Peltigera Membranacea

Day 177: You might wonder (as did I) why Peltigera membranacea is referred to as a "dog-lichen." One source claims that it is because the rhizines resemble a dog's fangs. Whether this is accurate or not, I can't say, but it makes sense in a left-handed sort of way. The Peltigeras were once believed to be a remedy for rabies, based on what is known as the "doctrine of signatures," i.e., that herbs and plants having a similar appearance to a body part can be used to treat an affliction of that part. The resemblance of the rhizines to canine fangs suggested being bitten by a dog, therefore the lichen must have been useful against hydrophobia. Kids, don't try this at home! The "doctrine of signatures" is a myth, and a dangerous one at that.

So, that said, the "fangs" of Membraneous Dog-Lichen are the clue to its identity. Other lichens may share the same upper-surface characteristics, but only Peltigera membranacea has rhizines growing singly rather than in clusters or groups. This can be seen quite clearly in the righthand photo. These structures are often mistaken for roots, but their true function is to attach the lichen to its substrate. Many lichens have rhizines, although some can't be seen without a magnifier.