Shoot Bud Histogenesis from Mature Embryos and Shoots of Eastern White Cedar (Thuja occidentalis L.) Cultured In vitro

The histological events associated with shoot primordia formation from mature embryos and in vitro cultured shoots of eastern white cedar, Thuja occidentalis, were examined. Axillary meristems were not present in the axils of the cotyledons. Embryos cultured on N6-benzyladenine medium developed two shoot buds from the apical position. The two shoot primordia arose from the peripheral region of the main shoot apex. Concomitant with the shoot bud initiation, the cells at the summit of the shoot apex began to accumulate phenolic substances. This region did not grow further and the apex was divided into two domes giving rise to two shoot buds. In the original shoot explants, small groups of cells with dense cytoplasm and large nuclei could be found in the axils of the leaves. These axillary meristems were initiated during leaf primordia differentiation. Little further development occurred in these meristems in the controls; as they remained as a small group of meristematic cells separated by vacuolated cells, they became "detached meristems." In the presence of zeatin, cell division in these detached meristems was initiated after 2-3 d of culture. Axillary buds became visible after about 25 d of culture.