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.