thallus

A thallus is the vegetative body of an organism that is not differentiated into stem, root, or leaf structures. The term is used in the study of certain algae, fungi, liverworts, hornworts, and lichens. In these organisms, the body may be thalloid (flat and sheet-like), filamentous, or more complex, and may lack vascular tissue typical of higher plants. The concept contrasts with the more differentiated bodies of seed plants, which have stems, leaves, and roots arranged in tissues and organs.

In algae, the thallus can be simple filaments or a branched, parenchymatous body. In liverworts and other bryophytes, the thallus is often dorsiventrally flattened and photosynthetic, sometimes with rhizoids for anchorage. In lichens, the thallus is a composite body formed by a fungal partner and photosynthetic partners (green algae or cyanobacteria), giving rise to crustose, foliose, and fruticose growth forms. The thallus bears reproductive structures in many species.

Etymology and usage: The term derives from Greek thallos, shoot or twig, reflecting its early use to describe plant bodies lacking typical organs. In modern botany and mycology, thallus remains a general term for the undifferentiated plant body or vegetative tissue.