1. Section of Plant Biology, 408 Mann Library, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA.
2. Section of Plant Biology, 412 Mann Library, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA.
3. Electron Microscope Facility, 7605 Remsen, Dartmouth College, 74 College Street, Hanover, NH 03755, USA.