The term 'SOURCE OF LITMUS' can be solved by the 6-letter word 'LICHEN'. Lichens are a type of composite organism that is composed of a fungus and an alga living together in a symbiotic relationship. They are found in many different habitats, including on rocks, trees, and soil. Lichens are often used as indicators of air quality, as they are sensitive to pollutants in the environment. The litmus test is a chemical test that uses lichens to determine the acidity or alkalinity of a solution.