The mutually beneficial association (see mutualism) formed between fungi and the roots of plants. This is a very common form of mutualism, estimated to occur in over 90% of terrestrial plants. The absorption of mineral ions, especially phosphate, by the plant is greatly enhanced—perhaps up to 1000-fold—by the presence of the network of fungal hyphae penetrating the finest soil pores. In return the fungus receives energy from the plant, in the form of products of photosynthesis. Ectotrophic mycorrhizas (ectomycorrhizas) form a network of hyphae around the root and grow into the extracellular spaces of the root. They occur in association with about 10% of plant families, mostly woody species such as pine, oak, and birch. The hyphae of endotrophic mycorrhizas (endomycorrhizas or arbuscular mycorrhizas) enter the cortical cells of the host roots, where they form branching structures called arbuscules, which invaginate to fill individual host cells, while remaining enclosed by the host cell’s plasma membrane. These are found in over 85% of plant species, including many crops. The vast absorptive surface area of mycorrhizal fungi is vital for optimal growth of most trees and crop plants, and in commercial production steps are taken to ensure the presence of suitable fungal spores in the soil. Indeed, mycorrhizas are likely to have enabled the colonization of the terrestrial environment by the earliest land plants. There is also evidence that plants transmit chemical defence signals to their neighbours via the network of hyphae. See also glomeromycota.