In 1980 Raven and coauthors reviewed the literature concerning fungal symbiosis in vascular plants. They reviewed two kinds of fungal-plant associations: ectomycorrhizal and endomycorrhizal. They reported that endomycorrhizal fungi, which penetrate plant cells, are found in 80% of all vascular plants, including ferns, gymnosperms, and angiosperms, and are found in forests of high species richness. On the other hand, ectomycorrhizal fungi, which do not penetrate plant cells, occur in forests of low species richness, are usually in temperate forests, or infertile soils of the tropics. Further, they hypothesized that ectomycorrhizal forests have expanded through the Middle Cretaceous at the expense of endomycorrhizal forests.