The spore producing layer of wood decay fungi is called the hymenium. Each particular decay fungus species will produce one of the different types of spore bearing types: gills, pores, teeth, daedaloid or lamellae, or smooth. The observation of the type, appearance, size and location of the hymenium is critical to proper identification. Most decay fungi produce their hymenium on the lower surface or underside of the fruiting structure.
Gills are found exclusively on the annual decay fungi that are typically referred to as mushrooms. Gills are found on the underside of the top or cap and can be different colored than the rest of the mushroom. In some cases it may be necessary to remove the top or cap and place the cap on a light colored piece of paper to obtain a spore print of the fungus (Photograph 2).
Several decay fungi have a spore producing layer that has teeth. Since there are relatively few decay fungi with this type of hymenium, its presence greatly narrows the number of species for identification.
The majority of decay fungi on urban trees have pores of varying sizes for their spore producing layer. The pores may range from barely visible to the naked eye to large and easily observable. The pores openings may have several characteristics such as being angular or round that can help in species identification. Other features of the pore layer such as its color and whether it bruises on touch are also important identifying features of some decay fungi.
This pore layer is also referred to as maze-like and is a cross between pores and gills. Several decay fungi have a true daedaloid spore layer, while other that appear somewhat maze-like are still grouped with the pored fungi.
Some wood decay fungi do not produce a noticeably different spore producing layer, or the spore producing structures are buried in stromatic or fungal growth so they cannot be seen with the naked eye. Several fungi are sterile and produce only a smooth conk without any spore bearing layer. All these fungi are grouped here under smooth pore layers as observed with the naked eye.