Looking through an old campcraft book I came across the following entry:
from the book Camp Cookery by Horace Kephart, published 1910
On the bark of maples, and sometimes of beeches and birches, in the northern woods, there grows a green, broad-leaved lichen variously known as lungwort, liverwort, lung-lichen, and lung-moss, which is an excellent substitute for yeast. This is an altogether different growth from the plants commonly called lungwort and liverwort---I believe its scientific name is Sticta pulmonacea. This lichen as partly made up of fungus, which does the business of raising dough. Gather a little of it and steep it over night in lukewarm water, set near the embers, but not near enough to get overheated. In the morning, pour off the infusion and mix it with enough flour to make a batter, beating it up with a spoon. Place this “sponge” in a warm can or pail, cover with a cloth, and set it near the fire to work. By evening it will have risen. Leaven your dough with this (saving some of the sponge for a future baking), Let the bread rise before the fire that night, and by morning it will be ready to bake.
It takes but little of the original sponge to leaven a large mass of dough (but see that it never freezes), and it can be kept good for months.