thalloid

Thalloid is a botanical term describing organisms that possess a thallus-like body: a vegetative form that is relatively undifferentiated and lacks true roots, stems, or leaves or extensive vascular tissue. In such organisms, the thallus is the primary photosynthetic and absorptive body, often flat, lobed, or branching, and it may be filamentous in places.

Thalli commonly lack differentiated organs. In liverworts (Marchantiophyta) and many hornworts, the body is thalloid, with rhizoids serving anchorage rather than true roots. Some simple green algae also have thalli that are not organized into leaves or stems; Ulva, for instance, has a sheet-like thallus. The term is descriptive and not a taxonomic name, used to distinguish thallus-bearing forms from those with clearly differentiated organs.

Reproduction may occur across the thallus through spores, pores, or specialized reproductive bodies. Thallus morphology can be variable, including crustose, filmy, or foliose forms in different groups, and habitats range from moist terrestrial to aquatic environments.

Etymology: thalloid derives from thallus, from Latin and Greek roots referring to a young shoot or green shoot; its use reflects early classifications that grouped thalloid organisms outside of a strictly differentiated plant body. See also thallus, thallophyte.