The branch-like tube that makes up the body of a fungus is called a hypha.
Hyphae are long, threadlike structures that grow, branch, and collectively form the fungus's main body. A dense network of hyphae is called a mycelium. This network may spread through soil, wood, decaying leaves, or other food sources, allowing the fungus to absorb nutrients over a large area.
Many hyphae are divided into cell-like compartments by walls called septa, while others lack these internal partitions and are described as coenocytic. Hyphae grow mainly at their tips, where new cell material is added. Branching lets a fungus explore its surroundings efficiently.
A common mix-up is confusing a hypha with a mycelium: one hypha is a single filament, whereas mycelium is the interconnected mass made from many hyphae. The visible mushroom is usually only the reproductive structure of a much larger hidden mycelial network.