Light micrograph showing a root nodule (egg-shaped) amongst the roots of a clover (Trifolium sp.) plant formed by Rhizobium sp. nitrogen-fixing soil bacteria. 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. Magnification: x20 when printed at 15cm wide.