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Specimens labelled as Quercus undulata appear in several oak collections. The name has been rather loosely applied to a complex of white oak hybrids from the southwestern United States, but should refer only to Q. gambelii × Q. grisea. All members of the complex have Q. gambelii Nutt. as one parent, but six other species (Q. arizonica, Q. grisea, Q. havardii, Q. mohriana, Q. muehlenbergii and Q. turbinella) are also involved. The complex includes not only the F1 hybrids, but also a series of other intermediates formed from backcrossing with one of the parent species (Tucker 1961, Tucker et al. 1961, Tucker 1963, 1970, 1971). The F1 hybrids typically have foliage intermediate between Q. gambelii and the other parent species (exemplar leaves are illustrated by Tucker 1961). Backcrossing features in the parentage of many cultivated specimens, however, and these may therefore not demonstrate typical F1 morphology. It is also possible that cultivated specimens of any of the seven parent species may actually have a hybrid origin. A number of selections have been made from the complex, and some are being propagated, especially by Pavia Nursery, Belgium.