Mycelium

The mycelium is the vegetative body of most fungi, the part that feeds and grows, as opposed to the mushrooms and other fruiting bodies that make spores. It is a mass of hyphae, tubular threads only a few micrometers wide with walls stiffened by chitin. In many fungi the hyphae are divided into cells by cross-walls with pores; in others, such as bread molds, each hypha is a continuous tube holding many nuclei.

Hyphae grow at their tips and branch again and again, so a mycelium can spread outward through soil, rotting wood, leaf litter or living tissue. The fungus digests its food outside its body: hyphae secrete enzymes into their surroundings and then absorb the small molecules released, and the huge surface area of the network makes this efficient. Because the threads are so fine, one mycelium can grow enormous; a honey fungus in an Oregon forest spreads through several square kilometers of soil and ranks among the largest organisms known.

Mycelia are central to ecosystems, as decomposers that recycle nutrients and as mycorrhizal partners that link plant roots and help them take up water and minerals. In the laboratory, the mycelium growing on agar is what microbiologists examine to identify a mold, and grown mycelium is now also made into packaging and leather-like materials.