Surface is smooth although there may be some connection to outside soil. Inner portion of sheath is connected to hyphae which ramify between cells of epidermis and outer cortex forming mycelial network called Hartig net . Fungus sually never penetrates endodermis and there is only occasional penetration of cortex cells. Anatomy of root profoundly changed by mycorrhizal infection. Auxins produced by fungus are responsible for some of these changes.
Infection is usually very obvious, visually.
Some ascomycetes and zygomycetes. Many can be grown in pure culture. Russula, Lactarius c. Seven families of ascomycetes, and a few zygomycetes. Methods and Principles of Mycorrhizal Research .