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Mycena

Field characters

Pileus: generally small. Conical to campanulate, often with an incurved margin, edges often striate.
Annulus: none
Stipe: small, fragile, often with lactiferous hyphae, usually hollow , often dextrinoid.
Stature: mycenoid
Ecology: saprophytic on humus, leaves, decaying logs, etc., but never on dung.

Micro characters

Basidiospores: smooth. amyloid or inamyloid
Hyphae: with clamp connections
Cystidia: cheilocystidia present
Spore Print: white to cream
Lamellae: sub-free to decurrent, sometimes with a differently colored margin (“marginate”)
Lamellar Trama: regular, usually dextrinoid, often with swollen elements
Pileipellis: a cutis or ixocutis, “with a layer of inflated cells directly below the surface layer (Largent and Baroni ).

Notes

Mycenas often are encountered in large troops in the forest. I have seen examples of hundreds of fruiting bodies (e.g. M. leariana, illustrated above) on dead logs. Large species’ caps are about 5 cm in diameter, but the great majority are much smaller (down to 2-3 mm diameter). Therefore, if you see a large cluster of smallish mushrooms, try Mycena first.

Keys will often differentiate Mycena from Marasmius by the fact that the former does not revive after drying while the latter does. This can be very frustrating to the beginner because you have to wait for some time to observe this.

Arora, 1986 gives some advice on how to recognize the genus from among the other smallish agarics:
1. not waxy as in the Hygrophoraceae
2. does not revive as in Marasmius
3. Collybia (Gymnopus) and Omphalina (and Gerronema) have plane or umbilicate cap shapes
4. “other small mushrooms with a conical or bell-shaped cap (Coprinus, Conocybe, Nolanea, etc. do not have white spore prints.

While Arora’s field characters might work most of the time,
The real characters that differentiate the genus are, however, microscopic, and to identify most of the species, a microscope is necessary. If I understand the delineation of the genus, these are the important characters:

1. dextrinoid lamellar trama
2. the layer of inflated cells below the cutis
3. amyloid or inamyloid spores

A.H. Smith wrote a monograph on the genus, in which he recognized 218 species.