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Density can affect attraction of pollinators, with rare plants receiving fewer pollinating visits compared with more common co-flowering species. However, if a locally rare species is very attractive in terms of the rewards it offers pollinators, it may be preferentially visited. Spiranthes romanzoffiana is a nectar rewarding, geographically rare, endangered orchid species which forms small populations in Ireland, co-flowering with more common, florally rewarding species. We examined visitation rates to S. romanzoffiana and two nectar rewarding co-flowering species (Mentha aquatica and Prunella vulgaris) in the west of Ireland. These three plant species were visited by three bee species (Bombus pascuorum, B. hortorum and Apis mellifera). B. pascuorum was the most common visitor, while A. mellifera was least common. Our results suggest that individual S. romanzoffiana inflorescences compete intraspecifically for visitation from pollinators at high densities. The relationship between visitation to S. romanzoffiana and total floral density appeared to be positive, suggesting interspecific facilitation of pollinator visitation at high densities. Nectar standing crop varied through the season, among species and between open and bagged flowers. Nectar standing crop was not correlated with visitation in S. romanzoffiana. Despite relatively high visitation, S. romanzoffiana produced no mature fruit during this flowering season. The lack of fruit maturation in this species may be a major factor causing its rarity in Europe.

Breeding systems in northeastern North American Spiranthes orchids were studied to provide background information for taxonomic and evolutionary interpretations. Pollination experiments revealed that Spiranthes casei var. casei, S. cernua var. cernua, S. magnicamporum (southwestern Ontario), and S. ochroleuca (southern Nova Scotia) are agamospermic, while S. lacera var. lacera, S. lacera var. gracilis, S. laciniata, S. magnicamporum (Illinois), S. ochroleuca (Pennsylvania, New York), S. romanzoffiana, S. tuberosa, and S. vernalis are sexual. In the sexual taxa, cross-pollination is followed by earlier dehiscence of capsules, a higher percentage of ovaries with seeds, and a higher percentage of seed coats with well-developed embryos than either self-pollination or geitonogamous pollination. This may be explained by partial self-incompatibility involving restricted pollen-tube growth and (or) inbreeding depression. In Spiranthes romanzoffiana and races of S. ochroleuca requiring pollination for seed development, the two or three layers of cells comprising the inner integument degenerate after fertilization. Seed produced by species requiring pollination (i.e., produced sexually) is monoembryonic with the embryo located centrally. Examination of stained serial sections in successive developmental stages indicated that agamospermy in northeastern taxa of Spiranthes is by adventitious embryony, the embryos (one or more) being formed by proliferation of the micropylar inner integument beginning at the time of the first division of the megasporocyte. Direct anatomical evidence of adventitious embryony was obtained for S. casei var. casei, S. casei var. novaescotiae, S. cernua var. cernua, and S. magnicamporum (Ontario). Adventitious embryony in northeastern Spiranthes taxa is autonomous but not necessarily obligate and can be readily detected because of its association with polyembryonic seed, extruded embryos, free embryos, and ruptured embryo sacs. Three northeastern taxa are characterized by autonomous agamospermy throughout their areas of distribution, while another three are agamospermic only at their northeastern range limits. These six comprise a taxonomically complex group. In contrast, correlative seed morphology indicates that eight taxonomically distinctive northeastern taxa are uniformly sexual, some clearly favouring outbreeding. Adventitious embryony in northeastern North American Spiranthes is associated with glaciated territory, northeastern range limits, and more or less isolated bioclimatic zones.

3. S.romanzoffiana Cham., Linnaea 3: 32 (1828). Roots cylindrical, relatively slender. Stem 12-25 cm, sparsely glandular-puberulent above. Leaves basal and cauline, linear-lanceolate to narrowly oblong, acute, suberect, the lower 7-12 cm x 5-13 mm, the upper cauline smaller but not reduced to scales, present during anthesis. Spike 3-6 x 2 cm, dense, with 12-35 flowers in 3 spiral rows. Bracts 10-17 mm, ovate, acuminate. Perianth c. 12 mm, white tinged with green or cream, all segments, including the labellum, connate in lower half. Labellum somewhat con­ stricted below the sharply deflexed, crenulate apex. Flowering in mid- to late summer. 2n=60. Damp, peaty ground and lake- shores. Ireland, W. Scotland, S. W. England. Br Hb. (North America.), Published as part of D. M. Moore (ed.), 1980, CCIII Orchidaceae, pp. 325-350 in Flora Europaea, Vol 5: Alismataceae to Orchidaceae, Camebridge :Cambridge University Press on page 330, DOI: 10.5281/zenodo.215522