Observation 105239: Physcia dimidiata (Arnold) Nyl.

When: 2012-08-03

Collection location: San Gabriel Peak, San Gabriel Mountains, Los Angeles Co., California, USA [Click for map]

34.2522° -118.0951° 1450m

Who: Jason Hollinger (jason)

Specimen available

HABITAT shady oak-conifer forest on north slope; SUBSTRATE granitic rock by trail; ASPECT shaded; NOTES cortex K+y, medulla K-, very scant soredia under crenulate lobe tips, lower cortex prosoplectenchymatous.


Physcia biziana below, Physcia dimidiata above.
Physcia biziana below, Physcia dimidiata above, all damp.
Lower surface of crenulate lobe tips, at 30x, stacked.
Upper surface of crenulate lobe tips, at 30x, stacked.
Upper surface of crenulate lobe tips, at 30x, stacked.
Lower surface of crenulate lobe tips, at 30x, stacked.

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Add Comment
This was a real surprise to me
By: Jason Hollinger (jason)
2012-08-14 04:28:43 CDT (-0400)

I thought for sure this was P. tribacia, but I had just read somewhere by K. Knudsen that P. dimidiata and P. tribacia could be very difficult to distinguish. Really? All my specimens of P. dimidiata have obvious soralia, all my specimens of P. tribacia are like this, with sparse granular soredia scattered just under the crenulate lobe tips. Both have K- medulla. P. dimidiata has variable pruina (just like P. biziana — usually pruinose, but sometimes with little or none), P. tribacia has no pruina and is typically shiny.

There is, however, one either-or, black-and-white difference between the two: the cellular structure of the lower cortex. In P. dimidiata (like P. biziana, its fertile counterpart), the lower cortex is a woven mesh of long hyphae (prosoplectenchymatous); in P. tribacia, it is composed of a brickwork of round cells (paraplectenchymatous).

In fact, this specimen is prosoplectenchymatous. I immediately double-checked all my other (many) alleged specimens of P. tribacia, and was relieved to discover that they were, in fact, paraplectenchymatous, and therefor identified correctly. But here it is: a form of P. dimidiata which looks identical (except for the dull pruinose surface) to P. tribacia.

Just when you think you’ve got a species figured out…

Created: 2012-08-14 04:16:07 CDT (-0400)
Last modified: 2012-08-14 04:16:14 CDT (-0400)
Viewed: 79 times, last viewed: 2017-09-02 09:43:37 CDT (-0400)
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