Flora of North America species comparison

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.)

The columns of Platanthera sparsiflora, P. brevifolia, and P. zothecina differ from those of superficially similar species in their comparatively large, diverging, and forward-directed rostellum lobes; they are typical of columns of Platanthera species that deposit pollinaria on insects’ compound eyes. The resulting broad column fills a large portion (generally about 2/3) of the width of the hood formed by the dorsal sepal and petals, and in live and well-prepared herbarium specimens this feature separates these species from their relatives with proportionately smaller, narrower columns. Selection for more generalized pollination mechanisms, as is known elsewhere in the genus, however, has apparently produced derivatives of P. sparsiflora with reduced columns that simulate members of the P. hyperborea complex. The use of column structure, as has been traditionally employed to delimit P. sparsiflora, therefore actually recognizes a pollination syndrome and may not be adequate to represent the evolutionary and systematic status of this group.

Platanthera sparsiflora is highly variable in both vegetative habit and flower structure. Plants may be rather short, stout, with numerous broad leaves scattered along the stem and a rather dense inflorescence of comparatively large flowers. Others are tall and very slender, with few small, ascending leaves widely spaced on the proximal portions of the stem and very lax inflorescences of small flowers (Limnorchis laxiflora Rydberg). Others, with long, usually dense inflorescences of small flowers distal to basal clusters of leaves, have recently been treated as Limnorchis ensifolia by W. A. Weber (1990). These last, also discussed at length by D. S. Correll (1950) under Habenaria sparsiflora var. laxiflora, are characterized by smaller columns in which the reduced rostellum lobes appear to be folded forward, replacing the typical hemispheric chamber with a narrower fissure. Flower size in some plants furthermore may vary greatly in different years. Spurs vary from uniformly tapering to clavate, extending stiffly along the ovary to curving forward, and in length range from slightly shorter than the lip to about 1.6 times its length. In both vegetative and floral characters various degrees of intermediacy and combinations of character extremes occur. The complex including the three species recognized here is currently under critical study.

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