Nostoc commune is a colonial cyanobacterium that begins as a small, hollow gelatinous sphere, which grows into a leathery, flattened, and convoluted mass when mature, often joining with adjacent colonies. Inside its thin sheath, it contains numerous unbranched hair-like structures known as trichomes, composed of short, bacteria-like cells in a linear arrangement. These cells lack a nucleus and internal membrane system typical of bacteria, and they reproduce by binary fission, splitting into two new cells. Along the trichomes, specialized nitrogen-fixing cells called heterocysts are interspersed among the ordinary cells. In wet conditions, Nostoc commune appears bluish-green, olive green, or brown, while in dry conditions, it transforms into a inconspicuous, crisp brownish mat.