Flora of North America species comparison

The greatly reduced herbage of Platanthera brevifolia is distinctive. Robust plants may lack developed leaves altogether, with merely a few reduced bracts at the base of the stem and an inflorescence comprising nearly the entire height of the plant. A strong mycotrophic mode is suggested by the occurrence in some colonies of truly albino plants without pigmentation.

Platanthera brevifolia largely replaces P. sparsiflora in Mexico and is very distinctive at its northern limit in the United States. Although P. sparsiflora is exceedingly variable in floral dimensions, in habit, and in the number, size, and placement of leaves, the pattern seen in P. brevifolia seems to lie outside this range of variation. With a generally longer spur and very different ecologic requirements, it is here treated at the specific level. Nonetheless, P. brevifolia is reported to intergrade with P. sparsiflora in Mexico (R. McVaugh and W. R. Anderson. 1974+, vol. 16, pp. 254–256), yet the latter species probably does not range there beyond a possible occurrence in Baja California (M. A. Soto 1988). 

(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.)