Plant Genera


Hexastylis

Family Code: ARISTO
Genus Code: HEXAS
Genus CN: heartleaf
Genus Authority: Raf.
Genus Summary: A genus of about 11 species, herbs, of se. North America, very possibly best expanded to include Asian taxa treated in Heterotropa and Asiasarum. Barringer (1993) and Kelly (1997, 1998) have recently employed a broad definition of Asarum, including Hexastylis. Over the last half-century various students of the group (emphasizing a range of fields of evidence) have arrayed themselves for and against the recognition of Hexastylis as a genus distinct from Asarum. A cladistic analysis (Kelly 1997, 1998) showed distinctive clades which could be interpreted as evidence for the recognition of Hexastylis (including the Asian Heterotropa), though the author preferred to recognize 2 subgenera. I choose here to follow the more traditional (at least in our area) separation of Hexastylis from Asarum, until and unless stronger evidence is presented for their combination. Electrophoretic and morphologic studies currently in progress validate the species / varietal level taxonomy presented, insofar as results are available (Z. Murrell, pers. comm., 2013; R. Wyatt, pers. comm.). A difficult genus, Hexastylis is made more frustrating by the fact that nearly all diagnostic features relate to the shape and size of the fleshy and brittle calyx characters which are difficult to describe and are largely lost when specimens are pressed. The difficulty of identifying herbarium specimens has sometimes been (apparently) used as a justification for reducing (often drastically, as in Cronquist) the number of taxa recognized. To those familiar with this genus in the field, however, the taxa here recognized form geographically distinctive populations. Size and (to a lesser degree) shape of individual flowers show considerable variation and can be altered by environmental factors; individual flowers or plants can be difficult to identify if taken out of context. Populations, however, are usually readily identifiable.
Genus Identification: Identification notes: The photograph (Figure 1) in Gaddy (1987a) of the flowers of all species other than H. arifolia and H. speciosa is highly recommended as an aid to identification. The calyx tube orifice is measured on the inside the diameter of the opening. The width of calyx lobes is measured from sinus tip to sinus tip.
Genus References: Whittemore & Gaddy in FNA (1997); Gaddy (1987a)=Z; Blomquist (1957)=Y; Barringer (1993)=X; Keener & Davenport (2015)=V; Gaddy (1987b); Gaddy (1986); Gaddy in Wofford (1989); Sugawara (1987); Huber in Kubitzki, Rohwer, & Bittrich (1993). Key adapted from FNA, Gaddy in Wofford (1989), and Gaddy (1987a).
Last Updated: 2019-11-29
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