Litmus is obtained from lichen spp. like Roccellatinctoria, Roccellamontagnei and Lasalliapustulata. Inferior kinds of litmus is obtained from species of Parmelia, Variolania and Lecanora etc. Litmus is used as acid-base indicator. Acid impart a red colour to blue litmus and alkalis cause reddened litmus to return its original blue colour.