Light micrograph of Nostoc sp., a blue-green nitrogen-fixing algae which grows in the absence of combined nitrogen and converts gaseous nitrogen into a usable form. Useful in enriching the soil, for instance in rice culture, it is a filamentous organism which forms chains of individual creatures. Aggregations of Nostoc form mucilaginous masses, usually seen resting on the soil. It is also being investigated for possible applications in biotechnology. Phase contrast micrograph. Magnification: X 160 at 35mm size, X 320 at 6x7cm size.