The veiled species of Hebeloma in the western United States / Alexander H. Smith, Vera Stucky Evenson, and Duane H. Mitchel.

Introduction

7

of cystidium could evolve by further overall inflation. The fusoidventricose type (fig. A13) apparently arises from the basidiole-type by
elongation of the apical region of the cell to produce a + narrow neck
tapered to a more or less obtuse apex. At this stage of development it
may be confused with the filamentous type unless one carefully examines the basal region to be sure it still retains some indication of its
original enlargement (see fig. A21). The distinctive aspect of this type
of cystidium, then, is that the widest part of the cell is below the area
where the walls become parallel. (Except for the fact that the walls of
a circle are parallel for an infinitesimal distance at points directly opposite each other, the above definitions are accurate.) To confuse the
issue further, this fusoid-ventricose type of cystidium, contrary to the
accepted definition of a cystidium as being an end cell, can form a
secondary cross-wall (see fig. A24). The differences between this divided cell and other fusoid-ventricose cystidia are: (1) there is no
clamp connection at the secondary septum and (2) the proximal portion of the cell is much wider (more ventricose) than the terminal (+
extended) portion. The recognition of this unusual cell structure, along
with the other variations previously described as the fusoid-ventricose
type of cheilocystidium, is very important since this type is indicative
of, and found characteristically in the subgenus Hebeloma. In some
species we find, rarely, a few cheilocystidia in which the apex forks or
branches once or twice (fig. A26). The significance of this occurrence
is not known.
In summary, cheilocystidia in subgenus Hebeloma appear to be in
an active state of evolution. Since mature basidiocarps occasionally exhibit filamentous, clavate, capitate, and fusoid-ventricose cystidia all on
the same gill edge, the cheilocystidium cannot be used to distinguish
species as arbitrarily as, for instance, in Mycena. We have, however,
demonstrated to our own satisfaction that as a rule the presence of a
veil, or veil remnants, and the finding of fusoid-ventricose cheilocystidia
are reasonably well correlated characters in subgenus Hebeloma. As a
corollary to this observation, fusoid-ventricose cheilocystidia are even
more uncommon to rare in the nonveiled species (subgenus Denudata)
in which filamentous to filamentous-capitate cheilocystidia are the rule.
These latter types of cystidia appear to be clearly related to droplet
formation on gill margins. For instance, our study indicates that nearly
all species of Hebeloma may develop droplets (weep) on the gill margin
and apex of the stipe especially when humidity is high. This tendency,
however, is very infrequent among veiled species (subgenus Hebeloma)
in which filamentous-capitate cystidia are few or absent, but is found
most consistently in the species of the subgenus Denudata which have
predominantly filamentous-capitate cystidia which have a tendency to
agglutinate. This agglutination process, apparently, is encountered