References ............................... 76 Subject Index ............................. 93 VIII Acknowledgments This study was funded by the Deutsche Forschungsgemeinschaft. I am indebted to Prof. Dr. W. Schlote for helpful advice and numerous discussions. I am also grateful to Dr. G. Kurz-Isler for her generous help in problems dealing with electron microscopy and to Mrs. B. Sabrowski for her careful preparation of the manuscript. The careful translation of T.C. Telger is gratefully acknowledged. The translation was financially supported by the Erwin Riesch Foundation. IX 1 Introduction...