When the vegetative mycelium of a fungus exhibits differentiation and a clear, determinate body plan of its own, it is called a thallus. Some book definitions more or less synonymize thallus with mycelium. But in practice, the term thallus is only used in the sense given here; the more common mycelia which just spread out indeterminately in all directions until they run into something are just called mycelia. In this sense, chytrids and lichens are the only fungi that typically form a thallus. In the case of the lichen, the term includes the alga symbiont enclosed by the thallus, not just the fungal component.
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