Friday, December 15, 2006
Biodiversity Corner Peppered Rock-Shield Lichen
Name.Xanthoparmelia conspersa is the scientific name for the peppered rock-shield lichen. Most of the 51 species of Xanthoparmelia in North America grow on rocks and are collectively referred to as rock-shields but there are some renegades in the genus that prefer soil and some true vagrants like the tumbleweed shield lichen. Up to 1974, the name, Parmelia, covered a very large number of lichens which have since been assigned to new genera. Xanth- means yellow, making Xanthoparmelia the yellow Parmelias — but where does that leave Flavoparmelia (since flav- also means yellow)? The unstoppable taxonomists are having too much fun, splitting and lumping, to assign common sense to the names.
When and where seen. The granite rocks in my front stone wall along School Street are peppered with the peppered rock-shield lichen. You can see this lichen all year round, all around town. Its typical habitat is siliceous rocks — usually granite — in open sunny places. It was one of the 66 lichen species recorded in Green Cemetery last year, on the lichen walk.
Lichen lingo. The body of a lichen is called a thallus and the main growth styles are foliose (leafy), fruticose (upright and shrubby), or crustose (flattened onto its substrate like a crust). It is almost impossible to identify lichens without looking at the reproductive structures for which there is a specialized vocabulary. Apothecia are typically cup-shaped or button-like structures which produce spores of the fungal component of the lichen; soredia and isidia are outgrowths that break off and allow the lichen to reproduce vegetatively. Soredia are little clumps of algal cells trapped by fungal threads. They have no outer skin or cortex so they tend to look dusty or powdery. Isidia are also little clumps of algal and fungal cells but the cortex of the lichen covers them, giving them a smoother appearance. Isidia are usually perpendicular to the thallus and stick up like little stubby fingers. The presence (or absence) and the location of apothecia, soredia and isidia are clues in lichen identification. Most can be seen with the naked eye but also most require a hand lens to discern the detail.
Distinguishing characteristics: The thallus of the peppered rock-shield lichen is what lichen people call yellow-green. The yellowness is not obvious until you see the specimen beside what lichen people call blue gray or mineral gray. It is a foliose lichen with lobes less than three millimeters wide. (Flavoparmelia has much larger lobes.) On the upper surface of the thallus there are brown-centered cuplike apothecia and lots of isidia. 'Isidiate' is the adjective for lichens with isidia and the peppered rock-shield lichen is densely isidiate. You can even find isidia on the rims of the apothecia. The isidia are globular to cylindrical and some are branched. The lower surface of the lichen is black which is all that distinguishes it from X. plittii. Many of the peppered rock-shield lichens on my stone wall are three or four inches across and some have blended with each other to create patches up to a foot across. In between the large specimens are smaller ones, less than an inch across, with no apothecia.
Sex, drugs, rock and roll. Sex, in the lichen world, is the responsibility of the fungal partner. In the unequal partnership composed of a fungus and a photobiont of some kind (usually an alga), it is only the fungal component that reproduces sexually. The hard-working, food-producing, celibate photobiont reproduces vegetatively. Lichens produce over 600 secondary compounds as by-products of metabolism. Most are unique to lichens and some have antibacterial properties. One reportedly inhibits HIV. Many cultures have used lichens medicinally. Peppered rock-shield lichen is said to have been used in east and southeast Africa to treat venereal disease and snakebite. So much for sex and drugs . . . what about rock and roll? Well, the peppered rock-shield lichen can't dance, and anyway I think it would prefer country music. It anchors itself to a rock to prevent it rolling around like its cousin, the tumbleweed shield lichen.
References. Lichens of North America by Brodo, Sharnoff and Sharnoff; Simplified Field Key to Maine Macrolichens, by Patricia L. Hinds and James W. Hinds.
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