Friday, December 12, 2008
The Scattered Rock-posy is a lichen. All lichens are dual organisms made up of a fungus (which like all fungi is unable to produce its own food) and a food-producing partner. The food producer is usually an alga but sometimes a cyanobacterium. In either case it uses photosynthesis to make enough food for both itself and the fungus. In return it gets a nice place to live – in the case of the Scattered Rock-posy, it gets a room with a view on a sunny rock wall where a moisture-loving alga could not survive on its own. These are very different organisms that have found a way to get along – and to derive almost equal benefit. It is not exactly a team of rivals but it does bring new meaning to reaching across the aisle.
Name. The Scattered Rock-posy is one of seven North American species of rock-posies and the only one we are likely to see here. The scientific name is Rhizoplaca subdiscrepans derived from the Greek rhiza for root and placos for spot.This is because some members of the genus have a single umbilicate attachment – or “root-spot” – anchoring them to their substrate. Another common name for members of the Rhizoplaca genus is Rockbright. I don’t know how to assess the brightness of a rock or a lichen but I do find these lichens dazzling although not in the literal sense. All lichens are named on the basis of the fungal partner since fungi of different kinds can team up with the same species of alga and produce a unique lichen.
When and where seen: This lichen is scattered all around town on the stone walls. It attaches itself to granite and likes to be in the open. You can find it on the stone retaining wall of the Old Burying Ground at the town center and there is a lot on parts of the rock wall along the Clark Farm trail between School Street and Concord Street.
Identifying characteristics. Many rock-dwelling lichens are difficult to identify to species in the field but this is one of the easier ones. The thallus or body of the lichen is a yellowish green or gray and divided into little bumps or lumps that sit on the surface of the rock. It is not tightly bonded to the rock surface. The fruiting bodies produced by the fungal partner are round, orange, and usually have a rim colored like the thallus. They are called apothecia; they produce spores; and in this species of lichen they are quite common. You can see this lichen with its apothecia all year round – so if you are out this winter cross-country skiing or just walking, stop and take a rest on a stone wall and see if you are sharing it with the Scattered Rock-posy. Chances are good that you are sharing the spot with many species of lichen, and if you are not deep in the woods, you may find this lichen.
References. Lichens of North America by Brodo, Sharnoff and Sharnoff; Macrolichens of New England by James W. Hinds and Patricia L. Hinds. ∆
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