Ontogenesis and Life Cycle of Charophytes

This chapter focuses on the development of the complex morphology of charophytes during their life cycle. Information about ontogenesis can allow the reader to undertake critical evaluation of the importance of morphological characters, especially semi-qualitative ones as, e.g., tylacanthous or aulacanthous cortication, for determination of species. It can also provide a better understanding of the recent concepts for evolution of the phylogenetic lineages. The chapter describes a full generation cycle, from germination of the oospore until formation of gametangia, for the most complex morphology, as represented by corticated species of the genus Chara. Important deviations from this general pattern are dealt with in footnotes or, if detailed explanations are required, in separate subchapters. Irrespective of the great number of very detailed investigations about the formation of almost all parts of the charophyte thallus presented here, there are still several gaps in knowledge and several uncertainties. The most prominent uncertainty is the debate about the exact place of meiosis, and this hampers the application of chromosome numbers for phylogenetic and taxonomic investigations. In any case the huge amount of knowledge about ontogenesis and aberrations of cortication and stipulode development provide important hints for exact determination of field and herbarium material as well as for physiological investigations. The very conservative pattern of post-mitotic cell development is an especially fascinating character of charophytes, which is worth further investigation.