A mycorrhiza is a symbiotic association of a fungus with a higher plant. Mycorrhizas (from the Greek mykes for "fungus" and rhiza for "root") occur commonly and are of two main types. These types are (1) endotrophic, in which the fungus is within the cortex cells of a root (e.g. orchid), and (2) endotrophic, in which the fungus is external, forming a sheath that completely invests the smaller roots (e.g. pine tree). Mycorrhizas are believed to constitute an example of a mutually beneficial symbiotic association. It has been shown (a) that mycorrhizal plants benefit from the association, e.g. under natural conditions the presence of the fungus partner is vital for the establishment and growth of seedling trees of a number of different species, e.g. pines, and (b) that associations of the fungus with the tree is necessary for development and reproduction by the fungus.