The North American species of Pholiota, by Alexander H. Smith and L. R. Hesler.

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The North American Species of Pholiota

occur in a more or less erect or even reclining position, and are seen on
the pileus as scales. In more rare instances, erect specialized cells, clavate
or cylindric, are present as pileocystidia. When the hyphae bearing these
elements are erect and numerous they form what is known as a trichodermium. This latter type occurs mostly in subgenus Flavidula in varying
degrees.
In viscid and glutinous species, a zone of the surface hyphae, when
wet, becomes gelatinized. Thus, the cuticle is gelatinous, and in sections
in KOH it is translucent, or glassy. The zone may be narrow or broad
(50-400 /u), and very distinct under the microscope even under low
magnification. In this gelatinous zone, hyphae or hyphal remnants persist
for a long time even in mature pilei. Shaffer (1966) has recently discussed the modifications which hyphal walls undergo to produce a viscid
or slimy layer (pellicle) and has pointed out how it is possible to have
hyphae imbedded in slime (secreted by the cell) without the cell wall
becoming involved through disintegration. In Pholiota both types
undoubtedly occur but in our studies we were unable to distinguish
clearly any cuticle of the pellicle type in which none of the gelatinization
came from hyphal walls. In Pholiota the common occurrence is for the
cuticle hyphae to have hyphal walls that gelatinize to some degree.
Usually, the gelatinous cutis rests on another zone, usually of brownish hyphae-the hypodermium. The hyphal components of it are often
periclinal in arrangement, but in some species are more or less radial,
and not infrequently their walls are encrusted. In some species a hypodermium is not differentiated. We regard the hypodermium in Pholiota
as the upper zone of the pileus context, not part of the cuticle. This is
because of the poorly defined nature of the zone, and too often the only
differentiation is the presence of a small amount of pigmentation.
The general type of pileus cutis-whether of repent, dry hyphae, or
of gelatinous hyphae-is highly important taxonomically at the subgenus,
and species levels, and has been used to a great extent in this study.
Pileocystidia. In a comparatively few species, single-celled elements
stand more or less erect on the pileus surface. These terminal cells, called
pileocystidia, are differentiated, and they may be distinguished morphologically from a mere hyphal tip. We found pileocystidia in tufts in
P. canescens and in P. minor. In the latter, they are similar to the
cheilocystidia (subfusoid). Likewise, pileocystidia were found rarely in
a few other species, but here they were scattered rather than in tufts. We
have assigned to pileocystidia only minor taxonomic value since they are
not a feature of very many species and those having them are not obviously closely related among themselves. In subgenus Flavidula they form
the terminal cell of the trichodermial hypha and are treated as part of
the trichodermium.
The stipe cutis, with few exceptions, is dry; in P. adiposa, however,
the hyphae of the scales on the stipe are gelatinous. This is a diagnostic
character for P. adiposa and a few other species. In a large number of