Mycorrhiza is not uncommon among many plants, but only orchids have specialised themselves to a point where they need it, at least in an immature stage. Later, they may be able to live without the fungus or keep it for their entire life, depending on the species.
Unlike the ectotrophic mycorrhiza of trees or other plants where the fungus hyphens are only covering the surface of the root cells, the endotrophic orchid mycorrhiza allows the hyphens to enter the cells, where parts of them are digested by the orchid. The picture to the left shows the infection of Listera cordata with a fungus, the right an active mycorrhiza of Neottia nidus-avis. If a fungus gets out of control, it could kill the orchid, so a good balance is vital.
