Caption

Macrophotograph of root nodules on the roots of white clover, Trifolium repens, caused by the nitrogen- fixing bacteria Rhizobium trifolii. The bacteria convert atmospheric nitrogen into a usable organic form, something the clover 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 renamed bacteroids. The nodule consists of a central region filled with bacteroids surrounded by a spongy region, the cortex. Magnification: x0.6 at 35mm size.