thallose

Thallose is a descriptive term used in biology to refer to a body plan in which an organism’s vegetative body, the thallus, is relatively simple and undifferentiated, lacking true stems, leaves, or roots. A thallus can be flat, sheet-like, ribbon-like, or filamentous, and its tissues are typically organized into a few layers rather than into distinct organs. The term is applied across several groups, including certain algae, liverworts, hornworts, and many lichens.

In algae, thalli constitute the main, photosynthetic body and can range from simple sheets to branching networks. In bryophytes, thalloid liverworts (such as Marchantia) and hornworts have thalli as their dominant vegetative form, whereas many mosses are leafy rather than thallose. In lichens, the thallus is the vegetative body formed by a symbiotic association between a fungal partner and photosynthetic cells (green algae or cyanobacteria); thalli can be flat and spread over substrates, and may be described as crustose, foliose, or fruticose depending on their overall morphology.

The concept of thallose emphasizes body organization rather than taxonomic grouping, contrasting with growth forms such as foliose (leaf-like), fruticose (shrublike), or crustose (crust-like). The term derives from the Greek thallos, meaning shoot or twig, reflecting the idea of a more or less undifferentiated, sexual-body structure in these organisms.