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Excerpts from Jim Conrad's
Naturalist Newsletter
from the February 9, 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; elevation ~1750m (~5750 ft); N29.62°, W99.86°; USA
PLACIDIUM LICHEN
About three miles up the canyon where the gravel road fords the little Dry Frio River there's a roadcut through a terrace of anciently deposited mud, gravel and fair-sized limestone rocks. A textbook-size portion of the roadcut's vertical face is shown below:
The ancient mud, rich in calcium carbonate from our area's limestone, now has hardened into caliche, a kind of natural cement typical of hot, arid zones. It's hard to pry rocks from the wall because they're cemented in place by the caliche. In the picture you can see that the caliche is covered with a grainy crust. The crust mostly is composed of a complex mixture of lichens, mosses and cyanobacteria. At the picture's bottom, right, notice the egg-size colony of brown lichens whose small, fingernail-like bodies, or thalli, grow flush with the caliche's vertical face. A close-up of the colony appears at the top of this page.
When that picture was taken I doubted that I could identify the lichen because the thalli bear no cuplike, spore-producing bodies, or apothecia. However, with a little browsing on the Internet the little, brown lichen revealed itself as somewhat famous. It's a Placidium lichen, Placidium being a genus name embracing several species.
Probably it's PLACIDIUM SQUAMULOSUM -- "probably," because in our area Placidium squamulosum is one of two nearly identical species, the other being P. lacinulatum, and they occupy similar habitats. The only essential difference between them is that the thalli of P. lacinulatum produce rootlike "rhizines" while P. squamulosum doesn't. I looked closely for rhizines, even soaking the area beneath the lichen with water after reading that dry rhizines might break off and go unnoticed, but I found none. Below, you can see a thallus cross section with no rhizines beneath it, perched atop a toothpick tip:
That picture is interesting for other reasons than showing an absence of rhizines. First, notice that inside the flat thallus's brown skin there's a green layer on top with a white layer below it. The top, green layer is where most of the photosynthesizing algal cells with their green chlorophyll concentrate, while the lower level, where less sunlight penetrates, consists mostly of non-photosynthesizing fungal hyphae.
Also interesting is the wet, spongy-looking mass below the two-colored lichen thallus. That's a dense network of fungal hyphae known as the "weft." Soil particles intermingle with weft hyphae, so one service the weft provides is to help anchor the thallus in the soil. Also the weft retains water like a ball of wet cotton.
When I wetted part of a lump of soil on which several Placidium Lichen thalli grew, it was surprising to see that within a minute or so the wet thalli turned green while they dry ones remained brown. You can see this below:
Placidium squamulosum mostly occurs on soil, especially soil with a high calcium content, like ours, worldwide.
With regard to Placidium Lichen being somewhat famous, part of the fame is based on what the LichenPortal.org website says: "Together with Placidium lacinulatum, P. squamulosum is the most common species in the Sonoran region," the Sonoran region being the Sonoran Desert of the US Southwest, and northern Mexico. The species is so abundant, and important, because it often contributes to the "biological soil crust" covering vast portions of desert.
More about that here.