"... mycelium, a material found in the root structure of mushrooms"
https://www.engineering.com/DesignerEdge/DesignerEdgeArticles/ArticleID/10787/RPI-Engineers-Develop-a-Mushroom-Based-Packaging-Material.aspx
Hehe. This like describing hens as a material found in the root structure of eggs ;) Mycelium is the being, mushrooms are just their fruiting bodies.