Botanical  history and taxonomic revision of the genus Washingtonia

Washingtonia (Arecaceae) is an American genus of palms composed of two species, W. filifera and W. robusta. The first one occurs naturally in Arizona, southern California, and north Baja California, while W. robusta is present in the Peninsula of Baja California, from latitude 30" to the Cape Region at 23", and in Sonora, mainland Mexico, where it has a very narrow distribution. Both palms have been an important element for the survival of native people even before the arrival of Jesuit missionaries to Baja California in the seventeenth century and continue to be today. Both species have been widely cultivated in California since 1874, and W. robusta is currently one of the most widely cultivated palms of the world. During the early years of cultivation, seeds of both W. filifera and W. robusta were being grown without knowledge neither of their taxonomic identity nor their geographic origin, due in part, to great morphological variation in both species. Poor understanding of its morphology led either to the description of numerous new species  (now mostly reduced to synonyms) or to an oversimplification of the genus resulting in the traditional 2-species circumscription. Accurate knowledge on the distribution of the genus is missing because of lack of fieldwork in its natural range, which is reflected in fragmentary herbaria collections. Washingtonia, in a way, has been the elephant of a safari: extensively photographed, but rarely collected. Variable taxonomic circumscription and imprecise species distribution has done little to clarify the identity of the palms brought into cultivation in the nineteenth century. This research is the first comprehensive review of the earliest horticultural records, letters, and nursery catalogs concerning Washingtonia in an attempt to clarify the taxonomic identity and distribution range of both species, since none of them have useful type specimens. The aims of my research are 1) to evidence the poor reliability of the most recent taxonomic treatment of the genus Washingtonia (Bailey, 1936), 2) challenge the long held idea that the genus is composed of two species, and 3) identify the origin of the seeds brought into cultivation using historical records.