The combination of protandrous flowers and acropetal inflorescence development in bee-pollinated species is thought to maximize cross-pollination because bees visit vertical inflorescences from the bottom up. However, incomplete protandry may allow bees to carry out geitonogamous pollinations. We examined the overlap in male and female phases in the rare orchid Spiranthes diluvialis, a plant with the above combination of characteristics. We found that unvisited male phase flowers proceed to a hermaphroditic phase, not a female phase, because each flower's single pollinarium remains viable and may be removed by pollinators throughout anthesis. Pollinator visitation rates, as estimated by pollinaria removal rates, varied among five populations in Utah and Colorado, USA. More hermaphroditic phase flowers accrued on inflorescences in populations with low visitation rates than in those with higher visitation rates. We conclude that the cross-pollination mechanism of S. diluvialis requires some minimum threshold of bee visits in order to work optimally. When bees are plentiful, male and female functions remain temporally separated and cross-pollination is maximized. In contrast, the potential for geitonogamy in this self-compatible species is much higher in populations with low visitation rates. We suggest that pollinator abundance may affect the mating system of other protandrous, acropetal, bee-pollinated plants if protandry depends upon the timely removal of long-lived pollen.
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Vol. 133 • No. 3