Pollination Ecology of Platanthera (Habenaria) Ciliaris and P. blephariglottis (Orchidaceae)

The pollination ecology of Platanthera (Habenaria) ciliaris and P. blephariglottis in Booth Lake Bog, Berrien County, Michigan, was investigated during 1973 and 1974. Platanthera ciliaris was pollinated mainly by Papilio troilus (spicebush swallowtail) and P. blephariglottis mainly by several species of moths; the main difference between these orchids was color. Butterflies apparently were attracted by the bright orange color of P. ciliaris, whereas moths appeared to be attracted more by scent than by color. The evaluation of capsule set indicated that in P. ciliaris half as many flowers were pollinated in a semiopen habitat compared with an open habitat, whereas in P. blephariglottis nearly equal pollination occurred in the open and semiopen habitats. Platanthera ciliaris had a higher mean number of flowers per raceme, which may indicate some selection pressure for larger racemes due to the visual requirements of the butterfly pollinators. Autogamy did not occur in these two Platantherans; however, artificial-pollination experiments showed that pollinator-induced self-pollination and hybridization can occur. On the basis of intermediate color, only 2.7% of the total orchid populations studied were hybrids.