Agar or agar agar is a gelatinous substance chiefly use as a solid substrate to contain culture medium for microbiological work. It is an unbranched polysaccharide obtained from the cell membranes of some species of red algae, primarily from the genuses Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema). Commercially it is derived primarily from Gelidium amansii.