False-colour scanning electron micrograph of a root nodule on the pea plant, Pisum sativum, caused by the nitrogen-fixing bacteria, Rhizobium leguminosarum. The bacteria convert atmospheric nitrogen into a usable organic form, something the pea cannot do itself, but which is imperative for its survival. Bacteria infect the plant through root hairs, forming an infection thread, which conveys them from the entry point to the nodule site. Here they divide repeatedly, swell and are known as bacteroids. The nodule consists of a central region filled with bacteroids surrounded by a loose, spongy region, the cortex, seen here. Magnification: x9 at 6x4.5cm size. Reference: MICROCOSMOS, figure 5.20, page 99.