REPRODUCTIVE BIOLOGY OF THE YELLOW‐FRINGED ORCHID, PLATANTHERA CILIARIS

Platanthera ciliaris is a butterfly‐pollinated, terrestrial orchid with a loose terminal raceme of 10–50 orange flowers, characterized by a long nectariferous spur. In the southeastern United States, P. ciliaris occurs in the Appalachian Mountains and Coastal Plain physiographic provinces, but rarely in the intervening Piedmont. Of 153 plants that flowered in a Mountain population in 1983, 54 (35.2%) flowered again in 1984; in a Coastal Plain population, only 51 of 306 (16.7%) flowered in both years. Experimental pollinations showed that P. ciliaris is self‐compatible but not autogamous. Nectar volume increased over the 6‐ to 7‐day life span of a flower, reaching a maximum of 4 μ l and 1.0 mg of sugar (Mountains) or 6 μ l and 1.2 mg of sugar (Coastal Plain). Nectar concentrations were relatively constant in flowers of all ages and averaged 23% in the Mountains and 19% in the Coastal Plain. Diurnal patterns of nectar secretion were not well‐defined, but most nectar appeared to be produced overnight. In the Mountains Papilio troilus (spicebush swallowtail) was the most frequent and effective pollinator, whereas in the Coastal Plain P. palamedes (palamedes swallowtail) was the predominant pollinator. These large butterflies were most active in the late morning (0930–1030) and early afternoon (1230–1400). Levels of pollinator service, as assessed by rates of removal and insertion of pollinia, were higher in the Mountains (0.81, 0.86) than in the Coastal Plain (0.63, 0.67). These values were reflected in higher levels of fruit‐set in Mountain (83.9% in 1983, 86.5% in 1984) than in Coastal Plain (63.8, 65.5%) populations. Combining these observations with the fact that hand pollination increased fruit‐set of Coastal Plain flowers 6.5–13.5% above open‐pollinated controls, it appears that fruit‐set in P. ciliaris is pollen‐limited.