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"Mycorrhizal (symbiotic) fungi growing in assoc- iation with the roots of a small-leaved lime tree, Tilia cordata. Symbiotic fungi are found growing on the roots of most trees; many will not grow properly unless the appropriate fungus is present. Mycorrhizal roots take up nutrients more efficiently than uninfected roots, while the fungi obtain carbohydrates and possibly B-group vitamins from the tree. The type of symbiosis seen here is known as endotrophic mycorrhiza; the fungal hyphae (white, fluffy structures) form a mantle around the tree roots and also penetrate into the soil. In endotrophic mycorrhiza the fungus lives mainly within the cells of the root cortex." (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JEREMY BURGESS)