The North American species of Psathyrella.

1972]

PSATHYRELLA

23

manner of Mycena corticola, but if it is on a stone it is obvious that an example
of migration has been encountered.
I have made no attempt to correlate pH with fruiting site, but it is evident
from what has been said already that a reading at the base of the basidiocarp
may not give accurate information on the pH where the vegetative growth is
taking place. Also, the basidiocarp can modify the pH slightly in its immediate
vicinity. In spite of a number of such possible pitfalls, it is a known fact that
certain species fruit on strongly calcareous soil and some only on rather acid
substrata. These however, are mainly in such genera as Inocybe, Cortinarius
and Rhodophyllus, i.e. mycorrhizal forming genera for the most part. I would
hazard a guess that in Psathyrella the pH requirements are about like those of
other genera of wood-inhabiting agarics.
As for most fleshy fungi, the amount of nutrient available appears to influence
the size and number of basidiocarps produced. Around a sawdust pile for instance,
species often normally growing in a gregarious or scattered manner have basidiocarps in large clusters and with larger basidiocarps than usual. However, the
colors, veil details, spore size, etc. are not likely to vary significantly. In these
respects the genus is like other genera of the order.
The alpine and subalpine species seem to show the same types of basidiocarp
modifications shown by other genera when they occupy the same habitat. The
basidiocarps are mostly small and usually remain hidden in the vegetation of
the meadow or the needle carpet. For Psathyrella these zones have not yet
been adequately explored. The process involves a maximum expenditure of
energy for results obtained. Most of the alpine and subalpine species have
very small slender basidiocarps and only a few are found at a time. It is curious
that one also finds species of similar stature in the hot dry areas such as sagebrush country, but they occur there when conditions of high humidity have been
maintained for 5-12 days. Here it seems that a set of factors opposite those prevailing in the high country seems to have produced species with basidiocarps
capable of quick development and sporulation. The species themselves, of course
are different.
PHYTOGEOGRAPHICAL CONSIDERATIONS
Not much can be said at this time relative to the distribution of Psathyrellae
over the world-we need to critically delimit the species first. But it is evident
from present knowledge that the genus is world-wide in distribution. It occurs
in tropical as well as subalpine and arctic regions. The degree of diversity, however, in the genus for the major land masses of the earth needs to be assessed more
critically before comparisons with various regions will be worth while. One of
the major needs, not limited to studies of Psathyrella, is better planned and
more intensive sampling in most areas.
Although the present work is the first, to my knowledge, which has involved
the above considerations, it must be candidly pointed out that the sample of the
Psathyrella flora of North America forming the basis for this work is minimal
for the proper understanding of speciation and species migration in North
America. For one thing, most of Canada remains unexplored. My own estimate
of the completeness of the present work is that it will fall between 50 and 75
per cent of the total flora.
Canada, Alaska, and Mexico have scarcely been touched. The areas in the
United States most adequately sampled are the mountains of the western United