Macrophotograph of root nodules on the roots of the broad bean, Vicia faba, caused by the nitrogen-fixing bacterium Rhizobium leguminosarum. The bacteria convert atmospheric nitrogen into a usable organic form, something the broad bean cannot do for itself, but which is nutritionally necessary for its survival. The bacteria infect the plant through its root hairs, forming an infection thread, which conveys them from the entry point to the nodule site.
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