Orchid mycorrhiza allowed for the identification of specific taxa forming symbiotic relationships which are of interest in the study, cultivation, and conservation of orchids. This is especially important in the trade and preservation endangered species or orchids of commercial value like the vanilla bean. There have been seen trends in the type of symbioses found in orchids, depending primarily on the life-style of the orchid, as the symbiosis is primarily of benefit to the plant. Terrestrial orchids have been found to commonly associate with Tulasnellaceae, however some autotrophic and non-autotrophic orchids do associate with several ectomycorrhizal fungi. Epiphytic fungi, however, may associate
Algebraic variety algebraic sets. Using the Nullstellensatz and related results, mathematicians have established a strong correspondence between questions on algebraic sets and questions of ring theory. This correspondence is a defining feature of algebraic geometry. An "affine variety" over an algebraically closed field is conceptually the easiest type of variety to define, which will be done in this section. Next, one can define projective and quasi-projective varieties in a similar way. The most general definition of a variety is obtained by patching together smaller quasi-projective varieties. It is not obvious that one can construct genuinely new examples of varieties in this way,