Thallus

The term "thallus" (plural: thalli) comes from the Latinized Greek thallos (θαλλός), meaning "a green shoot" or "twig." It refers to the vegetative tissue found in a variety of organisms, including algae, fungi, some liverworts, lichens, and the Myxogastria. Typically, a thallus describes the entire body of a multicellular, non-moving organism whose tissues are not organized into distinct organs. These organisms were once grouped together as thallophytes, though this is a polyphyletic category of distantly related species. Any organism or structure resembling a thallus can be described as thalloid, thalloidal, thalliform, thalline, or thallose.

Although thalli lack the organized parts—such as leaves, roots, and stems—found in vascular plants, they may still have analogous structures that serve similar functions or appear similar on a macroscopic scale. These analogies differ microscopically; for instance, no thallus contains vascular tissue. In rare cases, like the Lemnoideae, a vascular plant may have a thallus-like structure, which is then called thalloid or a thalloid.

While a thallus is largely undifferentiated in anatomy, visible and functional differences can exist. For example, a kelp's thallus can be divided into three regions: a holdfast (for anchoring), a stipe (supporting the blades), and blades (for photosynthesis).

In fungi, the thallus is usually called a mycelium, and in lichens, "thallus" commonly refers to the vegetative body. In seaweed, the thallus is sometimes called a frond. Additionally, the gametophyte of certain non-thallophyte plants—such as clubmosses, horsetails, and ferns—is termed a "prothallus."