siphonaceous (siphoneous) Applied to algae in which the thallus is not divided up by septa, i.e. the many nuclei are not compartmentalized into cells. The typical siphónaceous alga has a large central vacuole surrounded by a layer of protoplasm, containing nuclei and chloroplasts, which lines the cell wall.

siphonaceous (siphonous) Describing algae and other protoctists in which the thallus is coenocytic (see coenocyte) and often takes the form of a tubular structure.