1. Institute of Environmental Science and Research; PO Box 29-181 Christchurch New Zealand
2. Dep. of Applied Mathematics; Australian National Univ.; Canberra NSW Australia
3. Lincoln Environmental Research; PO Box 133, Lincoln Christchurch New Zealand
4. Plant and Food Research Lincoln; Private Bag 4704, Christchurch Mail Centre; Christchurch New Zealand
5. Lincoln Environmental Research; Private Bag 3062 Hamilton New Zealand