Received for publication 21 Jan. 1988. This work was supported by Academia Sinica and grant NSC-69B-0409-03(01) of the National Science Council, ROC. I thank S.P. Ho for her excellent technical assistance and Liang Li of Chia-Yi AES for providing sweet potato roots. Mention of a trademark or proprietary product docs not constitute a guarantee or warranty of the product by the Institute of Botany, Academia Sinica, nor docs it imply approval of the exclusion of other products that may be suitable. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.