 mm across. They are stiff and horn-like when dry but soften to a spongy texture after rain. Each stalk is irregularly ridged or angular, may divide a few times, and is riddled with narrow, window-like holes that give the surface a lace-like appearance. A thin, sometimes glossy cortex is dusted with minute crystals. Inside, the usual central cavity is almost completely packed with a loosely woven medulla that can be white near the tips but grades to charcoal grey or black lower down—one of the easiest field clues that the lichen is Rexiella rather than Cladia or Pulchrocladia. Reproduction occurs at the branch tips. Fertile pseudopodetia bear pin-head apothecia: tiny, shield-shaped discs 0.25–0.5 mm wide whose dark brown to black margins fuse with the hymenium (a structure termed "lecideine"). Inside each disc, the asci release single-celled, colourless ascospores about 25 × 5 μm. …