Origin of adventitious shoots in excised radiata pine cotyledons cultured in vitro

Abstract
Previous studies on shoot initiation in cultured excised cotyledons of Pinus radiata indicated that the process began early in culture on the side of the cotyledon in contact with the cytokinin-containing medium. In contrast to cotyledons cultured without cytokinin, organized structures, which have been termed promeristemoids, can be observed in cotyledons cultured with cytokinin after 5 days in culture. These structures arise from single cells in the first subepidermal cell layer at day 3. They are stable and their continued division leads to the formation of meristemoids, shoot primordia, and finally shoots with primary needles.