The effects of photoperiod (8, 12 or 16 h), mineral medium strength (dilutions of a tuberization medium, the T medium), sucrose (0, 2, 4, 8% w/v) and kinetin (0, 2.5 #M) on the development of roots, shoots and microtubers in shoot cultures of Dioseorea alata L. and D. bulbifera L. yams were evaluated. All. of the factors were found to have substantial effects on microtuber induction in these two species. The effects of high and low inorganic ammonium containing media on microtuberization of yam shoot cultures indicated that ammonium ions inhibited microtuber induction in D. alata but not in D. bulbifera. Microtubers ofD. alata were only formed on shoot cultures if these were held under 8-h days. D. bulbifera cultures on the other hand produced microtubers under this photoperiod treatment as well as under 16-h photoperiods provided that kinetin was present in media at 2.5/tM. Most microtuberization in D. alata shoot cultures occurred on full-strength T medium supplemented with 2% sucrose, 2.5/~M kinetin held under 8-h photoperiods at 25 °C, whereas most microtuberization in D. bulbifera shoot cultures occurred on full-strength MS medium supplemented with 4% sucrose, 2.5 ~M kinetin held under 8-h photoperiods at 25 °C. Under these two sets of conditions, yam shoot cultures consistently produced microtubers with individual weights in excess of 100 mg which were large enough to be capable of direct planting and subsequent growth in unsterilized soils.

A successful micrografting technique was developed for P&tacia vera. High levels of graft union were achieved when shoots from Stage II cultures of four-year-old P. vera. cv. Mateur were grafted onto in vitro-raised seedling rootstocks. Light and fluorescence microscopy investigations revealed that vascular continuity was established across grafts by three weeks.

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