example, develops a deeper blue color when it grows during cooler autumn weather. Optimal growth occurs during periods of frequent light rains and high humidity; if the habitat dries out, growth will stop, but may resume after further precipitation. This intermittent growth affects the fruitbodies of different species to variable extents, leading to large variations in form, surface texture, and color. The morphological variability of fruitbodies and the dependence of their appearance upon environmental conditions has made Hydnellum a difficult group to study. Canadian mycologist Kenneth A. Harrison, who described several new species from North America, noted "[t]he remarkable longevity of individual sporophores of many species and the changes in appearance that occur during the long period of their development have confused all workers studying this group." For example, H. aurantiacum, initially white, becomes in turn shades of orange, rusty-brown, and brownish-black. Its fruitbody initially has a turbinate (cushion-like) shape with a lumpy surface, later becoming flattened to funnel-shaped with a smooth to corrugated surface texture. The caps form from the top of the short stipe by the growth and expansion of a blunt margin and later as a thickening of the upper surface. Spines start to form when the cap hangs over the stipe slightly. They are white in many species, but become brown in maturity as the brown-colored spores accumulate on the surface. Habitat and distribution Hydnellum fungi are mycorrhizal, and are usually found in coniferous and mixed woods. Favored tree hosts include members of the Fagaceae and the Pinaceae. The genus is widely distributed in the Northern Hemisphere, particularly Europe and North America, but some species are also found in tropical Asia. Harrison identified a dozen new species from North America in the 1960s. Rudolph Arnold Maas Geesteranus recognized 16 European species in his 1975 treatment of the genus, to which H. dianthifolium has been recently added by Loizides and colleagues. Some Hydnellum species, including H. ferrugineum and H. scleropodium, form a tough mat of mycelia in the humus and upper soil of pine forests. This mycelial mat grows larger with old trees, and can cover an area of several square meters. These areas generally lack dwarf shrubs and promote the vigorous growth of mosses; reindeer lichens often occur in the center of large mats. The presence of the fungus changes the nature of the soil, resulting in a thinner humus layer, decreased groundwater penetration, decreased soil