Plant body of algae, fungi (see fungus), 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.

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