Most vascular plants have them. Only a few families don't. Probably the most important mutualistic symbiosis in plant kingdom. Mycorrhizal associations are mutualistic. Fungi cannot produce there own food.
Plants do, photosynthesis.
Plant- gets increased surface area for absorption of water and nutrients. How are the fungi associated with the plant root. Ectomycorrhizae-form a mantle around the outside of the root and a Hartig net around the epidermal and cortical cells as far in as the endodermis. Often these plants don't produce root hairs because the fungal hyphae act as root hairs.