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ABSTRACT
There has been question whether the taxon known as Yucca filamentosa (Agavaceae) is to be distinguished from the taxon known as Yucca flaccida. Much of the uncertainty lay in the absence of sure knowledge of the forms intended by their original authors. A neotype of Y. flaccida from Florida was selected in 2006. Here, the type locality in Virginia of Y. filamentosa is revisited and an epitype is selected. The two taxa, as represented by plants from these two type localities, differ markedly in a number of characteristics. It is suggested that Y. filamentosa and Y. flaccida be retained at specific rank.
INTRODUCTION In the eastern United States, the genus Yucca (Agavaceae) is an infrequent but easily recognized component of the flora. Few in number, the species byand-large are free of taxonomic and nomenclatural complexities. Yucca aloifolia L. with its erect stems and stiff, spiny-edged leaves occurs in local abundance on the coastal dunes and at widely scattered sandy locations inland; it well merits the common names Spanish-dagger or Spanish-bayonet. Yucca gloriosa L., the Mound-lily Yucca, is now very rare on its native coastal dunes; its smoothedged leaves and short, often-branched stems form attractive clumps when in cultivation. A third species - or small group of species - stands apart by having stemless basal rosettes and leaves with partially detached marginal fibers. These plants carry the names Yucca filamentosa L. or Yucca flaccida Haw., and are most often just called Yucca.
The nomenclatural history of the eastern species of Yucca has been adequately detailed elsewhere (Ward 2006). But no clear taxonomic understanding has been reached regarding the "Filiferae," those plants with marginal fibers on the leaves. The group has been stated to consist of only a single variable species, Y. filamentosa (Chapman 1897; Ahles 1964, 1968; Wunderlin 1998; Wunderlin and Hansen 2003), or two species usually called Y. filamentosa and Y. flaccida (Trelease 1902, Small 1933, Fernald 1944, Dress 1976, Duncan and Kartesz 1981, Clewell 1985, Godfrey 1988, Hess and Robbins 2002, Ward 2004). No field study has yet been conducted that gives strong evidence for the presence in eastern North America of one species, or two.
THE PROBLEM A central impediment to understanding the variability of the filiferous yuccas has been uncertainty...