The lichen is a plant that is made up of two different plant forms - a fungus and an alga.
The alga has millions of cells that are sensitive to light. These cells produce food for the fungus, which is carbohydrates, through photosynthesis. The fungus in its turn absorbs water vapour from the air for the alga and also protects it from strong sunlight.
So close is their relationship that they even reproduce together. The lichen produces tiny blisters that contain the tissues of both the alga and the fungus. The blisters gradually break away and the tissues develop into a new lichen.
The alga and the fungus thrive as long as they are together and can live for thousands of years.