POLLINATION BIOLOGY OF LISTERA CORDATA (ORCHIDACEAE)

Pollination biology of northern California populations of Listera cordata (L.) R. Br. is presented. The flowers are allogamous, self-compatible, and protandrous. At anthesis, access to the stigma is blocked by the flap-like rostellum. When an insect contacts the touch-sensitive trigger hairs of the rostellum, the pollinia are suddenly released and become cemented to the insect's body. One to two days after the pollinia are removed, the rostellum rises and exposes the receptive stigma. Visitors are attracted to the flowers by their foul odor and minute quantities of nectar presented on the surface of the labellum. Pollination efficiency is high with 61–78% of flowers from several localities setting fruit. The most important and abundant pollinators of L. cordata at our study sites are fungus gnats (Sciaridae and Mycetophylidae). Other less frequently encountered pollinators are members of these families: Braconidae, Ichneumonidae, and Tipulidae. At present, there is no compelling evidence that the floral characteristics of L. cordata have evolved as a result of an evolutionary interaction with any specific group of insects. Nor is there any indication that the flowers are sapromyophilous, despite their putrid odor.