The fixation of atmospheric nitrogen either by lightning (see nitrogen cycle
) or by free-living or symbiotic microorganisms. The association between many species of the Leguminosae and the nitrogen-fixing soil bacterium Rhizobium, which lives in nodules on the legume roots, has a significant effect on soil fertility. Hence much use has been made of leguminous crops in agriculture. There are different strains of Rhizobium, each specific to one or a group of closely related species. The bacteria are attracted to the legume by a growth substance and invade the root hairs. They then divide forming filaments that eventually reach and infect the plant cortex. The cortex is stimulated to enlarge and form a nodule the centre of which is red due to the presence of haemoglobin (see leghaemoglobin
). The bacterium fixes nitrogen by means of the enzyme *nitrogenase
. About 250 species of plants other than legumes also form symbiotic associations with nitrogen-fixing microorganisms. For example, bog myrtle (Myrica gale) and alder (Alnus glutinosa) have nitrogen-fixing root nodules that appear to contain fungi of the Plasmodiophorales. Gunnera and the water fern Azolla have blue-green algae in their roots. Free-living nitrogen-fixing microorganisms include the bacteria Azotobacter, Klebsiella, and Clostridium and all the blue-green algae and photosynthetic bacteria.