Root nodule. Coloured scanning electron micrograph (SEM) of a root nodule on a white clover plant (Trifolium repens) caused by the nitrogen-fixing soil bacteria Rhizobium trifolii. The plant and the bacteria have a symbiotic relationship. The bacteria converts (fixes) atmospheric nitrogen in the soil to ammonia. The plant cannot carry out this process itself, but it is vital for the production of amino acids, the building blocks of proteins. In return the plant passes carbohydrates produced during photosynthesis to the bacteria for use as an energy source. The bacteria enters the plant through its root hairs, where an infection thread leads it to the nodule. Magnification: x140 when printed 10 cm tall.