Thallus

Thallus from Greek θαλλός (thallos), meaning "green shoot," refers to the undifferentiated vegetative tissue in various organisms like algae, fungi, certain liverworts, lichen, and Myxogastria. This term typically describes the entire body of a multicellular, sessile organism lacking organ-level tissue organization. Many of these life forms were once classified as thallophytes, a now-obsolete polyphyletic group. Structures or organisms resembling a thallus are described as thalloid, thallodal, thalliform, thalline, or thallose, reflecting their simple, undifferentiated nature.