thalle

Thalle, often encountered as a variant spelling of thallus in older literature, refers to the vegetative body of many non-vascular organisms, including certain algae, fungi, and lichens. The term thallus arises from Greek thallos, meaning a twig or shoot. In modern usage, thallus is the standard form, with thalli as the plural.

A thallus is typically undifferentiated and lacks the distinct roots, stems, and leaves of higher plants. It can be flat and sheet-like (crustose or foliose forms), lobed, or branched and bushy (fruticose forms). Thalli may be photosynthetic when they contain algae or cyanobacteria, or non-photosynthetic in parasitic relationships.

In lichens, the thallus is a composite structure formed by a symbiotic association between a fungal partner and photosynthetic partners (usually algae or cyanobacteria), and its morphology is a key feature in lichen taxonomy. In many algae, the thallus constitutes the main body of the organism, as seen in seaweeds like Ulva. Liverworts and some other bryophytes also display thalloid bodies, as do certain non-vascular fungi in historical descriptions.

The concept of the thallus is important in biology for describing a body plan that is not organized into true plant-like organs. In current English usage, thallus is the preferred term; thalle is a much rarer, older, or cross-linguistic variant.