Experiments were done with the objective of describing floral induction in tobacco. A short-day mutant, grown in controlled-environment cabinets, was used to define the stages of development, and the results were used to interpret the behaviour of both short-day and day-neutral plants grown in the field. The shoot apex passed through an apparent juvenile phase, characterized by a progressive increase in its size. Next, in the absence of floral induction it entered an equilibrium stage during which its size, staining properties, and activity remained constant. After 10 inductive cycles, apices of short-day plants became committed to flower. The rate of leaf inception increased, the apical meristem became domed, and the region of intense pyronin staining (indicative of RNA) spread to the central zone. Differentiation of the inflorescence followed, and the terminal flower was recognizable about 20 days after the start of induction. The apices of field plants remained juvenile for very much longer than did apices of cabinet-grown plants, and floral induction did not take effect until after recovery from transplanting. Apices of day-neutral plants then passed directly to the induced state, whilst those of the mutant remained indefinitely vegetative in the equilibrium stage, natural day lengths at the time being non-inductive. The protracted juvenility of field plants was attributed to the stresses of seed-bed conditions and transplanting damage.

SummaryThe growth of the vegetative shoot of tobacco, Nicotiana tabacum L., and the associated changes in dry weight of the whole plant and its major parts are described. By means of serial reconstruction, the volume changes of successive young leaf primordia are followed, and this information is integrated with the dry weight data for older leaves.Initially the relative growth rates of the primordia are very high (about 1· 6 per day), but fall rapidly for 2 or 3 days to about 1·1 per day. At this level growth is exponential for a few days. The relative growth rate starts to fall again before leaf emergence, and then falls asymptotically to zero during leaf expansion.Correlations are noted between major changes in relative growth rate and vascular differentiation in the primordium. The eighth leaf is described in detail in terms of increase in cell number in both lamina and vein tissue. At first the primordium consists only of vein-like tissue; the lamina is initiated after 2 or 3 days, and its relative growth rate is higher than that of the veins for a considerable time. The proportion of cells in the vein tissue at leaf emergence is greater at higher leaf positions.The pattern of leaf growth is contrasted with that for the wheat leaf, and it is suggested that the two patterns may be dependent on different modes of vascular differentiation in the shoots.