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Unlike plants, fungi cannot make their own food. They absorb their
food from dead organic matter, or other living organisms. The source of
their food is called a substrate if it is dead, and a host if it is
living.
The body (mycelium) of a fungus is made of threads
called hyphae. Hyphae absorb nutrients from the substrate, spread, grow,
and produce fruiting bodies. The fruiting body of the fungus is the part
that we can see, and touch.
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Mycorrhizae result from the symbiosis (partnership) between a
fungus' hyphae and a living plant root. Symbiotic partnerships occur only
between living organisms. If a fungus obtains its carbohydrates from dead
organic matter it is a "decomposer" and not symbiotic.
Symbiotic partnerships are formed in three ways. A parasitic
relationship benefits the fungus but harms the host. In mutual symbiosis,
both fungus and host benefit from their partnership. A neutral symbiosis
benefits the fungus with no harm or benefit to the host.
Mycorrhizal symbioses are mutual and two types are recognized:
ectomycorrhizae and endomycorrhizae.
In ectomycorrhizal symbiosis, the hyphal threads coat the tips of tree
roots. Some threads spread from the root coating into the soil to capture
minerals. Others spread inward between the root cells. These hyphae make a
path from the soil into the tree for the minerals the tree needs, and
allow the fungus to feed in turn upon the carbohydrates produced by the
tree's photosynthesis.
The importance of the ectomycorrhizal
partnership is shown in the close physical relationship between the fungus
and its host tree. It is very difficult to separate hyphae from the host
cells. Ectomycorrhizal hosts include many trees: oaks, pines, firs,
hemlocks, beeches, birches, and more. Many large forest fungi form
ectomycorrhizal partnerships, both truffles with underground fruiting
bodies and fungi with the umbrella-shaped fruiting bodies known as
mushrooms.
Endomycorrhizal partnerships are the most common type of mycorrhizal symbiosis. Endomycorrhizal fungi do not coat the root. Only a few hyphae spread across the root's surface and those that pierce the root enter into the root cells. The hosts for this symbiosis are green plants that are not ectomycorrhizal. This type of fungus usually produces single spores in the soil for reproduction, not large fruiting bodies.
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Last
update: 7 Oct 96. © 1996, Robert Fogel,
Ivins, UT 84738. Edited by Patricia Rogers. Morchella and Amanita photographs
courtesy of R.L. Shaffer. Ectomycorrhiza photographs courtesy of J.M.
Trappe and U.S. Forest Service.