It is normal for the roots of most plant species, including Angiosperms, Gymnosperms, Pteridophytes, and some Thallophytes, to form mutualistic associations with different groups of soil fungi. Charged to investigate the culture of truffles in the Kingdom of Prussia, Frank (1885) instead discovered and described such mutualistic associations in temperate forest trees and named them mycorrhiza (fungus root). He believed that the fungi performed the function of root hairs, which were lacking in these much modified dual structures. The term mycorrhiza has since been adopted for many other mutualistic associations between soil fungi and below-ground plant organs, some of which, notably the protocorms of orchids, are not even roots. The only common feature of these diverse, sometimes highly specialized, mycorrhizal associations is the low degree of pathogenicity exhibited by the fungus towards its host. In the vesicular-arbuscular mycorrhiza-the most widespread and probably the least specialized of all mycorrhizal association-this has led to complete dependence of the fungus on a living host and inability of the fungus to grow on synthetic media. Conversely, some hosts lacking chlorophyll depend largely on their associated fungi for reproduction and growth.