Collection location: Carnaxide, Oeiras, Portugal [Click for map]
This observation is very much related with observation 110992. Reading the comments there, instead of “collect the whole tree” and in this way “kill the goose that lays the golden eggs” I decided to wait and observe the specimens from time to time. During more than three years very few happened. Recently I noticed that some apothecia were best visible, because some black dots could better seen (though it is difficult with the naked eye, due to size) due to degradation of the thallus. Finally I decide to get a sample and analyze it.
Meanwhile, in Sep. 2014 Jason post observation 179005 with a specimen named Arthonia susa that I found very similar to my specimens (compare the photos). There we also discussed a bit of the taxonomy: while N.A. people place arthoniod specimens in the genus Arthonia, European people call them Arthothelium, at least the British Flora does but refers that this genus in not universally accepted.
From the microscopy we can see that the above suppositions were right. From the sections we can see the arthonioid characteristic apothecia and, moreover, the spores are muriform, though a bit smaller than those found by Jason in observation 179005; I got the following dimensions:
(20.3) 22.1 – 26.1 (26.5) × (8) 8.8 – 11.57 (11.6) µm
Q = (2.2) 2.25 – 2.6 (2.7) ; N = 10
Me = 23.7 × 10 µm ; Qe = 2.4 .
Taking into account that the spores have not an individed enlarged upper cell and (at least some) of the spores has more than one longitudinal septa, according to the Bristish Flora there are to possibilities: either Arthotellium orbiliforum, if a photobiont is absent, or A. ruanum, if the photobiont is Trentepohilia. Since these specimens have an algae layer in apothecia, I think that more likely they belong to Arthotellium ruanum, although I’m not completely sure that the alga is a Trentepohlia. I must also refer that some parts of its description apply as a glove to these specimens; quoting: " Apothecia … irregularly rounded to blunty stellate, often desintegrating in places and regenerating to give the appearance of a crowded swarm of punctiform apothecia, black, not pruinose (but often long remaining covered by bark cells) …".
This for me is like: Mission accomplished!
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sum(score * weight) /
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