Interpreting the Function of Saccate Pollen in Ancient Conifers and Other Seed Plants

Abstract

Saccate pollen is present in many ancient seed plants and in two families of extant conifers. In living conifers, saccate pollen occurs in taxa that combine erect ovulate cones at the time of pollination, inverted ovules, and pollination drops. Sacci function primarily to float pollen grains through a liquid pollination drop and concentrate them near the nucellus of the ovule. The morphology and anatomy of sacci are crucial to the proper operation of this pollination mechanism, and similar size proportions in fossil and modern saccate grains suggest that ancient pollen may have functioned in the same way as modern forms. Multivariate analysis of discrete pollen characters performed on data from in situ grains of several groups of fossil seed plants indicates that modern saccus morphology and structure appear as early as the Upper Carboniferous in some nonconiferous seed plants but later in the evolutionary history of the conifer lineage. Combined evidence from pollen morphology suggests that sacci may have functioned in their modern capacity by the Upper Carboniferous in at least some groups of seed plants, which is consistent with the occurrence of potentially inverted ovules in many extinct groups producing saccate pollen. This study also suggests that modern saccus morphology has evolved independently several times over the course of seed plant evolution.