A protein found in the centre of root nodules of leguminous plants infected with the nitrogen-fixing bacterium Rhizobium. It has been found that the haemoglobin is coded for by a legume gene but that synthesis only occurs in the presence of the bacterium. The haemoglobin is believed to transport oxygen to the bacterium (which respires aerobically) in such a way that the activity of the nitrogen-fixing enzyme, *nitrogenase
(which is destroyed on exposure to oxygen), is not affected. Haemoglobin is not found elsewhere in the plant kingdom.