Shoot Histogenesis in Cotyledon Explants of Radiata Pine

The histological events associated with shoot primordium formation in cultured excised cotyledons from germinated seed of radiata pine (Pinus radiata D. Don) were examined. Cytological changes in the explants were observed by day 1 in culture. Mitotic activity, initially random, became restricted to the epidermal and subepidermal cell layers closest to the medium. This led to the formation of meristematic tissue along the entire length of the cotyledon during the first 3 wk in culture. Within this meristematic zone, meristemoids, shoot primordia, and finally shoots with well-developed apical meristems, needles, and needle primordia were formed.