Peltigera Britannica

Day 136: Peltigera britannica is fairly easy to distinguish from other similar lichen species in the field when it is fruiting. Its cephalodia (layman's term: "little black spots") are easily detached. In fact, they are so easily detached that a good rainstorm can complicate identification by removing them. Each cephalodium is capable of reproducing another lichen, and large masses of Peltigera are not uncommon where conditions are ideal. This lichen attaches to its substrate by means of rhizines which, although they resemble roots, do not absorb nutrients. They serve solely to hold the lichen in place on rock, tree trunks or soil. The same species can exhibit a brown upper surface, many times growing side-by-side with a spring-green companion.

Cephalodia Scrape-Test

Day 128: New word for you: cephalodia (SEF-uh-LO-dee-uh, singular "cephalodium"), i.e, "a small gall-like growth that contains cyanobacteria and occurs within the tissues or on the surface of some lichens with green algal photobionts." What the definition neglects to mention is that each cephalodium is a reproductive structure, in essence a mini-lichen which when detached, can reattach to the substrate and become a full-sized specimen.

"Detach" is the keystone word in this story. When I first discovered this colony, it was quite dry and the cephalodia were firmly attached to the thallus. Consequently, I misidentified it as Peltigera aphthosa. After the lichen had rehydrated some months later, I happened to brush against one of the lobes while examining something else, and noticed as I drew back that the cephalodia had fallen from one section. I tested another area with a swipe of my finger. Sure enough, the little black dots wiped away easily. "Well!" says I. "You're britannica after all!" (Parkie friends are used to me talking to natural objects.)

Compare the areas circled in red in these two photos. Although lightly attached, the cephalodia were easily scraped off by my thumbnail, leaving behind small white dots (the green algal photobiont came away with them). This is a quick and easy field test which will separate these two confusing Peltigeras. Yes, I cleaned them out from under my thumbnail and left them where they could continue to reproduce.

Paired Pelts

Day 25: Lichens can be very selective as to their preferred habitat, but at this particular location, the dominant genus is Peltigera. But which species? That's been puzzling me for some time as I viewed them in a less-than-optimal state of dryness. I had thought the grey, crisp mass might hold both young and old specimens of a single species since there were subtle differences, but I was surprised when, after a good soak in our abundant October rains, two Pelts made themselves apparent. Peltigera britannica (left) is characterized by its bright green colour and dark cephalodia which are easily scraped off the surface of its lobes (a feature which distinguishes it from P. apthosa), and Peltigera membranacea (right) by its thin tissue and rope-like rhizines. These two colonies seem to be campaigning for dominance of the rock face on which they are growing. It's much more to my liking to ponder which will become the next President of Lichenopolis.