A thallus; thallium.
[Greek thallos, green shoot.]
A thallus is a plant body that is not differentiated into stem and leaves, typical of algae, fungi, lichens, and some liverworts. A thallophyte (Greek phuton, a plant) is a plant that consists of a thallus; linked adjectives are thalloid and thalline. The element thallium was given its name by its discoverer Sir William Crookes (1832–1919) because of a bright green line in its spectrum; the adjective thallous refers to that metal in its monovalent state (as in thallous oxide, Tl2O), while thallic refers to trivalent thallium (as in thallic chloride, TlCl3).