<P> A mycorrhiza is the symbiotic association between a green plant and a fungus . The plant captures the energy coming from the sun by means of its chlorophyll and supplies it to the fungus, and the fungus supplies water and mineral nutrients taken from the soil to the plant . Mycorrhizas are located in the roots of the plant . Most plant species form mycorrhizal associations, though some families like Brassicaceae and Chenopodiaceae cannot . There is fossil evidence that early land plants formed arbuscular mycorrhizal associations . Different forms for the association are detailed in the next section . The most common is the arbuscular type that is present in 70% of plant species, including many crop plants such as wheat and rice . </P> <P> Mycorrhizas are commonly divided into ectomycorrhizas and endomycorrhizas . The two types are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane . Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza, while arbutoid mycorrhizas can be classified as ectoendomycorrhizas . Monotropoid mycorrhizas form a special category . </P> <P> Endomycorrhizas are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizas . Arbuscular mycorrhizas, or AM (formerly known as vesicular - arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae enter into the plant cells, producing structures that are either balloon - like (vesicles) or dichotomously branching invaginations (arbuscules). The fungal hyphae do not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginate the cell membrane . The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell cytoplasm to facilitate the transfer of nutrients between them . </P> <P> Arbuscular mycorrhizas are formed only by fungi in the division Glomeromycota . Fossil evidence and DNA sequence analysis suggest that this mutualism appeared 400 - 460 million years ago, when the first plants were colonizing land . Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many crop species . The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of carbon in the soil . Arbuscular mycorrhizal fungi have (possibly) been asexual for many millions of years and, unusually, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis). </P>

Mycorrhizal fungi have a special relationship with most flowering plants which includes