Ultrastructural Aspects of Shoot Initiation in Tobacco Callus Cultures

An ultrastructural investigation of shoot initiation in tobacco ( Nicotiana tabacum L. var. W. 38) callus cultures was made. Zones of preferential division were observed in the basal portion of the tissue by eight days in culture and these led, sequentially, to meristemoids, primordia, and shoots. During the initial stages of meristemoid formation, protein inclusions and large accumulations of plastid starch were present in the cells, while vacuoles were filled with membranous and cytoplasmic protrusions. At later stages of meristemoid development, these features were not observed in the cells, which were also smaller in size and possessed numerous small, peripheral vacuoles. It appears that the membranous and cytoplasmic protrusions are involved in vacuolar reduction during meristemoid formation. It would also appear that the storage materials supply the energy and other reserves needed for the organogenetic process. By contrast, tissue cultured under nonshoot‐forming conditions and nonmeristemoid regions of shoot‐forming tissue remained parenchymatous over the same time period.