Light micrograph of a root nodule of the broad bean, Vicia faba, a leguminous plant. The nodule develops in response to invasion by the bacteria Rhizobium sp. The bacteria convert atmospheric nitrogen into an organic form essential to the plant's survival. Bacteria enter the plant through root hairs and pass via an infection thread to the nodule site. Once there, they lose their cell walls and divide repeatedly, swelling as they do so. They are now known as bacteroids. The nodule consists of a central region (purple) containing bacteroids surrounded by a loose, spongy region called the cortex. Magnification: x40 at 35mm size.