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

8

THE BOLETES OF MICHIGAN

ornamentation involves warts or a mesh of ridges formed (possibly) by
outer-wall material. In Strobilomyces this ornamentation correlates with
the blackish to chocolate spore deposit and the globose to subglobose
spore shape to define a clearly delimited genus at least as far as our
flora is concerned.
In many species, as already intimated, there may be an outer and
an inner wall, the latter usually being the thickest. The remains of the
outer wall may not always be readily visible. The thickest of the walls,
however, may at times show canals or lines extending through it and
filled with a -substance different from that of the wall proper. The ends
of these canals will show as irregularly shaped spots on the spore surface
under an oil-immersion lens. We do not regard this type of ornamentation as being as significant taxonomically as the 2 types already discussed, for in species such as Tylopilus gracilis one often finds part of
the spores in a deposit unornamented and part ornamented. The electron microscope is now helping to solve the problem of the differentiation present in the spore wall in such species. We believe that spore
ornamentation in fleshy fungi generally has arisen "de novo" many
different times and that all boletes, for instance, with ornamented spores
are not necessarily more closely related to each other than some in the
group may be to species with unornamented spores. This is why we
regard the family Strobilomycetaceae as defined by Singer as "artificial."
The wall in most bolete spores is around 0.2, thick, which means
it is too thin to measure accurately by the light microscope. It is rather
seldom that one encounters spores with walls much over 0.5 A thick.
Also it makes some difference at times if the spores are mounted in
KOH-the wall swells more than it does in Melzer's. For these reasons
the thickness of the wall does not figure prominently in our description
of species in the boletes. Of greater significance is. the presence of apical
differentiation-in the form of a notched apex (fig. 113) caused by
inflation of the wall around it, or by a discontinuity (germ pore) in an
apical position. The latter is a rare occurrence in boletes, though a
character of major taxonomic importance in the gill fungi.
Recently, it has been found that spores of boletes may give a
bluish reaction in Melzer's, but this feature is not as clear as in the
agarics because of certain complications, the first being the rather highly
colored spore wall. This obscures the reaction. It has also been found
that spores taken from dried hymenophore tissue are dingy blue
mounted directly in Melzer's but soon change to reddish tawny (dextrinoid) in at least the distal half of the spore, the proximal half merely
fading to yellowish. This reaction has been termed a "fleeting-amyloid"