Plants have a relationship with fungi in two ways: In endomycorrhiza, the hyphae of the fungus penetrate the outer cells of the plant root and extend into the surrounding soil; in ectomycorrhiza, the hyphae surround but do not penetrate the roots. Endomycorrhiza are much more common than ectomycorrhiza. The most common plants associated with ectomycorrhiza are trees and shrubs growing in temperate regions, including pines, firs, oaks, beeches, and willows. These plants tend to be more resistant to extreme temperatures, drought, and other harsh environmental conditions. In addition, some ectomycorrhizal fungi may provide protection from acid precipitation (for more about acid pollution, see the chapter “Environment and Ecology”).