Mycorrhizae are the example of:

Mycorrhizae are symbiotic relationships that form between fungi and plants. The fungi colonize the root system of a host plant, providing increased water and nutrient absorption capabilities while the plant provides the fungus with carbohydrates formed from photosynthesis. Mycorrhizal fungi encompass many major groups of the fungus Kingdom and in the past were divided into two non-evolutionarily related groups: the ectotrophic and endotrophic based upon the position of the fungal hyphae in relation to the tissues of a plants roots. Ectomycorrhizal fungi ensheath the root cells but usually do not penetrate them (extracellular). Endomycorrhizal fungi penetrate and enter the cells of a plant root