INTERPRETING THE FUNCTION OF SACCATE POLLEN IN ANCIENT SEED PLANTS

Saccate pollen is present in many ancient gymnosperm taxa and in two families of extant conifers. Sacci do not occur on the spores of wind-dispersed pteridophytes, the pollen of anemophilous angiosperms, or the pollen of many anemophilous gymnosperms, suggesting sacci may not perform an essential function in wind dispersal. In living gymnosperms, saccate pollen is consistently found in taxa with erect ovulate cones at the time of pollination, inverted ovules, and pollination droplets. Sacci primarily function as floats which move pollen grains through a pollination droplet and concentrate them in the micropyle of the ovule. The morphology and anatomy of sacci are crucial to the proper operation of this specialized gymnosperm pollination mechanism. A similar relationship between saccus height and corpus size in ancient and modern taxa suggests that ancient saccate pollen may have functioned in the same way as modern forms. In order to more fully compare the morphology of ancient and modern gymnosperms, a multivariate analysis of discrete pollen characters was performed on data from in situ grains of walchian, voltzialean, and extant conifer taxa, as well as several taxa from non-conifer gymnosperm groups. This analysis indicates that the morphology of ancient conifer pollen is significantly different from modern saccate pollen. Modern saccus morphology and anatomy do not appear until relatively late in conifer evolutionary history. However, essentially modern saccate pollen morphology appears as early as the Upper Carboniferous in some non-coniferous gymnosperms. The results of this study suggest that sacci may have functioned in their modern capacity by the Upper Carboniferous in at least some gymnosperm groups. The prevalence of inverted ovules in fossil gymnosperm groups having saccate pollen provides further support for the operation of this pollination mechanism in ancient taxa. This study also suggests modern saccus morphology may have independently evolved several times in gymnosperms.