Observation 208029: Physcia dimidiata (Arnold) Nyl.

When: 2015-06-28

Collection location: Atascadero, California, USA [Click for map]

35.4513°N 120.6443°W 282m

Who: J-Dar

Specimen available

Common to abundant on valley oak (Quercus lobata) bark in semi-arid open woodland rural site. Does not seem to occur on live oak (Quercus agrifolia), but may occur on blue oak (Quercus douglasii).

White pruinose (crystals?) thallus with narrow lobes, no apothecia, and marginal soralia. Underside pale with concolorous rhizines, simple to fibrous or squarrose. Thallus upper cortex K+Y, medulla white and seemingly K-. Thallus thin, difficult to expose medulla for K test.

Taking a few leaps of faith in the keys (K- medulla, lower cortex prosoplectenchymatous, and pruina dense enough to be calcium oxalate crystals), it would come to Physcia dimidiata, considered to be the sorediate form of Physcia biziana. J. Hollinger Observation 105238 shows the two species occurring together, which may be why I have been struggling with this one. One collection from this location is non-sorediate with uncommon apothecia, and co-occurs intermingled with the sorediate form, but otherwise they both look identical in thallus coloration, pruina, and rhizines. See Observation 208032


Thallus K+Y.

Proposed Names

28% (1)
Used references: Nash Sonoran Flora V1
Based on chemical features: Medulla K+Y
76% (2)
Recognized by sight

Please login to propose your own names and vote on existing names.

= Observer’s choice
= Current consensus


Add Comment
I didn’t want to say…
By: Jason Hollinger (jason)
2015-06-28 19:19:43 PDT (-0700)

But the “pruina” of P. biziana and P. dimidiata is extremely variable. Blah. Fortunately you got a typical one with abundant pruina. It almost looks frosted with granular sugar at that extreme, and that’s pretty unmistakable. Compare the texture of this pruina with that of Physconia for example, or Physcia aipolia (if you’ve had the pleasure, I forget). That’s why Moberg calls the biziana pruina crystals. Probably all of the above are crystals, but when it’s that coarse, it really looks crystalline.

When you find P. biziana or P. dimidiata in shaded cracks or sheltered rock faces, expect it to have much less pruina. There you might have trouble separating P. dimidiata from something like P. dubia or P. tribacia. Physcia can get really tricky!

By: J-Dar
2015-06-28 18:58:52 PDT (-0700)

Takes time! So many little conventions that need to be worked through and memorized, like what level of pruina constitutes crystals as represented in the keys. But, 2 more Physcia species under my belt, can’t complain!

I think you’re right
By: Jason Hollinger (jason)
2015-06-28 16:18:53 PDT (-0700)

This looks like P. dimidiata to me, too. It is unfortunate Moberg asks about the anatomy of the lower cortex so early in his key. Yes, for some species (e.g., P. dubia vs P. tribacia in your area) you really need this character until you learn the gestalt. But for this species it is entirely unnecessary. Pruinose with K- medulla means either P. biziana or P. dimidiata in southern California, nice and easy. (Although, look out for P. stellaris, also K- medulla, but a much smaller species and usually not so pruinose.)

Created: 2015-06-28 15:32:23 PDT (-0700)
Last modified: 2015-06-29 07:55:14 PDT (-0700)
Viewed: 40 times, last viewed: 2019-03-11 07:04:13 PDT (-0700)
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