Excerpts from Jim Conrad's
from the February 2, 2014 Newsletter issued from the Frio Canyon Nature Education Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA
Having identified most of our larger lichens, to find a species new to me I needed to start focusing on the smaller ones and, sure enough there they were, just waiting for me. One found this week formed fairly large, conspicuous colonies, but its individual, leaf-like bodies, or "thalli," were very small. Below, you can see a pale gray colony covering a narrow ridge of bark on a mature Mesquite tree standing in the open:
In that picture you can barely make out the tiny, cup-shaped reproductive bodies, the apothecia, which don't grow larger than 2mm across (1/20th inch). A close-up at the top of this page shows how the apothecium spore-producing surface is pale gray at first but darkens with age. At the bottom of that picture you can see the lichen's flat, leafy thalli, which branch as they advance across the bark, and which also are no wider than 2mm across. This foliose-type lichen has exceptionally small parts.
The smallness of its slender, flat thalli, its whitish gray color, the interesting way the apothecia spore-producing surface darkens with age, and its presence on tree bark, were important field marks enabling this lichen to be identified as PHYSCIA AIPOLIA. Because when species grows on broad, smooth patches of bark its colonies form expanding circles or rosettes of intricately branching, lacy-looking thalli, sometimes it's called the Rosette Lichen. Our Rosette Lichen atop its narrow ridge of deeply fissured Mesquite bark couldn't form its rosette.
Rosette Lichens are widely distributed in the Americas and Eurasia and are described as growing on trees in fairly open situations, which was precisely the case with ours.