The annual thallus is typically globose, hollow at maturity, olive brown in coloration and 3-5 cm in diameter.  The wall of the balloon shaped thallus has parenchymatous structure and is usually 5-6 cell layers thick with smaller cells towards the exterior. The stipe morphology varies depending on the host plant and the locality along the Pacific coast (see range above).  If Soranthera is found on Rhodomela, the stipe is usually spherical, thick walled and broadly attached to the host plant, whereas on Odonthalia, the stipe has a thicker diameter towards the apex is thin walled and narrowly attached to the host plant.  (These varying features dependent on the host plant can be easily viewed on the habitat page). Dark brown bumps that are evenly distributed may be observed on the surface of the plant which are the reproductive structures called sori. (Abbott, 1976)

Top picture: Immature. hollow thallus growing on Odonthalia with air bubbles present inside

Bottom picture: Mature thallus with sori present and stipe

Abbott, Isabella A., Hollenberg, George J.. Marine Algae of California.  Stanford, California: Stanford University Press, 1976.