: a plant or plantlike body (as of an alga, fungus, or moss) that lacks differentiation into distinct members (as stem, leaves, and roots) and does not grow from an apical point
Plant body of algae, fungi (seefungus), and similar simple, plantlike organisms. Composed of filaments or plates of cells, a thallus ranges in size from a single-celled structure to a complex treelike form. The photosynthetic and supportive cells tend to be organized linearly, but a thallus lacks such differentiated specialized structures as stem, leaves, and conducting tissue. Most thallus plants are currently classified as complex protists.