Tubes are a surface on the underside of the cap of the fruiting body of mushrooms and bracket fungi. Similar to lamellae/gills, tubes hold and disperse spores. The many small cavities create large surfaces where the spores are exposed to the outside. Tubes occur on basidiomycetes, which thus have basidia as spore-forming organs.
Source
Mylifeatplaytime – Tubes – Anatomy of a Mushroom with Parts Explained 
April 2023, TÄNKOM | Revised February 2024 RETHINK
