thal·lus / ˈ[unvoicedth]aləs/ • n. (pl. thal·li / ˈ[unvoicedth]alī/ ) Bot. a plant body that is not differentiated into stem and leaves and lacks true roots and a vascular system. Thalli are typical of algae, fungi, lichens, and some liverworts.DERIVATIVES: thal·loid / ˈ[unvoicedth]aloid/ adj.

thallus A primitive type of vegetative plant body that is not differentiated into stems, leaves, and roots, although analogous structures may be present. The term is used mainly of non-vascular plants, e.g. algae, fungi, lichens, and liverworts. See also THALLOPHYTE.

thallus A relatively undifferentiated vegetative body with no true roots, stems, leaves, or vascular system. It is found in the algae, fungi, mosses, and liverworts and in the gametophyte generation of clubmosses, horsetails, and ferns.