Friday, February 3, 2006
Common toadskin lichen
When and where seen: The common toadskin lichen grows on non-calcareous rocks and boulders. There are large colonies of it in the Conant Land, sometimes sharing the boulder with its prolific cousin, the smooth rock tripe (Umbilicaria mamulata). I also see it in the Towle Woods and Great Brook Farm State Park. As its name suggests, the common toadskin lichen is common, but I only see it on rocks in the woods and not on roadside stone walls - even though it tolerates sun.
Distinguishing characteristics: This is an easy lichen to identify for several reasons. It is umbilicate, which means it is attached to the rock by a central holdfast. It looks warty and bumpy like toad skin. The bumps on the upper surface have corresponding depressions on the lower surface. It's large — you don't need a hand lens. The upper surface is greenish when wet and dries to gray or brown. The color change occurs because the cortex is transparent when wet and the green color of the algae inside shows through. The upper surface usually has lots of little black dots scattered around the bumps. These are the spore-producing structures, called apothecia. The lower surface is pale brown. Common toadskin tends to grow in colonies where an individual lichen is typically two to four inches across, although they can grow as large as six inches. The right hand specimen in the photo is three inches.
Lookalikes: Toadskins are closely related to the rock tripes which are also umbilicate (their genus name is Umbilicaria) and grow in the same habitat. While some rock tripes are a bit warty and toad-like, they do not have the corresponding depressions on the lower surface. The common toadskin has a red form but so far I have only seen it in books.
Lichen chemicals: Lichens produce an astonishing number of chemical compounds. Some are the primary metabolites that you might expect from the photosynthesizing algae, but over 600 additional compounds or secondary metabolites are produced by the fungal component of the lichen. Their purpose is not fully understood. Some may deter animals from eating the lichen; some may help shield UV light; some create air spaces within the lichen and improve the gas exchange necessary for photosynthesis; some protect the lichen from bacterial attack and have been used as antibiotics by humans. The presence of these lichen chemicals is commonly used to identify lichens. For example, if you scrape a little piece of the cortex on the common toadskin lichen and expose the medulla, then apply a drop of household bleach, you will get a red color. You can identify the common toadskin without this test, but other lichens are much trickier and require a series of chemical spot tests.
Lichen dyes: Some lichens produce chemicals which people have used as a source of dye for thousands of years. The common toadskin lichen can be used for a pink dye — after 16 weeks of fermentation in an ammonia bath. The active chemical is gyrophoric acid. A good book for anyone interested in natural dyes from lichens is Lichen Dyes by Karen Diadick Casselman.
References: Lichens of North America, by Brodo, Sharnoff and Sharnoff; Lichens Above Treeline, by Ralph Pope; Lichen Biology, by Thomas H. Nash.
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