Some types of orchid are non-photosynthetic; others only produce chlorophyll when they have grown past the seedling stage. In all cases, the plant depends on sugars derived from a fungal partner for at least part of its life. The minute orchid seeds, with negligible nutrient reserves, will not germinate unless a fungus infects them, although the seeds can germinate aseptically if supplied with the 'fungal sugar' trehalose.
These mycorrhizas are unusual because, in effect, the plant parasitises the fungus that invades it. The fungi in these associations resemble the common plant pathogen Rhizoctonia solani, but recent taxonomic studies have assigned them to several related genera. They are mainly saprotrophic - they grow by degrading organic matter in soil - but they might obtain trace elements or some other factor from the plant.