8. Beitrag zur Wuchsform und Biologie der Gefäßpflanzen des hercynischen Raumes: Zur Lebensgeschichte von Salvinia natans

Abstract

Dörfelt, H., Schmidt, A. R.: 8. Contributions to the growth form and biology of Hercynian vascular plants: On the life history of Salvinia natans. – Hercynia N.F. 39 (2006): 195–221.Hydrophobic floating-layers, largely consisting of sporangia of Salvinia natans, were observed from December 2005 to March 2006 in stagnant waters of the River Elbe flootplain in central Germany. A phenomenon like this is not previously mentioned in the extensive literature on Salvinia natans. Upwelling and emergence of sporangia from the ground to the water surface, usually expected in spring, happened already in late autumn and was probably caused by the relatively high temperature in autumn 2005 due to global climatic change. The sporangia-floating-layer largely consisted of microsporangia, but also megasporangia, Chara-zygotes, statoblasts of freshwater bryozoans and plant remnants of Lemnaceae were found among them. Since both types of Salvinia-sporangia contained germinable spores, the whole development of sporangia, prothallia and young sporophytes could be observed in detail. Doing this we could clarify some previous controversial and unclear illustrations on the life history of Salvinia natans. Newly observed details are, for example, that the secondary prothallium may develop three (usually two) leaflets, and that the hairs of the sporophyte may have rounded (usually cone-shaped) tips at the apical cells. Also the arrangement of the 64 microspores in immature microsporangia was undescribed to date. The microspores are initially arranged in eight groups of eight spores each. New insights could also be derived regarding the reorganisation of the tapetum from a primary cellular structure into the foamy matrix. Apart from intact megasporangia with germinable spores, the floating-layer also yielded remnants of destroyed megasporangia. Megaspores with a surrounding ring-shaped matrix produced by the tapetum were developed by these sporangia and are hereby of particular interest. This is because these megaspores probably show a genetically determined ancestral structure. Observing these free megaspores we were probably allowed to comprehend the structure of megaspores of floating fern ancestors. In Earth history, these ancestors might have had free megaspores with floating bodies developed by the tapetum as found in modern Azolla-megaspores. We interpret this structure to be homologous to the floating bodies of the Azollaceae, the floating fern’s sister group, developing prothallia outside the spores. Thus, global warming modified the life cycle of the European annual floating fern and gave new insights into ontogeny and history of floating ferns.