Flora of North America species comparison

The key will permit determination of most specimens of Platanthera orbiculata and P. macrophylla. More precise discrimination of ambiguous specimens is possible by use of the formula from A. H. Reddoch and J. M. Reddoch (1993): spur length + (2 × pollinarium length), which for P. orbiculata is less than 38 and for P. macrophylla is equal to or greater than 38.

Considerable variation in size and shape of leaves occurs, and although to some extent regional in nature, intergradation is complete; recognition of infraspecific taxa is unwarranted. A few collections from isolated areas on the Pacific Coast of Canada are noteworthy, however. Those are small, few-flowered plants with rather narrow leaves borne alternately or suboppositely toward the base of the stem, as in some Asiatic species. They are in some respects very similar to Platanthera freynii Kränzlin, an Asiatic species distinguished primarily by its abruptly narrowed petals, in contrast to the generally broader, but variable, petals in North American plants. These western plants warrant further study to establish their identity and to elucidate relationships between North American and Asiatic species.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

The rostellum lobes in Platanthera cristata normally are only slightly curved, presenting the viscidia in a generally forward position. Considerable variation is apparent, however, and sometimes a marked curvature similar to that in P. chapmanii is seen, apparently presenting the viscidia downward. These orientations are difficult to assess in distorted herbarium material, and the species is in need of further study. The triangular mouth of the spur of P. cristata is distinctive, however, and can aid in determination of confusing material (see also below and the discussion under 29. P. chapmanii).

Platanthera pallida P. M. Brown was recently proposed to accommodate two populations of pale-flowered plants on Long Island, New York. Their distinctive nature has long been apparent. In the field, pollination normally is rapid and the flowers senesce while small and yellowish, but in cultivation in the absence of pollinators, flowers remain open for an extended period during which the lips elongate markedly and the flowers fade to a dull white. Hence they suggest P. ×canbyi (Ames) Luer [P. blephariglottis × P. cristata], but they differ in their much shorter spurs.

Plants of Platanthera pallida were reported to differ from P. cristata in shorter spurs, entire dorsal sepals, and lateral sepal and lip orientation. The range of spur lengths in P. cristata, however, is much greater than specified and in fact includes the range of P. pallida. Dorsal sepals with entire margins are commonly seen also in P. cristata, and those of P. pallida may be deeply emarginate. Lateral sepal and lip orientation tend to differ in the two taxa, but the conditions typical of each taxon can be found in the other. The keyhole shaped orifice of the spur in P. pallida mentioned by Brown is a unique feature, but it is present in only a small percentage of individuals and completely intergrades to the triangular orifice of P. cristata. Furthermore, superficially similar pale-flowered plants from farther west on Long Island are more similar to typical P. cristata and hence possibly indicative of a cline. The plants described as P. pallida therefore appear to represent neither a distinct species nor are they merely hybrids; they seem to be partially stabilized introgression products and potentially useful subjects for evolutionary study.

Hybrids of Platanthera cristata with P. blephariglottis are P. ×canbyi (Ames) Luer and with P. ciliaris are P. ×channellii Folsom; until recently the latter was confused with P. chapmanii.

(Discussion copyrighted by Flora of North America; reprinted with permission.)