1. CSIRO Tropical Agriculture, 306 Carmody Road, St. Lucia, Q 4067, Australia
2. Department of Chemistry, University of York, Heslington, York YO1 5DD, U.K.
3. Centre de Recherches sur les Macromolécules Végétales (Affiliated with the Joseph Fourier University), CNRS, BP 53, F-38041 Grenoble Cedex 9, France