Plantlet formation in black and white spruce. II. Histological analysis of adventitious shoot formation in vitro

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Abstract
Histological analysis of development events during adventitious shoot formation in cultured epicotyl explants of black and white spruce was carried out. Cells towards the periphery of the explants 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 explant. After about a month of culture, the entire explant became meristematic and showed evidence of additional de novo shoot developent. Several rings of bud primordia were found at the periphery of the explant 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 explant was covered with well-developed shoots having organized apical meristems, needle primordia, and vascular systems.