This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
The plants belonging to this family are characterized especially by a honeycombed fruiting surface, that is, the under surface of the plants possesses numerous tubes or pores which stand close together side by side, and except in a very few forms these tubes are joined by their sides to each other. In Fistulina the tubes are free from each other though standing closely side by side. In Merulius distinct tubes are not present, but the surface is more or less irregularly pitted, the pits being separated from each other by folds which anastomose, forming a network. These pits correspond to shallow tubes.
The plants vary greatly in consistency, some are very fleshy and soft and putrify readily. Others are soft when young and become firmer as they age, and some are quite hard and woody. Many of the latter are perennial and live for several or many years, adding a new layer in growth each year. The larger number of the species grow on wood, but some grow on the ground; especially in the genus Boletus, which has many species, the majority grow on the ground. Some of the plants have a cap and stem, in others the stem is absent and the cap attached to the tree or log, etc., forms a shelf, or the plant may be thin and spread over the surface of the wood in a thin patch.
In the genus Daedalea the tubes become more or less elongated horizontally and thus approach the form of the gills, while in some species the tubes are more or less toothed or split and approach the spine-bearing fungi at least in appearance of the fruit-bearing surface. Only a few of the genera and species will be described.
The following key is not complete, but may aid in separating some of the larger plants:
Tubes or pores free from each other, though standing closely side by side,..........Fistulina.
Tubes or pores not free, joined side by side, . . 1. 1. - Plants soft and fleshy, soon decaying,.....2.
Plants soft when young, becoming firm, some woody or corky, stipitate, shelving, or spread over the wood,..............Polyporus.
Tubes or pores shallow, formed by a network of folds or wrinkles, plants thin, sometimes spread over the wood, and somewhat gelatinous, . . Merulius. 2 - Mass (stratum) of tubes easily separating from the cap when peeled off, cap not with coarse scales, tubes in some species in radiating lines, .... Boletus. Stratum of tubes separating, but not easily, cap with coarse, prominent scales,.........Strobilomyces.
Stratum of tubes separating, but not easily, tubes arranged in distinct radiating lines. In one species (B. porosus) the tubes do not separate from the cap,..................Boletinus.
This last genus is apt to be confused with certain species of Boletus which have a distinct radiate arrangement of the tubes. It is questionable whether it is clearly distinguished from the genus Boletus.