Certain soil fungi form associations with roots of vascular plants. The association appears to benefit both members because studies with radioactive tracers have shown that the fungus receives carbohydrates from the green plant while the roots obtain mineral nutrients that the fungus has absorbed from the soil. These mycorrhizal associations are widespread throughout the vascular plant kingdom and so far have been found in over 80% of the flowering plants examined.

(A) Ectomycorrhizae and Endomycorrhizae

The two common types of mycorrhizae are ectomycorrhizae and endomycorrhizae. Ectomycorrhizae are characteristic of certain temperate trees such as oaks, willows, and conifers. They form a sheath surrounding the root tips with hyphae penetrating between the cells of the root cortex. Root tips with ectomycorrhizae take on a characteristic appearance, shorter and stubbier than noninfected roots and often forming "Y" branches. Ectomycorrhizal fungi are usually basidiomycetes, or occasionally ascomycetes. Endomycorrhizae are commonly found on herbaceous plants, many tropical and some temperate trees. The mycelium penetrates the cells of the plant root cortex in this association and forms large vesicles or branching systems within the cells. Endomycorrhizal fungi are zygomycetes.

Certain flowering plants (the Sierran snow plant and the eastern indian pipe), were formerly considered to be saprophytes (they lack chlorophyll). In one study, however, radioactive tracers added to adjacent trees turned up in the "saprophytes" at such a rapid rate that only a mycorrhizal connection directly between the snow plant and the pine could explain it. Henceforth it becomes necessary to reconsider whether flowering plants that were formerly called saprophytes are in fact "mycotrophs".

Examine the demonstration of ectomycorrhizae. Note the mantle. Carefully examine how thick the mantle is. Are there any differences between roots with or without mycorrhizae? What processes are fungi commonly involved in which would provide something of benefit to a host vascular plant? Are there environmental circumstances you can think of where this association would be critical to the success of a vascular plant?