STUDIES ON THE GROWTH OF EXCISED STEM TIPS OF STACHYS SIEBOLDII I. FORMATION OF TUBER IN VITRO

In autumn the subterranean stems (stolons) of Stachys sieboldii Miq. form tubers on their tips, resembling the formation of tubers in potato plants. However, if the tips of stolons were cut at various lengths and were cultured in artificial medium at the end of summer, these segments continued to grow. They gradually lost the original characters of the stolon and developed into the normally upright stems: Meanwhile, numerous adventitious roots were formed at the base of the cuttings, until a well-developed root system was established. The initiation and development of adventitious roots were intimately related to the length of the original stem-tip segments. The highest percentage of the initiation of the adventitious roots was obtained when the segments undertaken contained 5—6 internodes, whereas those with only 3 internodes showed no apparent growth. After four weeks of culture, a full plant was developed from the isolated segment. This newly formed seedling was covered with fairly dense epidermal hairs. The stem became square from which dark-green decussate foliage leaves developed. A well-developed root system was formed from the adventitious roots near the cut surface. After eight weeks in culture, the apexes of these already established upright stems became swollen and transformed into tubers of various sizes. Such tuber formation is different from that under natural conditions. At present, it is rather difficult to interpret such phenomenon by any known hypotheses of tuber forma-tion.