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thallus thal·lus (thāl'əs)
n. pl. thal·lus·es or thal·li (thāl'ī)
A plant body or fungus undifferentiated into stem, root, or leaf.
Plural thalli (thāl'ī)
A type of body found among plants and fungi that is not differentiated into roots, stems, or leaves. Thalli are found among lichens, mosses, liverworts, and many algae, as well as the gametophyte generations of horsetails and ferns, which have rhizoids but not true roots.