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

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THE BOLETES OF MICHIGAN

Section SUBTOMENTOSI Fries
Epicr. Syst. Mycol. p. 412. 1838
The diagnostic characters are given in the key to sections.
Type species: Boletus subtomentosus.
The majority of the species included here have been placed in
Xercomus by modern authors, and we also have tried to recognize this
group as distinct from Boletus at times but have finally given it up.
Watling (1968) has also recognized that on a world basis the genus is
untenable. In the past various characters have been used in attempts to
circumscribe it such as: Do the tubes separate from each other when the
pileus is broken downward through the tubes, or do the tubes split in
such a way that you see the hymenial surface as you view the tube
layer? We have found this distinction unusable and so cannot consider it
of importance in the recognition of a genus. As most boletes have the
pileus cuticle in the form of a trichodermium which often collapses to
an interwoven tangled layer of hyphae, this feature is of little help at
the generic level although the rather coarse development of a trichodermium is a feature of the type species of Xerocomus and its closest
relatives. It is now known that the trichodermium and the ixotrichodermium are common to most of the genera of the Boletaceae. In fact it
is one of the basic types for the family.
Our approach is to divide the section as here defined into numerous smaller groups of obviously closely similar species in order to contrast species sufficiently to allow them to be fairly readily recognizable.
In this section, however, it is always an open question as to whether one
is dealing with relationships as compared to accidental parallelisms. The
major problem, in spite of all that has been done in this group, is to
clearly define the species.
KEY TO SUBSECTIONS
1. Basidiocarps attached to basidiocarps of the gastromycete Scleroderma............................................... Subsect. Parasitici
1. Habitat terrestrial or lignicolous................................ 2
2. Spores often over 20 p long; pileus granulose-squamulose at maturity.................................................................... Subsect.Mirabiles
2. Not as above........................................ 3
3. Spores short (7-9 x 3-3.5 u); habitat on decayed wood..... Subsect. Sulphurei
3. Spores longer than in above choice or habitat terrestrial............. 4
4. At least some spores notched or truncate at apex..... see Sect. Truncati
4. Spores lacking above-mentioned differentiation at their apex......... 5