Shoot Histogenesis in Cotyledon Explants of Radiata Pine

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Abstract
The histological events associated with shoot primordium formation in cultured excised cotyledons from germinated seed of radiata pine (P. 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 shoots with well-developed apical meristems, needles and needle primordia were formed. [Since P. radiata D. Don (radiata or Monterey pine) is a conifer of considerable economic importance in several Southern Hemisphere countries, there has been much interest in propagating the species vegetatively.].