Spiranthes incurva (Jenn.) M.C. Pace - Sphinx Ladies' Tresses


 

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Spiranthes incurva - (image 1 of 7)

 

Taxonomy

Family: Orchidaceae

Habitat

Fens, bogs, wet to xeric roadsides and prairies, alvar, lake dunes.

Associates

 

 Distribution

Middle, northern, and eastern Interior Lowlands, Western and Northern Appalachian Mountains and Adirondacks, Great Lakes Basin, and Prairie Peninsula from New Brunswick, southern Ontario, and southern Quebec, west to MN, central NE, and eastern KS.

Morphology

Perennial to 40 cm. Leaves occasionally remaining until anthesis and withering shortly after; flowers 5 mm or longer, slightly tubularly campanulate, slightly ascending to moderately nodding, white to pale ivory; lateral sepals flattened, linear-lanceolate, acute, straight to slightly upwardly falcate, the tips often meeting the dorsal sepal and petals; labellum abaxially white or very pale yellow, centrally smooth and yellowish (sometimes faintly), margin crisped and lacerate, abaxial glands conical and reduced.

Notes

Flowers late August to September

Wetland indicator: NA

This species is part of the Spiranthes cernua complex. It has been described as an ancient hybrid of S. cernua and S. magnicamporum. This taxon is most similar to S. cernua which mostly replaces it along the coast and to the south. It differs from coastal and southern S. cernua in having a noticeably thickened labellum that is often centrally yellowish and has smaller callosities, though these characteristics seem to intergrade with S. cernua as one moves south and east. The above photos were taken in southeast WI and were originally used on the page for S. cernua. The treatment of this as a distinct species is something that probably needs further scrutiny.

The type location that is the basis of the name Ibidium incurvum Jennings is an island in Lake Erie in Erie County, PA. The specific epithet "incurvum" of Jennings refers to the long, incurved callosities of plants from this site. Typical S. cernua s.s. also has long callosities and the degree to which they curve inward seems to vary considerably and this was later recognized by Jennings who indicated that I. incurvum could not be recognized. As described by Pace and Cameron, S. incurva has callosities that are very small, conical to rounded, and 0.3-0.8 mm tall.

Genetics show that plants from the western Great Lakes region across the Great Lakes regions of OH, PA, and Ontario (also northern VT) are the result of hybridization of S. magnicamporum with S. cernua and recovered as monophyletic using Bayesian tree reconstruction based on combined five-locus chloroplast data. Plants from MI, NY, NE, IA also appear to be of hybrid origin, however these were not included in the combined phylogenetic reconstruction (they are included in other tree constructions in Pace's dissertation but do not seem to form a monophyletic group).

The existence of intermediate forms and their likely hybrid nature was first recognized by Sheviak who called these plants the "low prairie" race and later, after recognizing how widespread these plants were, referred to them as the "prairie complex". Sheviak did not go so far as to treat these as a distinct species given that they represent a "diverse assemblage of variably intermediate forms". He hypothesized that they were the result of unidirectional gene flow from S. magnicamporum to S. cernua and formed, in part, the basis of treating S. cernua as a compilospecies. While the influence of S. magnicamporum is evident in many populations in central NY and northeastern PA, it appears to be strongest closer to the range of S. magnicamporum. Therefore S. incurva might be more appropriately treated as a variety or subspecies of S. cernua.

Update March 2020.

When this page was originally created, field observations seemed to indicate that plants in central NY down through northeastern PA exhibited some degree of introgression with S. cernua s.s. While this is still apparent to some extent, preliminary genetics seems to show that several plants originally determined to be intermediates of S. incurva and S. cernua are actually S. arcisepala that have lateral sepals that are straight or sweeping upward rather than falcate. Therefore the long and rather pointed lip appears to be one of the more important diagnostic characteristics.

References

Pace, M.C. and Cameron, K.M. 2017. The systematics of the Spiranthes cernua species complex (Orchidaceae): Untangling the Gordian Knot. Systematic Botany, 42(4):1-30.

 

Sheviak, C.J. 1982. Biosystematic study of the Spiranthes cernua complex. New York State Museum Bulletin No. 448.

 


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 Michael Hough 2017