1. Institut des Sciences Végétales, C.N.R.S.,UPR40, 91198 Gif/Yvette cedex, France,
2. RMC Centre for Electron Microscopy, School of Biological and Molecular Sciences, Oxford Brookes University, Oxford OX3 0BP, U.K.,
3. DuPont Company, Science and Engineering Laboratories, Wilmington, DE 19880‐040, U.S.A.