Thallus (plural:
thalli), from
Latinized Greek θαλλός (
thallos), meaning "a green
shoot" or "
twig," is the
undifferentiated vegetative tissue of some
organisms in diverse groups such as
algae,
fungi, some
liverworts,
lichens, and the
Myxogastria. Many of these organisms were previously known as the
thallophytes, a
polyphyletic group of distantly related organisms. An organism or structure resembling a thallus is called
thalloid, thallodal, thalliform, thalline, or
thallose.