Bracket fungi include the tough, woody, shelf-like growths on the trunks of dead trees. Some species are serious parasites of living trees. The upper side often shows concentric striations that represent successive years of growth. Ages of 50-70 years have been recorded for some species. The lower surface is composed of numerous minute pores through which astronomical numbers of spores are released. Some of the largest and thickest bracket fungi are called conks. Artist's conk of the Ganoderma applanatum group can be up to three feet (0.9 m) across and eight inches (20 cm) thick. According to David Arora (Mushrooms Demystified, 1986), large conks may liberate 30 billion spores a day for a period of six months. This is 5,000,000,000,000 or 5 trillion spores annually.