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 vary greatly in size, form, and consistency. Some of them are very large, some quite small, some are fleshy in consistency, some are woody, corky; some membranaceous; and if we include plants formerly classed here, some are gelatinous, though there is a tendency in recent years on the part of some to place the gelatinous ones among the trembling fungi. The special character which marks the members of this family is the peculiarity of the fruiting surface, just as a number of the other families are distinguished by some peculiarity of the fruiting surface. In the Hydnaceae it covers the surface of numerous processes in the form of spines, teeth, warts, coarse granules, or folds which are interrupted at short intervals. These spines or teeth always are directed toward the earth when the plant is in the position in which it grew. In this way the members of the family can be distinguished from certain members of the club fungi belonging to the family Clavariaceae, for in the latter the branches or free parts of the plant are erect.
In form the Hydnaceae are shelving, growing on trees; or growing on the ground they often have a central or eccentric stem, and a more or less circular cap; some of them are rounded masses, growing from trees, with very long spines extending downward; others have ascending branches from which the spines depend; and still others form thin sheets which are spread over the surface of logs and sticks, the spines hanging down from the surface, or roughened with granules or warts, or interrupted folds (see Phlebia, Figs. 193, 194). In one genus there is no fruit body, but the spines themselves extend downward from the rotten wood, the genus Macronella. This is only distinguished, so far as its family position is concerned, from such a species as Clavaria mucida by the fact that the plant grows downward from the wood, while in C. mucida it grows erect.