Nostoc, the genus of blue-green algae with cells arranged in beadlike chains that are grouped together in a gelatinous mass. Reproduction is by fragmentation. A special thick-walled cell has the ability to withstand desiccation for long periods of time. Like most blue-green algae, Nostoc contains two pigments, blue phycocyanin and red phycoerythrin, as well as chlorophyll, and has the ability to fix nitrogen in specialized cells called heterocysts.