Growth and Morphology of Tobacco Tissue Cultures In vitro

The growth in darkness of tobacco stem callus tissue on nutrient agar medium was investigated. Growth was studied during a period of 9 weeks in cultures taken from stock subcultures which were 6 weeks old. The experimental cultures were prepared by cutting the stock cultures into uniform cubes of two different sizes by a technique which is described. 2. Increase in size of the cultures resulted from the proliferation of small knoblike protuberances in which division and enlargement of cells was occurring very close to the surface. Knobs increased in number in two ways: (a) by cleavage and (b) by the formation of new growing centers in the subsurface of older knobs. An account of the histology of the growing cultures is presented. 3. Though uniform in initial size and under constant cultural conditions, the cultures within each series grew at different relative rates. Thus there were differences inherent in the initial culture pieces which determined the subsequent rates of growth of the cultures. 4. Over and above the large amount of variation in growth rate of cultures started from pieces of the same size, there were differences in growth rate attributable to the initial size of the explant. The smaller the explant, the greater was the relative growth rate. 5. For each series growth curves were plotted by reference to the largest culture at any time. In these terms the cultures increased exponentially with time for 5-6 weeks. Growth rates are described in terms of Blackman's formula.