Rhizocarpon alpicola
From Wikipedia, the free encyclopedia
| Rhizocarpon alpicola | |
|---|---|
 | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Class: | Lecanoromycetes |
| Order: | Rhizocarpales |
| Family: | Rhizocarpaceae |
| Genus: | Rhizocarpon |
| Species: | R. alpicola |
| Binomial name | |
| Rhizocarpon alpicola | |
| Synonyms[1] | |
|
List
| |
Rhizocarpon alpicola is a species of saxicolous (rock-dwelling), crustose lichen in the family Rhizocarpaceae.[2] The lichen grows as a patch-like crust up to 15 cm across, composed of small yellowish-green tiles that develop black, button-like reproductive structures. A member of the Rhizocarpon geographicum species complex, R. alpicola can usually be distinguished from related species by its large and smooth areoles.[3]
R. alpicola is often utilised in lichenometry, a dating technique used to estimate the length of exposure of rock surfaces.[4] Research on Norwegian glacier forelands has shown that R. alpicola typically establishes on rock surfaces after they have been exposed for about a century, with individual colonies growing slowly over many decades.
Rhizocarpon alpicola forms a broad, patch-like crust that can reach roughly 15 cm across. The edge of each colony is outlined by a conspicuous black prothallus—a band of fungal tissue that develops before the main body develops. The thallus itself is divided into tiny "tiles" (areoles) up to about 1.5 mm wide; these range from contiguous to widely scattered and are matt green-yellow because the cortex concentrates rhizocarpic and psoromic acids. Individual areoles are flat to gently domed and often crack finely across their surface. Internally, the medulla shows no iodine reaction (I–).[5]
Reproduction is via black, button-like apothecia up to 1.5 mm in diameter. They sit flush with or slightly above the thallus and may be round or angular. The surrounding wall (true exciple) is hard to discern externally but, when sectioned, reveals a red-brown inner layer that turns purplish-red with a drop of potassium hydroxide solution. The uppermost tissue (epithecium) is pale to mid-brown and may give a faint purple tint with the same K test. Beneath lies a clear hymenium, and lower still a dark red-brown hypothecium. Each club-shaped ascus contains eight ascospores; the mature spores measure 20–33 μm by 9–17 μm, are divided by a single cross-wall (occasionally a couple of extra thin ones), and darken to a deep brown as they age. No specialised asexual propagules have been observed in this species.[5]
A quick chemical spot test helps separate R. alpicola from look-alikes such as R. atroalbescens: the medulla of R. alpicola remains unresponsive to potassium hydroxide (K–) but flashes bright yellow with para-phenylenediamine (Pd+), whereas R. atroalbescens gives different colour changes because it produces alternate acids.[5]