Lichens are often distinguished by growth form and by their photosymbiont. Crust lichens include crustose and aerolate lichens that are appressed to the soil substrate, squamulose lichens with scale- or plate-like bodies that are raised above the soils, and foliose lichens with more “leafy” structures that can be attached to the soil at only one portion. Lichens with algal symbionts can fix atmospheric carbon, while lichens with cyanobacterial symbionts can fix nitrogen as well. Lichens produce many pigments that help protect[citation needed] them from radiation.