Thallos

Thallos is a Greek root meaning “shoot” or “twig,” from which the botanical term thallus is derived. In early and modern usage, thallos denotes a simple, undifferentiated plant body that is not organized into true stems, leaves, or roots. The form thallus, derived by adding the Latin suffix -us, is used across a range of groups to describe such bodies.

In practice, a thallus can be flat and sheet-like (thalloid), filamentous, or otherwise simple in structure, depending on the organism. In algae, the thallus is the main vegetative body, as seen in seaweeds where the thallus is the photosynthetic, recognizable mass. In bryophytes like some liverworts, a thallus refers to a flat, sheet-like body that lacks the differentiated vascular tissues of higher plants. In fungi and lichens, the term is also used to describe the vegetative body, which may be mycelial or a cohesive, non-woody thallus.

The term thallos is largely encountered today in etymology, historical texts, or discussions of plant morphology, whereas thallus is the common term used in contemporary descriptions of organisms with undifferentiated or minimally differentiated bodies. The concept distinguishes simple, body-wide organization from the more complex organization seen in plants with distinct roots, stems, and leaves. See also thallus, thallophyta.