An order of algae in which the thallus is parenchymatous and growth is mostly diffuse. Scytosiphon lomentaria is a common British species, consisting of loosely tufted, tubular fronds. Petalonia has flat, leaf-like fronds and is found mainly on rocks on the lower shore. Colpomenia has a thallus consisting of thin-walled hollow spheres. See also oyster thief.
Subjects: Plant Sciences and Forestry.