DICHOGAMY IN ANGIOSPERMS

We obtained information on dichogamy and other aspects of the biology of over 4200 species of angiosperms from several hundred published and unpublished sources. We used this information to describe patterns of occurrence of dichogamy and to test specific hypotheses relating dich ogamy to other characteristics of plants or their environments. Protan dry was more common than protogyny at the intrafloral level, but the r everse was true at the interfloral level. Patterns of dichogamy varied significantly among major taxa, with protogyny more common among mono cotyledons and primitive dicotyledons, and protandry expecially common in the Asteridae. Arctic species tended to be less dichogamous and mo re protogynous than temperate and tropical species. Aquatic and alpine species were especially protogynous. Patterns of dichogamy varied amo ng sexual systems, with gynomonoecious and gynodioecious species espec ially protandrous, and monoecious species highly protogynous. Autogamo us and self-compatible species were disproportionately protogynous. Fl owers of intraflorally dichogamous species were slightly larger than t hose of adichogamous species, owing to the presence of many autogamous species in the latter group. Species with interfloral protogyny bore much smaller flowers than did species with interfloral protandry. Earl y-blooming species in north-temperate and polar regions were dispropor tionately protogynous. Sexual structures that abscised, shriveled or m oved after completion of their function tended to be presented first, and those that facilitated the other sexual function were presented se cond. A negative association existed between type of intrafloral and i nterfloral dichogamy in diclinous species. Most animal-pollinated flow ers were protandrous, except beetle-pollinated and refuge and trap blo ssoms. Wind pollination was markedly associated with protogyny. Vertic al inflorescences visited by upwardly-moving vectors were protandrous. Our results suggest that three primary factors may be involved in pro moting dichogamy: selection for avoidance of pollen-pistil interferenc e, selection for avoidance of self-fertilization, and selection for sy nchrony of pollen discharge and stigma receptivity in the different fl ower types of diclinous species. In contrast to many earlier workers w e reject the thesis that avoidance of self-fertilization is the univer sal or even the most important force in the evolution of most forms of dichogamy. We attribute the prevalence of intrafloral protandry to se lection for avoiding interference between pollen export and pollen rec eipt. Intrafloral protogyny was associated with imprecise pollen trans fer, where other means of avoiding pollen-pistil interference (e.g., h erkogamy) are likely to be of limited value. The prevalence of interfl oral protogyny seems to reflect the smaller size of unisexual flowers than bisexual flowers, the absence of intrafloral pollen-pistil interf erence in diclinous species, and selection for synchrony of pollen dis charge from one flower type with stigma receptivity in the other.