Forming Attachments

Day 48: At some point in my illustrious career, I identified this lichen as Parmelia hygrophila, and I choose to believe that I would not have ventured out onto a limb without being 99% sure. This post, however, is not so much about one particular species, but about forming attachments, and trust me, lichens are amazingly good at it. They can grab onto almost anything: wood, rock, bone...even metal or plastic, as can easily be seen on my garden gate (above) and the vinyl siding on my house. All it takes is a film of dust and a tiny bit of texture/porosity, the latter not something one generally associates with painted surfaces. The trick to their tenacity lies in rhizines, root-like structures on the underside of their lobes. In many cases, these rhizines are too fine to be seen with the naked eye. Unlike roots, though, rhizines are not concerned with nutrient uptake. They're there solely to hang on, and if you were to try to peel this delicate Parmelia away from its metal friend, you'd be in for an argument which might surprise you with its vehemence.

They'll Grow Anywhere

Day 163: I have often said that if you hold still for very long in the Pacific Northwe't, you'll find yourself colonized by lichens. Some species can take hold in the very smallest amount of growing medium, in this case, dust and pollen which has collected on the surface of my metal garden gate. At least two species are present here, Parmelia hygrophila dominating at the junction of the two ornamental curls. I have also found the same species growing on the manufactured siding of my house where it found a toehold (rhizine-hold?) in dust trapped in the simulated wood grain. Of course, any organic material is fair game. Lichens have been found on bone, leather and old cloth, and certain species exhibit preferences for one substrate or another. Fortunately for us, they do not seem to have an agenda which includes global domination, although they appear to be fully capable of taking over the world.

Dichotomously Branched Rhizines

Day 1 of YEAR EIGHT! I'm going to start you off with a couple of those big "science words" I throw around so freely because it's crucial to know what to look for if you're trying to identify this particular common lichen. Parmelia hygrophila grows on tree bark and is grouped with others as "shield lichens," meaning that they form a mass which adheres closely to the substrate (i.e., the bark, in this case). We refer to this as being "tightly appressed," but that's a feature shared with a lot of other lichens. The words I'd like to introduce are "rhizine" and "dichotomous." "Rhizine" refers to the root-like structures on the back side of the thallus (body). Sometimes these structures are so fine that you need a microscope to see them. At the very least, you need a hand lens to tell how those of the Parmelias branch. Enter "dichotomously," i.e., they form two-pronged forks ("di-" means "two").

Looking at these under a 20x stereo microscope, it's easier to identify individual rhizines if you view a piece of the thallus from the top and observe where they stick out along the sides. Viewed from the bottom, they just look like a mass of tangled threads. Note the inset in the upper left. See how some of the threads have split ends? That's "dichotomous branching." Parmelia hygrophila exhibits dichotomous branching, as opposed to its look-alikes P. saxatilis (unbranched) and P. sulcata, which has squarrulose ("bottlebrush") rhizines. We'll save "squarrulose" for another day.