Histological analysis of developmental events during adventitious shoot formation in cultured epicotyl explants of black and white spruce was carried out. Cells towards the periphery of the expiants and between the bases of the cotyledons responded to cytokinin and became meristematic. Cell division in these regions led to the formation of meristemoids and ultimately intercotyledonary adventitious shoots. These shoots developed a vascular system, which became connected to the main vascular system of the expiant. After about a month of culture, the entire expiant became meristematic and showed evidence of additionalde novoshoot development. Several rings of bud primordia were found at the periphery of the expiant after about 40 days of culture. These primordia that developed in the subepidermal layers could easily be distinguished from the surrounding cells by their distinct boundaries and intense staining for nucleic acids and proteins. After 2 months of culture, the entire expiant was covered with well-developed shoots having organized apical meristems, needle primordia, and vascular systems.
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