Certain symbiotic nitrogen fixing bacteria invade roots of leguminous plants and initiate nodule formation. Nodule formation is stimulated by auxin produced by cortical cells and cytokinin liberated by invading bacteria. The infected cells enlarge. Bacteria stop dividing and form irregular polyhedral structures called bacteroids. In an infected cells, bacteroids occur in groups surrounded by host membrane. The host cell develops a pinkish pigment called leghaemoglobin (Lb). Leghaemoglobin has ten times more affinity for oxygen as compared to blood pigment haemoglobin. Leghaemoglobin acts as oxygen scavenger. It protects nitrogen fixing enzyme nitrogenase from oxygen.