Melanohalea elegantula or exasperatula

I have never taken much notice of the underside of Melanohalea species. Perhaps I should investigate one day but I suspect the isidia provide better diagnostic characteristics than trying to judge the darkness of the underside.

I dug out a specimen each of this pair and have attempted to take some close-up shots to illustrate the differences of the isidia.

First we will look at M. elegantula. The isidia develop first into mainly cylindrical pegs, then start to become coralloid (and may eventually become somewhat flattened).

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Looking at an isidium that has been sliced with a razor in extreme close-up, we see that the tissue inside is rather loose but nevertheless there are hyphae which form a continuous tissue across the centre.

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Now to look at M. exasperatula. In this case the isidia soon take on a swollen appearance, becoming like partially inflated flattened balloons. They have a tendency to look as if they are flopping over in places, leaning against each other. They have less of a tendency to become intricately coralloid.

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Now to look at isidia that have been sliced. The isidia are seen to be truly hollow, with a central void that is not really spanned by anything that would be interpreted as continuous tissue.

[attachment=0]exasp1.jpg[/attachment]

I hope these will help in your deliberations and I think you will decide that the identity of your lichen is M. elegantula. I am taking on trust that we really are dealing with a species of Melanohalea (it looks right) and that you have satisfied yourself that the medulla is thin and wispy (compared with Melanelixia spp.) and that you have used strong fresh C to test the medulla.

Mark

I agree with Neil about the difficulty of interpreting the undersides of parmelioids. Of the genera that I encounter frequently it is perhaps in Punctelia that the darkness of the undersides is most useful but even here there is scope for being led astray. LGBI2 and other sources claim that the underside of P. subrudecta is ‘pale brown or partly white’. In reality, there is often a zone of very dark brown verging on black beneath the innermost parts of the marginal lobes. If we study the gradation of colour from margin to interior, we find in P. subrudecta that it often goes from mid brown to dark, almost blackish brown but then becomes paler again in central parts. By contrast P. borreri goes from mid brown near the margin and then rapidly darkens without becoming paler again, becoming jet black underneath and remaining so in central parts. Many people merely lift up an outer lobe of P. subrudecta, see that it is becoming blackish underneath and mistakenly assume P. borreri.

I compiled a twitter thread which hopefully illustrates some of the most important points to consider when attempting to identify Punctelia species.

https://twitter.com/obfuscans3/status/1210862789666988034

I find the pruinosity of the upper surface of P. borreri to be a good and reliable character (but like all parmelioid characters best considered in conjunction with all other reliable characters available).

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The pruinosity of P. borreri is widespread on the upper surface and is not to be confused with the very restricted but usefully diagnostic pruina on the lobe margins of P. jeckeri.

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Another word of warning about P. subrudecta concerns relatively young thalli. These may have developed the dark zone but, even in their immature central parts, not yet become paler brown. Hence some people think they are being diligent in looking at the centre of the thallus but still misled by the darkness they find there.

Some people have tried to distinguish between P. borreri and P. subrudecta on the supposed difference in tone of red given by gyrophoric and lecanoric acids with C. My experiments with specimens of each, tested side by side, indicate to me that any such difference cannot be reliably interpreted.

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Sorry to divert the subject of this post to Punctelia; it is intended to continue the theme that interpreting the underside of parmelioids need real care.

Mark

Mark

No a very useful digression into Punctelia, I was aware when I wrote my last post, that Punctelia subrudecta s. str./Punctelia borreri was one split where the underside did matter but was still confusing. I will have a good look at both in the light of your really useful observations.

Neil