1. Institute of Plant Biology, University of Zurich, CH-8008 Zurich,1
2. Institute of Physiology, University of Zurich, CH-8057 Zurich,2
3. Laboratory for Electron Microscopy, ETH-Zentrum, CH-8092 Zurich,3 and
4. Laboratory for Electron Microscopy, University Hospital, CH-8091 Zurich,4 Switzerland