For nearly eighteen years the problem of the fixation of nitrogen by the root nodules of leguminous plants has been the subject of investigation by numbers of workers in many parts of the world, and steady progress has been made towards realising the conditions under which the organism works in the nodule. Artificial nutrient media have been prepared closely resembling the food supplied by the plant, and the organism itself has in recent years been induced to grow in artificial culture in the characteristic “bacteroid” forms approximating to those which are found in nature.