In traditional taxonomical literature, the size of a given fungal structure is shown as static numbers, within certain statistical limits. In nature, however, everything happens in the dimension of time. The dynamics of growth contains important biological information.

We have studied the fruiting body growth of four taxa - Boletus edulis, Hypholoma capnoides, Kuehneromyces mutabilis and Armillaria sp. - using the cap diameter and fruiting body dry weight as main variables.

Our data illustrates that the growth of a single species may take numerous alternative routes depending on, e.g., the weather. The optimal range of growth should be told apart from various degrees of suffering. It is also not self-evident, how young or over-aged sporocarps should be included in size definitions. All this calls for better definitions of ”size” as a taxonomical character.

When cap expansion data and weight/diameter data are combined, it is possible to calculate other features such as gain of mass, or increase of hymenium area, in relation to time. Several sources of error need to be considered, however.

Students of mycology can easily contribute to the knowledge of fungal dynamics by collecting datasets of fruiting body growth in the vicinity of their homes.