Nostoc sp., a filamentous blue-green algae (Cyanobacteria) which forms chains of individual organisms, strung together like beads on a necklace. Large aggregates of these chains form mucilaginous masses on the surface of the soil, as in this example from the chalk downs near Betchworth, Surrey, England. Nostoc is a nitrogen- fixing organism which grows in the absence of combined nitrogen; it converts gaseous nitrogen from the atmosphere into a usable form, thus liberating it to the benefit of plants in the surrounding soil. It is of considerable importance as a means of biologically upgrading the quality of soil.