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.
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