DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
Nitrogen-fixing nodules on roots of the garden pea Pisum sativum. Nodules of this type are produced when roots are infected by the soil bacterium Rhizobium leguminosarum. The relationship is one of symbiosis; the bacterium benefits from an amenable environment within the plant, and the plant benefits from the bacterial ability to fix atmospheric nitrogen within the nodules.The picture shows the roots of a pea plant with scattered small nodules. The pink tinge on some of the nodules is due to the presence of leghaemoglobin, a protein involved in the process of nitrogen fixation.
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