The North American species of Psathyrella.

1972]

PSATHYRELLA

15

spore is truncate at the base in face view because of the peculiar shape of the
spore.
The thickness of the colored endosporium varies with the size of the spore,
being thickest, usually, in large spores. Thickness of the spore wall is not of
much practical importance in the taxonomy of the genus simply because most
species have spore walls 0.2-0.4 p thick. In some species, as already pointed out,
the spore wall swells appreciably in KOH.
The Hymenium
Basidia. The principle and most abundant element forming the hymenium
is the basidium. It may be defined as the end-cell of a subhymenial hypha or
of one of its branches, in which meiosis takes place. In Psathyrella it is typically
clavate but in some species varies to elliptic. In it the two nuclei of the dicaryon
fuse to form a single large nucleus which usually soon undergoes meiosis to
produce 4 nuclei by two successive divisions. In some genera a third division
takes place with 8 nuclei formed only 4 of which are functional. The spores are
produced following the nuclear divisions and typically one nucleus migrates into
each young spore.
Our interest in the basidium in this work is mainly in any morphological or
chemical features which it has which might be of use in the identification of
the various species. The shape of the basidium and its width-length ratio vary
greatly with the location on the gill-whether near the edge or in the area
between the gills, for instance. Hence measurements can be misleading unless
one is careful to include the position of the basidium in his description. The
basidia are often twice as long between the gills as on the gill edge. The typical
shape for the genus is narrowly clavate; broadly clavate basidia are almost
as numerous. In fact, in some species they fall in the first category in relatively
young basidiocarps, and in the latter category in old pilei. In some species they
finally become almost elliptic and in some subcapitate. In fact it may be said
that the morphology of the basidium in Psathyrella is of little help in the
taxonomy of the genus.
If one considers the evolution of the coprinoid type of hymenium, however,
the differences in the shape of the basidia of Psathyrella take on more meaning.
The Coprinus-type of hymenium is the most highly specialized in the Agaricales
both morphologically and physiologically. Concerning the question of which
type in Psathyrella is possibly the most primitive, the narrowly clavate type is
here so regarded since it is the least differentiated from a typical hyphal end
cell. In the Coprinus hymenium the basidia, depending on the species, may project different distances when sporulating in such a pattern as to facilitate the
free fall of the spores after discharge. Thus, function is directly correlated to
the morphology of the hymenial elements. It is not uncommon in Psathyrella
to note, in various species, that the basidia project slightly at the time of sporulation. In Psathyrella this condition appears to be a forerunner of the type featured
by Coprinus. In only a few species such as P. praeatomata is the wall of the
basidium thickened. In this species the pedicel is not only slightly thickened but
the wall is colored dark brown the same as for the hymenophoral tissue. This
feature, to my knowledge has never become involved as a main feature of evolution in any line in Psathyrella.
Basidioles and Brachybasidioles. In the hymenium many young basidia do
not mature and end up merely as sterile cells helping to give rigidity to the