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When you look at them side by side, the bigger one’s apothecia is seen as a little disk with a naked eye, while the smaller one is seen as black dot. Now that said, the ID of the bigger one is still in flux as I understand, per our discussion in observation 185404. But whatever it is on the sandstone as common as dirt around here, its apothecia are the size of S.regularis, and S.privigna looks Lilliputian next to it. I just got myself a ruler, so next time I will try to show the sizes of what I photograph.
How do you distinguish the two side-by-side?
It’s probably more common that once thought but not noticed due to its small size. Jim also let me compare S.privigna and S.regularis growing on the same rock (different specimen – not in this observation), where the latter dominates due to its larger size (I mean apothecia). And yes, they’re both on sandstone, right next to each other.
But I’ve read that most acids should work in a pinch, e.g., vinegar or maybe even lemon juice. Strongly calcareous substrates like limestone should bubble away vigorously. The tricky ones will be weakly calcareous sandstones. Only thing to be careful of is that porous sandstone might produce a few bubbles even just soaking up a drop of tap water. Try it out on something you’re sure of, see if I’m right.
Okay, so it’s regularis on calcareous and privigna on siliceous. I see your point. There’s no easy way to tell which it will be, except by pruina (which of course is always variable!) At leastyour options are limited. One or two Lecidea and two Sarcogyne. I’ve seen much worse, e.g., Utah sandstone has a host of options, some apparently not even described yet. And the same problem you’re talking about with never knowing until you put a drop of HCl on it whether any particular chunk of sandstone is calcareous or not. Ugh.
according to him S.regularis grows on calcareous rocks. Those are usually cloudy-blue around here with pruina, and grow on dolomites. S.privigna, according to him, grows on granites and sandstones (probably non-calcareous but he doesn’t mention). Problem with sandstones in Wisconsin is that most of the Cambrian sandstones were covered, at one time or another, by Silurian dolomite. But lots of time passed, some lands shifted, things changed, basic substances may or may not still be permeating the sandstones on locations, and there is no way to know, without chemical analyzes, whether particular formation is acidic or calcareous.
If these are all on calcareous(?) sandstone, then odds are good that you’re right that they’re all Sarcogyne privigna. There are Lecidea with endolithic thallus (e.g., L. laboriosa, ubiquitous on granites in California). There are a couple of other Sarcogyne, too, e.g., S. regularis, but they may be pruinose or grow on siliceous sandstone or something, I forget the technical differences. Otherwise, the candidate pool is dwindling rapidly…
I have many collections that look like this. Whether they’re the same species or not – they will need to be reexamined. Jim Bennett identified one of them as S.privigna, and I assigned the rest of them the same name. But according to Thomson, S.privigna is very rare in Wisconsin. These “buttons”, in contrast, are the feature of each and every sandstone outcrop one can encounter. To be continued…
Created: 2014-11-27 16:28:25 EST (-0500)
Last modified: 2015-05-10 18:43:33 EDT (-0400)
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