Scanning electron micrograph of a root nodule on the white clover plant, Trifolium repens, caused by the nitrogen-fixing bacteria Rhizobium trifolii. The bacteria convert atmospheric nitrogen into a usable organic form, which the clover cannot do itself, but which is imperative for its survival. Bacteria infect the plant through root hairs, forming a nodule site. Here they divide repeatedly, swell and become known as bacteroids. The nodule consists of a central region filled with bacteroids surrounded by a spongy region, the cortex, seen here. Magnification: x175 at 8x10 inch size.