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Date: October 1, 2015. Location: Ciha fen (map)

Comments: Orchids are especially well adapted to interact with certain species of fungi. Like all plants orchids tissues that are dead or dying are consumed by many strains of fungi called saprophytes. The better term is saprotrophs (organisms nourished by dead or decaying matter). The suffix -phyte (plant) is no longer considered appropriate for fungi.

Orchids, like most plants, have root associations with mycorrhizal fungi (myco=fungus, rhizal= root). For most plants the association with fungal mycelia provides an increased surface area for the absorption of water and nutrients performing and enhancing the role of the plants own root hairs. The fungus in return, receives carbon based molecules which the photosynthetic plants provides. Since the numerous threads of a fungal mycelium may be intimately connected to the roots of may different plants, forming a mycorrhizal network, it is sometimes conjectured that the mycelium may provide a channel for chemical communications between interconnected plants.

Orchids have evolved a means of entering these mycelial networks and acquiring photosynthetic products transported through them. The orchid may also receive the nutritive products the fungus acquires from acting as a saprotroph. The fungus receives no apparent benefit from its association with the orchid so the orchid is sometimes called a “cheater”. The orchid benefits by having a reduced investment in the mechanisms of photosynthesis and the complexities of root structure. Through some parts of their life cycle orchids are achlorophyllous (lack chlorophyll) and consequently non-photosynthetic. Orchids are called myco-heterotrophs, which means they derive nutrients from fungi (myco) and other (hetero) sources. The “hetero" usually refers to photosynthesis at certain stages of the orchids development. A reduced dependance on making photosynthetic products also allows some orchids to grow in difficult environments such as those under very low light conditions.

Orchids enlist the aid of fungi for seed development. Typically orchids will produce thousands of tiny seeds (called dust seeds - see photos above) in a single capsule. The seeds can be small and numerous because the orchid doesn’t have to spend its resources on storage nutrients to support the seeds development. Instead, it has found a way to use fungal resources for that purpose (see Germination of Seed). While the odds are against an individual seed finding the right conditions for germination and development to maturity, enough plants are produced to make orchids one of the largest groups of plants on the planet. Most prefer moist conditions and are found in the tropics.

Sometimes evolution finds more than one path to efficiency. Orchids are monocots that have evolved a symbiotic relationship with fungi. Among the dicots a similar efficiency has evolved in the achlorophyllous plant Monotropa uniflora. It too, augments its root functions with fungi and it produces smaller seeds that require a fungal assist for germination. The plant is completely white and can grow in dark areas where it is sometimes called the ghost plant.

None of the orchids are directly parasitic on other plants (at least not to my knowledge). It could be said that many are parasitic on a fungus which is in turn parasitic on another plant as its host. There are other plants however, that are directly parasitic on a host plant. The Cuscutta glomerata (rope dodder) plant for example, entwines its host’s stem and inserts its haustora into the vascular tissue of the host from which Cuscutta derives its nutrients.