Though not well-known to those outside botany, mycorrhizal fungi exist as a critical part of Earth's ecosystem. These microorganisms form a symbiotic association with many plants, allowing the plants access to more nutrients than they could obtain from the soil directly. In poor soil conditions, this can mean the difference between a plant surviving or dying. Additionally, the network they grow binds soil particles together, significantly slowing the effects of erosion.

Before getting into the topic, a brief clarification of terminology may aid the reader:
"Mycorrhizal fungi" or, plural, "Mycorrhizae", refers to a colony of individual fungal cells, similar to a mushroom (though not for reproductive purposes). It does not refer to a particular species, but rather, to the particular relationship these organisms have with a plant.
"Hyphae" refers to individual fungal cells, in the form of thread-like filaments that tangle together in a larger multicellular mass (ie, the mycorrhizae).

Conceptually, mycorrhizal fungi act like an extremely fine terminal branching of the plant's own root systems, producing a microscopic web between and around each individual soil particle.

The peripheral hyphae, called the soil network, absorb inorganic nutrients from the soil itself and from decaying organic matter in the soil, including larger objects such as dead insects. These nutrients travel through channels between individual hyphae to the mantle, a small but visible fungal coating around the roots of the plant. Finally, numerous hyphae interface with the root cells themselves, in structures called a Hartig net. Here, the mycorrhizae transfer inorganic nutrients to a plant, and in return, the plant provides the mycorrhizae with carbon in the form of simple sugars, benefitting both the plant and the fungus.

According to Michael Miller, a US Department of Energy senior soil scientist, "90 percent of the world's vascular plants belong to families that have symbiotic associations with mycorrhizal fungi".


Sources:
http://www.ucfv.bc.ca/biology/biol210/1999/Mutualism/Mycorr.htm
http://dmsylvia.ifas.ufl.edu/mycorrhiza.htm
http://www.planthealthcare.com/fungi.html
http://www.mastergardeners.org/newsletter/myco.html
http://www.consrv.ca.gov/OMR/smara/newsletter/winter2001.pdf

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