Flora of New Zealand Series

Description : Thallus smooth, continuous, dull buff-brown, olive-green or greenish black, 60–120 μm thick, slightly gelatinous when moist, delimited by a wavy black line of prothallus. A brown-black basal layer well-developed up to 60 μm thick, or discontinuous, rudimentary or lacking. Punctae black, 20–60 μm wide, present on thallus surface, often abundant close to perithecia, scattered or abundant elsewhere. Perithecia numerous, 0.16–0.26 mm diam., showing a range of involucrellum development depending on thallus thickness and degree of melanisation – perithecia tend to remain immersed in thicker thalli and have a poorly developed involucrellum, while in thinner thalli (60–80 μm thick) perithecia usually occupy hemipsherical swellings on the thallus and have an involucrellum 35–50 μm thick that may penetrate to and merge with the dark basal layer. Exciple colourless to medium-brown at base, darkening towards apex. Ascospores ellipsoidal, 8.5–14.5 × 4.5–7 μm.

S: Otago (The Remarkables, Rock & Pillar Ra., Trotter's Gorge). On ±immersed rocks and boulders in streams, lowland and alpine. Known also from aquatic siliceous rocks in Great Britain, Scandinavia, central and northern Eurasia and North America (Swinscow 1968; Hawksworth et al. 1992; Nimis 1993; Santesson 1993; Scholz 2000; Hafellner & Türk 2001; Valcárcel & Carballal 2002; Santesson et al. 2004). First collected in New Zealand (also the first record for the Southern Hemisphere) from Trotter's Gorge by James Murray in 1959 (McCarthy 1991e: 285). Subsequent alpine collections made by Peter Johnson. Also in New South Wales (McCarthy 2003c, 2006).

Cosmopolitan

Illustrations : Zschacke (1934: 228, fig. 100); Swinscow (1968: 46, fig. 5 – as Verrucaria kernstockii); Dobson (2000: 404; 2005: 454); Valcárcel & Carballal (2002: 263, fig. 7C).

New Zealand specimens of V. rheitrophila consist of mosaics of individuals displaying broad continua of plasticity in thallus colour, thallus thickness, the development of the melanised basal layer and punctae, perithecial emergence, and the thickness and penetration of the involucrellum. Darker thalli tend to be thinner, possess a well-developed, dark basal layer and punctae, and to be dark-pigmented in the uppermost 10–15 μm. This thinness also exposes otherwise immersed perithecia that acquire an inevitably well-defined involucrellum. Immersed fruits invariably lack or have a vestigial involucrellum; emergence is usually accompanied by the progressive development of this protective structure (McCarthy 1991e).