1. Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences University of Glasgow Glasgow G12 8QQ, U.K.
2. Centre for Plant Research, University of British Columbia Vancouver British Columbia V6T 1Z4, Canada; E-mail: dmpercy@interchange.ubc.ca (D.M.P.)
3. Institute of Cell and Molecular Biology, University of Edinburgh Kings Buildings, Mayfield Road, Edinburgh EH9 3JH, U.K.
4. Royal Botanic Garden 20A Inverleith Row, Edinburgh EH3 5LR, U.K.