STRUCTURE AND EVOLUTION OF METARETICULATE POLLEN

In a palynological study of the Amaranthaceae, a peculiar type of reti culate pollen was found that is characterized by the presence of a por ate aperture in each of the meshes of the reticulum. Previously, this type of pollen has been described as ''reticulate''. However, closer i nvestigations show that the reticulum in pollen of Amaranthaceae is co mposed of mesoporia and pores. Consequently, this kind of reticulum re presents a fundamentally different type, and is not homologous to the well known examples of pollen grains with a true reticulum (e.g. in Br omeliaceae, Lamiaceae). Therefore, the term ''metareticulate'' is prop osed (i.e., pantoporate pollen with a reticulum-like structure of meso poria and pores). The new term allows to distinguish between metaretic ulate and truely reticulate pollen, what is important in phylogenetic studies. Metareticulate pollen occurs only within lineages characteriz ed by pantoporate pollen, and is found to be derived from pantoporate pollen in a cladistic analysis. Apart from the Amaranthaceae, metareti culate pollen evolved parallel in Vivianiaceae and Zygophyllaceae. In Caryophyllaceae and Convolvulaceae only a trend towards a metareticula tion is observed. Metareticulate pollen is suggested as representing t he highest developmental level in successiformy, which is one of the m ajor patterns in pollen evolution leading from tricolpate to pantopora te grains.