MAXIMIZATION OF REPRODUCTIVE SUCCESS BY EUROPEAN ORCHIDACEAE UNDER CONDITIONS OF INFREQUENT POLLINATION

The pollination biology of a group of European orchids (Dactylorhiza, Ophrys, Orchis, Plantanthera, Goodyera, and Serapias species) are inve stigated, and their anthecological characteristics considered in relat ion to natural levels of reproductive success. Pollen:ovule (P:O) rati os of the European orchids surveyed range from 10:1 (Goodyera repens) to 24:1 (Platanthera chlorantha). Average pollen-load : ovule ratios a re consistently lower than P:O ratios. Naturally occurring pollen lend s range from 1 massula to > 1 pollinium. Even the smallest pollen load is sufficient to stimulate embryogenesis in experimentally pollinated Dactylorhiza purpurella flowers, although more seeds are set with lar ger loads. Pollen tubes grow rapidly through the stylar canal and into the top of the ovary within 2 or 3 days of pollination, and grow down either side of the 3 parietal ridges in the ovary. Fertilisation occu rs throughout the length of the ovary but its distribution is non-rand om, especially when pollen loads are limiting, with more seeds being s et at the sty lar end. All species of Dactylorhiza, Ophrys, and Orchis studied are highly self-compatible, In the absence of pollination, Op hrys and Orchis flowers remain open and fertile for at least 3 weeks. Pollinated flowers remain receptive to further pollinations for at lea st 8 days. Some fruits can even be obtained on selfing 20-day-old unpo llinated Orchis morio flowers. Excised pollinia retain germinability f or a long time, up to 51 days in Dacrylorhiza purpurella. The arrival of pollen on the stigma hastens floral senescence, but post-pollinatio n changes are relatively slow when compared with those reported for tr opical orchid species. It is concluded that characteristics of the pol lination biology of European orchids act to maximise reproductive succ ess by (1) prolonging the opportunity for effective pollen deposition both pre- and post-pollination, (2) increasing the likelihood of wides pread dispersal, (3) reducing pollen wastage, and (4) increasing seed quality by promoting some pollen competition. As most European orchids are xenogamous and require pollen to arrive on the stigma before seed can be set, reproductive maximisation is of particular adaptive advan tage because many of them art infrequently visited by insects so that the probability of successful pollination can be very low.