False-colour transmission electron micrograph of the nitrogen-fixing bacteria (red), Rhizobium leguminosarum, in the cell of a root nodule of the pea plant, Pisum sativum. The large structure (yellow/green) is the cell nucleus. It is surrounded by Rhizobium bacteroids, enlarged forms of the bacteria that occur once they have entered the root of the plant. The bacteroids release an enzyme, nitrogenase, which converts atmospheric nitrogen into a usable organic form - something the plant cannot do for itself. Around each bacteroid is a transparent envelope (blue) provided by the plant tissue. Magnification: x2400 at 6x4.5cm size.