False-colour scanning electron micrograph of Nostoc sp. , a filamentous blue-green algae (Cyanophyta), criss-crossing the surface of a small rock. Nostoc has the ability to fix atmospheric nitrogen, thereby playing a major role in soil fertility. It has a wide distribution & is commonly found in soil, on mud, rocks, & other plants. It also forms the algal partner of a number of lichens. Blue green algae are considered to be exceedingly primitive organisms, dating back to pre-Cambrian times. Magnificaton: x125 at 6x4.5cm size, x70 at 35mm size.