Observation 18991: Agaricus micromegathus Peck
When: 2009-02-28
Herbarium specimen reported

Notes:

Photos 4 and 5 compares A. micromegathus and A. semotus side-by-side. The habitat is in-situ for A. micromegathus only. The A. micromegathus were found on an open, sloped trailside mostly carpeted in Douglas-Fir needles with scattered grassy tufts and sprouting oak stumps.

The A. semotus were found in a layer of oak leaves on level ground near a grassy area.

Notice the differences in size and stature, pileus color, and intensity of staining (oranger and more pervasive in A. semotus). Another interesting feature to pay attention to is the strong orange color at the juncture of the stipe and lamellae in A. micromegathus (photos 2 and 3).

The buttons can be differentiated by the color, shape, and texture of the pileus, staining reactions, shape of the base, presence of copious mycelial cords, and the scurfiness of the stipe below the annulus.

Images

37829
Copyright © 2009 Christian F. Schwarz
37827
Copyright © 2009 Christian F. Schwarz
Note orange at juncture of stipe with lamellae.
37828
Copyright © 2009 Christian F. Schwarz
Mature vs. young gill color.
37831
Copyright © 2009 Christian F. Schwarz
A. semotus at left, A. micromegathus at right.
37832
Copyright © 2009 Christian F. Schwarz
Large basidiome at left and two at top, A. semotus; four in second row, A. micromegathus.

Proposed Names

28% (1)
Eye3
Recognized by sight: This species is also pictured.
57% (1)
Eye3 Eyes3
Based on microscopic features

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Eye3 = Observer’s choice
Eyes3 = Current consensus

Comments

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Okay…
By: Christian (Christian Schwarz)
2009-03-11 11:33:39 PDT (-0700)

Well, I was reading my notes on this collection again, and something just didn’t sit right, so I consulted Agaricales of CA and looked at the color plate for A. diminutivus, as well photos here and on Mykoweb – they all looked somehow coherently different.

However, the color plate in Agaricales of CA for A. micromegathus looked much closer, so I did a line-by line comparison of the descriptions.

Aside from habitat, there were only 2 microscopic differences that varied enough to be useful:
1) The presence of orange-staining velar inclusions (in KOH) for A. diminutivus
2) a difference in size and aspect ratio of the basidia: smaller and with a low Q value ( less than 2, and more like 1.6 for A. diminutivus), larger, and with a higher Q (greater than 3) for A. micromegathus.

I examined tissues from my dessicated specimens, and I found no trace of orange inclusions in the velar tissue, and the Q avg for the basidia was 3.3. (L avg. 21.3 microns, W avg 6.4 microns).

Given this, I reexamined the habitat for the collection, and decided that it could be interpreted as “grassy” (scattered large clumps), and definitely “open”, and exposed trailside with little vegetation other than the grass and old oak stumps.

Anyways, I’m not completely convinced, but feel stronger about this ID – discuss away!

By: Christian (Christian Schwarz)
2009-02-28 19:49:07 PST (-0800)

Side-by-side comparison with A. semotus is included.

Created: 2009-02-28 18:26:34 PST (-0800)
Last modified: 2010-08-28 18:01:59 PDT (-0700)
Viewed: 160 times, last viewed: 2016-10-02 11:09:59 PDT (-0700)
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