The boletes of Michigan, by Alexander H. Smith and Harry D. Thiers.

20

THE BOLETES OF MICHIGAN

such that meaningful generic separations cannot be made by using one
or several in combination to subdivide Suillus further into smaller
genera. These are: the presence of a veil and whether it leaves an
annulus (ring) on the stipe. The veil in Suillus grows over the tube
cavity from the margin of the pileus, and as we (1964) showed, it is
present in some degree on species with wide pores as well as on species
with minute pores and is present on specimens with a dry pileus as well
as on those in which the pileus is viscid or slimy. Some species (Suillus
sibiricus, for example) have a veil which part of the time leaves an
annulus and the remainder of the time adheres to the margin of the
pileus.
A second character involves clamp connections. For a time it was
thought that this was a very important feature. Studies in pure culture,
however, showed that for most species clamps could be found on the
mycelium; but they are often rare, in some occasional, and in some
readily detected, as in S. cavipes. Some authors have tried to maintain
the genus Boletinus for species with clamps, a dry pileus, and a stipe
which becomes hollow in the base. But Suillus sibiricus, a viscid species,
also tends to have a hollow stipe base. S. pictus has a dry pileus and
lacks clamps on the basidiocarp hyphae. In S. lakei, a western species,
one can hunt an hour to find a clamp-but the clamps are present. One
variety of this species has in the pileus such a poorly defined gelatinous
layer that it often goes undetected. In the type variety, however, it is
much more distinct. This interlocking pattern of characters among these
fungi convinced us that Boletinus should not be ranked as a genus. We
rebel at having "first-class" and "second-class".genera. Consequently, we
maintain Suillus as we defined it in 1964. It has, in this concept, all the
requirements for a truly natural genus, particularly since within it one
can follow the evolution of a number of characters.
Let us now examine the important features of Suillus as a genus
and see if evolutionary changes in certain of its characters can be recognized in various other genera. In Singer (1963) the Gomphidiaceae are
recognized as a family of gill fungi considered related to the Boletaceae,
particularly in the area of Suillus. We accept the idea of this relationship. But why do we believe that 2 such distinct groups of fungi are
indeed related? Obviously, they have certain features in common: (1)
elongate spores of the type known as boletoid; (2) greatly elongated
pleurocystidia which in some react in KOH, giving characteristic color
changes, and which are more like the cystidia of Suillus than of any
other group of fleshy fungi; (3) caulocystidia are basically similar also
and even occur in bundles as in typical species of Suillus.