Polysiphonia devoniensis grows as a tuft of erect, fine, terete branches up to 2.5 cm long and brown in colour. Each branch consists of an axis of central cells surrounded by 4 perixial cells, all of the same length surrounding the central cells and are ecorticate. Trichoblasts are formed near the tips of the branches and rhizoidsgrow from the periaxial cells of the prostrate axes and trichoblasts from the apex. The rhizoids remain open with the periaxial cells.
Reproduction
The plants are dioecious. Spermatangia are borne in loose clusters in clusters. The cystocarps are ovoid or round or slightly urceolate with a large ostiole. The tetrasporangia occur spiral series in the branches.