A thallus is a simple, undifferentiated plant body that lacks true roots, stems, and leaves. It is commonly found in non-vascular plants such as mosses, lichens, and algae. The thallus can take various forms, including flat, branched, or filamentous structures, and it plays a crucial role in photosynthesis and nutrient absorption. In fungi, the term thallus can also refer to the vegetative part of the organism, which is often composed of a network of filaments called hyphae. This structure allows fungi to efficiently absorb nutrients from their environment. Overall, the thallus is essential for the survival and growth of these simple organisms.