annual

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annual, perennial

Image of Fomes fomentarius from Christian Gottfried Daniel Nees von Esenbeck (1816 - 1817) Das System der Pilze und Schwämme

A fruiting body is termed annual when it receives no further nutritive support from the somatic hyphae after it has sent off its spores. A perennial fruiting body is (after its initial beginning) built up in a long-term ongoing process, through layers of tissue that are deposited on top of the existing fruiting body. Thus, a perennial fruiting body in cross-section exhibits distinct "growth-rings", like a tree, marking the extent of each period of growth.
You can see these layers as steps on the top of the top of the pileus of the picture of Fomes fomentarius, and in the lower left of the picture is a cross-section of the fruiting body, showing the "growth-rings".

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The terms annual and perennial are used by analogy with flowers, which either come up year after year in the same place, or come up once and then die. De Bary specifically warned against tying these terms to the seasonal cycle of plants, as fungi are not tied to the same requirement of sunlight for nutrition:

Persoon and Fries call the zones of these Polypori annual zones. They may no doubt be correctly compared in certain points to the annual rings of Dicotledons, but it has never been distinctly proved that only one new zone is formed each year in these [fungi]. There is no doubt that many zones are formed in the course of a year in most of the other zoned mushrooms. J. Schmitz has shown this in detail for Stereum hirsutum, and there are a certain number of many-zoned pilei in the Hymenomycetes which only last one year.
De Bary (1887) p. 57

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