Nitrogen fixing root nodules. Macrophotograph of red nitrogen-fixing nodules on the roots of a black alder tree ( IAlnus glutinosa i). The nodules contain symbiotic IFrankia sp. i bacteria, which take nitrogen from the air and convert it into forms the tree can use for nutrition. In return, the bacteria feed on sugars produced by the tree. This symbiosis (mutually-beneficial partnership) means that the alder can grow in less fertile soils than many other plants. Magnification: x2.5 at 35mm size.