1. Graduate School of Agriculture, Hokkaido University, Sapporo 060‐8589, Japan
2. Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK
3. Laboratory of Plant Systematics, Institute of Botany and Microbiology, K.U. Leuven, Kasteelpark Arenberg 31, B‐3001 Leuven, Belgium