Thallus (pl.:thalli), fromLatinizedGreekθαλλός (thallos), meaning "a greenshoot" or "twig", is the vegetative tissue of someorganisms in diverse groups such asalgae,fungi, someliverworts,lichens, and theMyxogastria. A thallus usually names the entire body of a multicellular non-moving organism in which there is no organization of thetissues intoorgans.[1] Many of these organisms were previously known as thethallophytes, apolyphyletic group of distantly related organisms. An organism or structure resembling a thallus is calledthalloid,thalloidal,thalliform,thalline, orthallose.
Even though thalli do not have organized and distinct parts (leaves,roots, andstems) as do thevascular plants, they may haveanalogous structures that resemble their vascular "equivalents". The analogous structures have similar function or macroscopic structure, but different microscopic structure; for example, no thallus hasvascular tissue. In exceptional cases such as theLemnoideae, where the structure of a vascular plant is thallus-like, it is referred to as having a thalloid structure, or sometimes as a thalloid.
Although a thallus is largely undifferentiated in terms of its anatomy, there can be visible differences and functional differences. Akelp, for example, may have its thallus divided into three regions. The parts of a kelp thallus include the holdfast (anchor), stipe (supports the blades) and the blades (for photosynthesis).[2]
The thallus of afungus is usually called amycelium. The term thallus is also commonly used to refer to thevegetative body of a lichen. Inseaweed, thallus is sometimes also called 'frond'.
Thegametophyte of some non-thallophyte plants –clubmosses,horsetails, andferns is termed "prothallus".
