Answer and Explanation:
During a primary succession, lichens grows on a bare rock at first. Lichen is a symbiotic relationship between fungi and algae. The fungi grows on the weaker parts of the rock (e.g. cracks) and derive nutrients. The availability of nutrients would allow algal species to perform photosynthesis and produce carbohydrates upon which fungi can survive. Fungi also help algae to anchor the rock material. The process usually occurs very slowly but, once the right conditions are established, mosses and grasses start appearing on the surface. To understand further this scheme, I have attached an image as well.