A thallus is a simple, undifferentiated plant body that lacks true roots, stems, and leaves. It is a characteristic feature of certain groups of organisms, including:

* Algae: Thalli of algae can be filamentous, sheet-like, or even complex, like the giant kelp.

* Fungi: The body of a fungus is also a thallus, often referred to as a mycelium. It is made up of a network of hyphae.

* Lichens: These symbiotic organisms are formed by a fungus and an alga. The fungal partner forms a thallus that provides structure and protection, while the alga provides nutrients through photosynthesis.

* Some liverworts and mosses: These non-vascular plants have thalli that are more differentiated than those of algae or fungi, but still lack the true vascular tissues of higher plants.

Key characteristics of a thallus:

* Lack of vascular tissue: This means that water and nutrients are transported through the thallus by diffusion or other simple mechanisms.

* Undifferentiated structure: It doesn't have specialized tissues or organs like roots, stems, or leaves.

* Variable size and shape: Thalli can be microscopic or large, and come in various forms.

The thallus is a fundamental part of the life cycle of these organisms, playing a role in photosynthesis, nutrient absorption, reproduction, and even protection from the environment.